[L2TP]: Changes to existing ppp and socket kernel headers for L2TP

[linux-2.6/linux-acpi-2.6/ibm-acpi-2.6.git] / fs / nfs / direct.c

/*
 * linux/fs/nfs/direct.c
 *
 * Copyright (C) 2003 by Chuck Lever <cel@netapp.com>
 *
 * High-performance uncached I/O for the Linux NFS client
 *
 * There are important applications whose performance or correctness
 * depends on uncached access to file data.  Database clusters
 * (multiple copies of the same instance running on separate hosts)
 * implement their own cache coherency protocol that subsumes file
 * system cache protocols.  Applications that process datasets
 * considerably larger than the client's memory do not always benefit
 * from a local cache.  A streaming video server, for instance, has no
 * need to cache the contents of a file.
 *
 * When an application requests uncached I/O, all read and write requests
 * are made directly to the server; data stored or fetched via these
 * requests is not cached in the Linux page cache.  The client does not
 * correct unaligned requests from applications.  All requested bytes are
 * held on permanent storage before a direct write system call returns to
 * an application.
 *
 * Solaris implements an uncached I/O facility called directio() that
 * is used for backups and sequential I/O to very large files.  Solaris
 * also supports uncaching whole NFS partitions with "-o forcedirectio,"
 * an undocumented mount option.
 *
 * Designed by Jeff Kimmel, Chuck Lever, and Trond Myklebust, with
 * help from Andrew Morton.
 *
 * 18 Dec 2001  Initial implementation for 2.4  --cel
 * 08 Jul 2002  Version for 2.4.19, with bug fixes --trondmy
 * 08 Jun 2003  Port to 2.5 APIs  --cel
 * 31 Mar 2004  Handle direct I/O without VFS support  --cel
 * 15 Sep 2004  Parallel async reads  --cel
 * 04 May 2005  support O_DIRECT with aio  --cel
 *
 */

#include <linux/errno.h>
#include <linux/sched.h>
#include <linux/kernel.h>
#include <linux/file.h>
#include <linux/pagemap.h>
#include <linux/kref.h>

#include <linux/nfs_fs.h>
#include <linux/nfs_page.h>
#include <linux/sunrpc/clnt.h>

#include <asm/system.h>
#include <asm/uaccess.h>
#include <asm/atomic.h>

#include "internal.h"
#include "iostat.h"

#define NFSDBG_FACILITY         NFSDBG_VFS

static struct kmem_cache *nfs_direct_cachep;

/*
 * This represents a set of asynchronous requests that we're waiting on
 */
struct nfs_direct_req {
        struct kref             kref;           /* release manager */

        /* I/O parameters */
        struct nfs_open_context *ctx;           /* file open context info */
        struct kiocb *          iocb;           /* controlling i/o request */
        struct inode *          inode;          /* target file of i/o */

        /* completion state */
        atomic_t                io_count;       /* i/os we're waiting for */
        spinlock_t              lock;           /* protect completion state */
        ssize_t                 count,          /* bytes actually processed */
                                error;          /* any reported error */
        struct completion       completion;     /* wait for i/o completion */

        /* commit state */
        struct list_head        rewrite_list;   /* saved nfs_write_data structs */
        struct nfs_write_data * commit_data;    /* special write_data for commits */
        int                     flags;
#define NFS_ODIRECT_DO_COMMIT           (1)     /* an unstable reply was received */
#define NFS_ODIRECT_RESCHED_WRITES      (2)     /* write verification failed */
        struct nfs_writeverf    verf;           /* unstable write verifier */
};

static void nfs_direct_write_complete(struct nfs_direct_req *dreq, struct inode *inode);
static const struct rpc_call_ops nfs_write_direct_ops;

static inline void get_dreq(struct nfs_direct_req *dreq)
{
        atomic_inc(&dreq->io_count);
}

static inline int put_dreq(struct nfs_direct_req *dreq)
{
        return atomic_dec_and_test(&dreq->io_count);
}

/**
 * nfs_direct_IO - NFS address space operation for direct I/O
 * @rw: direction (read or write)
 * @iocb: target I/O control block
 * @iov: array of vectors that define I/O buffer
 * @pos: offset in file to begin the operation
 * @nr_segs: size of iovec array
 *
 * The presence of this routine in the address space ops vector means
 * the NFS client supports direct I/O.  However, we shunt off direct
 * read and write requests before the VFS gets them, so this method
 * should never be called.
 */
ssize_t nfs_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov, loff_t pos, unsigned long nr_segs)
{
        dprintk("NFS: nfs_direct_IO (%s) off/no(%Ld/%lu) EINVAL\n",
                        iocb->ki_filp->f_path.dentry->d_name.name,
                        (long long) pos, nr_segs);

        return -EINVAL;
}

static void nfs_direct_dirty_pages(struct page **pages, unsigned int pgbase, size_t count)
{
        unsigned int npages;
        unsigned int i;

        if (count == 0)
                return;
        pages += (pgbase >> PAGE_SHIFT);
        npages = (count + (pgbase & ~PAGE_MASK) + PAGE_SIZE - 1) >> PAGE_SHIFT;
        for (i = 0; i < npages; i++) {
                struct page *page = pages[i];
                if (!PageCompound(page))
                        set_page_dirty(page);
        }
}

static void nfs_direct_release_pages(struct page **pages, unsigned int npages)
{
        unsigned int i;
        for (i = 0; i < npages; i++)
                page_cache_release(pages[i]);
}

static inline struct nfs_direct_req *nfs_direct_req_alloc(void)
{
        struct nfs_direct_req *dreq;

        dreq = kmem_cache_alloc(nfs_direct_cachep, GFP_KERNEL);
        if (!dreq)
                return NULL;

        kref_init(&dreq->kref);
        kref_get(&dreq->kref);
        init_completion(&dreq->completion);
        INIT_LIST_HEAD(&dreq->rewrite_list);
        dreq->iocb = NULL;
        dreq->ctx = NULL;
        spin_lock_init(&dreq->lock);
        atomic_set(&dreq->io_count, 0);
        dreq->count = 0;
        dreq->error = 0;
        dreq->flags = 0;

        return dreq;
}

static void nfs_direct_req_free(struct kref *kref)
{
        struct nfs_direct_req *dreq = container_of(kref, struct nfs_direct_req, kref);

        if (dreq->ctx != NULL)
                put_nfs_open_context(dreq->ctx);
        kmem_cache_free(nfs_direct_cachep, dreq);
}

static void nfs_direct_req_release(struct nfs_direct_req *dreq)
{
        kref_put(&dreq->kref, nfs_direct_req_free);
}

/*
 * Collects and returns the final error value/byte-count.
 */
static ssize_t nfs_direct_wait(struct nfs_direct_req *dreq)
{
        ssize_t result = -EIOCBQUEUED;

        /* Async requests don't wait here */
        if (dreq->iocb)
                goto out;

        result = wait_for_completion_interruptible(&dreq->completion);

        if (!result)
                result = dreq->error;
        if (!result)
                result = dreq->count;

out:
        return (ssize_t) result;
}

/*
 * Synchronous I/O uses a stack-allocated iocb.  Thus we can't trust
 * the iocb is still valid here if this is a synchronous request.
 */
static void nfs_direct_complete(struct nfs_direct_req *dreq)
{
        if (dreq->iocb) {
                long res = (long) dreq->error;
                if (!res)
                        res = (long) dreq->count;
                aio_complete(dreq->iocb, res, 0);
        }
        complete_all(&dreq->completion);

        nfs_direct_req_release(dreq);
}

/*
 * We must hold a reference to all the pages in this direct read request
 * until the RPCs complete.  This could be long *after* we are woken up in
 * nfs_direct_wait (for instance, if someone hits ^C on a slow server).
 */
static void nfs_direct_read_result(struct rpc_task *task, void *calldata)
{
        struct nfs_read_data *data = calldata;
        struct nfs_direct_req *dreq = (struct nfs_direct_req *) data->req;

        if (nfs_readpage_result(task, data) != 0)
                return;

        spin_lock(&dreq->lock);
        if (unlikely(task->tk_status < 0)) {
                dreq->error = task->tk_status;
                spin_unlock(&dreq->lock);
        } else {
                dreq->count += data->res.count;
                spin_unlock(&dreq->lock);
                nfs_direct_dirty_pages(data->pagevec,
                                data->args.pgbase,
                                data->res.count);
        }
        nfs_direct_release_pages(data->pagevec, data->npages);

        if (put_dreq(dreq))
                nfs_direct_complete(dreq);
}

static const struct rpc_call_ops nfs_read_direct_ops = {
        .rpc_call_done = nfs_direct_read_result,
        .rpc_release = nfs_readdata_release,
};

/*
 * For each rsize'd chunk of the user's buffer, dispatch an NFS READ
 * operation.  If nfs_readdata_alloc() or get_user_pages() fails,
 * bail and stop sending more reads.  Read length accounting is
 * handled automatically by nfs_direct_read_result().  Otherwise, if
 * no requests have been sent, just return an error.
 */
static ssize_t nfs_direct_read_schedule(struct nfs_direct_req *dreq, unsigned long user_addr, size_t count, loff_t pos)
{
        struct nfs_open_context *ctx = dreq->ctx;
        struct inode *inode = ctx->dentry->d_inode;
        size_t rsize = NFS_SERVER(inode)->rsize;
        unsigned int pgbase;
        int result;
        ssize_t started = 0;

        get_dreq(dreq);

        do {
                struct nfs_read_data *data;
                size_t bytes;

                pgbase = user_addr & ~PAGE_MASK;
                bytes = min(rsize,count);

                result = -ENOMEM;
                data = nfs_readdata_alloc(nfs_page_array_len(pgbase, bytes));
                if (unlikely(!data))
                        break;

                down_read(&current->mm->mmap_sem);
                result = get_user_pages(current, current->mm, user_addr,
                                        data->npages, 1, 0, data->pagevec, NULL);
                up_read(&current->mm->mmap_sem);
                if (result < 0) {
                        nfs_readdata_release(data);
                        break;
                }
                if ((unsigned)result < data->npages) {
                        nfs_direct_release_pages(data->pagevec, result);
                        nfs_readdata_release(data);
                        break;
                }

                get_dreq(dreq);

                data->req = (struct nfs_page *) dreq;
                data->inode = inode;
                data->cred = ctx->cred;
                data->args.fh = NFS_FH(inode);
                data->args.context = ctx;
                data->args.offset = pos;
                data->args.pgbase = pgbase;
                data->args.pages = data->pagevec;
                data->args.count = bytes;
                data->res.fattr = &data->fattr;
                data->res.eof = 0;
                data->res.count = bytes;

                rpc_init_task(&data->task, NFS_CLIENT(inode), RPC_TASK_ASYNC,
                                &nfs_read_direct_ops, data);
                NFS_PROTO(inode)->read_setup(data);

                data->task.tk_cookie = (unsigned long) inode;

                rpc_execute(&data->task);

                dprintk("NFS: %5u initiated direct read call "
                        "(req %s/%Ld, %zu bytes @ offset %Lu)\n",
                                data->task.tk_pid,
                                inode->i_sb->s_id,
                                (long long)NFS_FILEID(inode),
                                bytes,
                                (unsigned long long)data->args.offset);

                started += bytes;
                user_addr += bytes;
                pos += bytes;
                /* FIXME: Remove this unnecessary math from final patch */
                pgbase += bytes;
                pgbase &= ~PAGE_MASK;
                BUG_ON(pgbase != (user_addr & ~PAGE_MASK));

                count -= bytes;
        } while (count != 0);

        if (put_dreq(dreq))
                nfs_direct_complete(dreq);

        if (started)
                return 0;
        return result < 0 ? (ssize_t) result : -EFAULT;
}

static ssize_t nfs_direct_read(struct kiocb *iocb, unsigned long user_addr, size_t count, loff_t pos)
{
        ssize_t result = 0;
        sigset_t oldset;
        struct inode *inode = iocb->ki_filp->f_mapping->host;
        struct rpc_clnt *clnt = NFS_CLIENT(inode);
        struct nfs_direct_req *dreq;

        dreq = nfs_direct_req_alloc();
        if (!dreq)
                return -ENOMEM;

        dreq->inode = inode;
        dreq->ctx = get_nfs_open_context((struct nfs_open_context *)iocb->ki_filp->private_data);
        if (!is_sync_kiocb(iocb))
                dreq->iocb = iocb;

        nfs_add_stats(inode, NFSIOS_DIRECTREADBYTES, count);
        rpc_clnt_sigmask(clnt, &oldset);
        result = nfs_direct_read_schedule(dreq, user_addr, count, pos);
        if (!result)
                result = nfs_direct_wait(dreq);
        rpc_clnt_sigunmask(clnt, &oldset);
        nfs_direct_req_release(dreq);

        return result;
}

static void nfs_direct_free_writedata(struct nfs_direct_req *dreq)
{
        while (!list_empty(&dreq->rewrite_list)) {
                struct nfs_write_data *data = list_entry(dreq->rewrite_list.next, struct nfs_write_data, pages);
                list_del(&data->pages);
                nfs_direct_release_pages(data->pagevec, data->npages);
                nfs_writedata_release(data);
        }
}

#if defined(CONFIG_NFS_V3) || defined(CONFIG_NFS_V4)
static void nfs_direct_write_reschedule(struct nfs_direct_req *dreq)
{
        struct inode *inode = dreq->inode;
        struct list_head *p;
        struct nfs_write_data *data;

        dreq->count = 0;
        get_dreq(dreq);

        list_for_each(p, &dreq->rewrite_list) {
                data = list_entry(p, struct nfs_write_data, pages);

                get_dreq(dreq);

                /*
                 * Reset data->res.
                 */
                nfs_fattr_init(&data->fattr);
                data->res.count = data->args.count;
                memset(&data->verf, 0, sizeof(data->verf));

                /*
                 * Reuse data->task; data->args should not have changed
                 * since the original request was sent.
                 */
                rpc_init_task(&data->task, NFS_CLIENT(inode), RPC_TASK_ASYNC,
                                &nfs_write_direct_ops, data);
                NFS_PROTO(inode)->write_setup(data, FLUSH_STABLE);

                data->task.tk_priority = RPC_PRIORITY_NORMAL;
                data->task.tk_cookie = (unsigned long) inode;

                /*
                 * We're called via an RPC callback, so BKL is already held.
                 */
                rpc_execute(&data->task);

                dprintk("NFS: %5u rescheduled direct write call (req %s/%Ld, %u bytes @ offset %Lu)\n",
                                data->task.tk_pid,
                                inode->i_sb->s_id,
                                (long long)NFS_FILEID(inode),
                                data->args.count,
                                (unsigned long long)data->args.offset);
        }

        if (put_dreq(dreq))
                nfs_direct_write_complete(dreq, inode);
}

static void nfs_direct_commit_result(struct rpc_task *task, void *calldata)
{
        struct nfs_write_data *data = calldata;
        struct nfs_direct_req *dreq = (struct nfs_direct_req *) data->req;

        /* Call the NFS version-specific code */
        if (NFS_PROTO(data->inode)->commit_done(task, data) != 0)
                return;
        if (unlikely(task->tk_status < 0)) {
                dprintk("NFS: %5u commit failed with error %d.\n",
                                task->tk_pid, task->tk_status);
                dreq->flags = NFS_ODIRECT_RESCHED_WRITES;
        } else if (memcmp(&dreq->verf, &data->verf, sizeof(data->verf))) {
                dprintk("NFS: %5u commit verify failed\n", task->tk_pid);
                dreq->flags = NFS_ODIRECT_RESCHED_WRITES;
        }

        dprintk("NFS: %5u commit returned %d\n", task->tk_pid, task->tk_status);
        nfs_direct_write_complete(dreq, data->inode);
}

static const struct rpc_call_ops nfs_commit_direct_ops = {
        .rpc_call_done = nfs_direct_commit_result,
        .rpc_release = nfs_commit_release,
};

static void nfs_direct_commit_schedule(struct nfs_direct_req *dreq)
{
        struct nfs_write_data *data = dreq->commit_data;

        data->inode = dreq->inode;
        data->cred = dreq->ctx->cred;

        data->args.fh = NFS_FH(data->inode);
        data->args.offset = 0;
        data->args.count = 0;
        data->res.count = 0;
        data->res.fattr = &data->fattr;
        data->res.verf = &data->verf;

        rpc_init_task(&data->task, NFS_CLIENT(dreq->inode), RPC_TASK_ASYNC,
                                &nfs_commit_direct_ops, data);
        NFS_PROTO(data->inode)->commit_setup(data, 0);

        data->task.tk_priority = RPC_PRIORITY_NORMAL;
        data->task.tk_cookie = (unsigned long)data->inode;
        /* Note: task.tk_ops->rpc_release will free dreq->commit_data */
        dreq->commit_data = NULL;

        dprintk("NFS: %5u initiated commit call\n", data->task.tk_pid);

        rpc_execute(&data->task);
}

static void nfs_direct_write_complete(struct nfs_direct_req *dreq, struct inode *inode)
{
        int flags = dreq->flags;

        dreq->flags = 0;
        switch (flags) {
                case NFS_ODIRECT_DO_COMMIT:
                        nfs_direct_commit_schedule(dreq);
                        break;
                case NFS_ODIRECT_RESCHED_WRITES:
                        nfs_direct_write_reschedule(dreq);
                        break;
                default:
                        nfs_end_data_update(inode);
                        if (dreq->commit_data != NULL)
                                nfs_commit_free(dreq->commit_data);
                        nfs_direct_free_writedata(dreq);
                        nfs_zap_mapping(inode, inode->i_mapping);
                        nfs_direct_complete(dreq);
        }
}

static void nfs_alloc_commit_data(struct nfs_direct_req *dreq)
{
        dreq->commit_data = nfs_commit_alloc();
        if (dreq->commit_data != NULL)
                dreq->commit_data->req = (struct nfs_page *) dreq;
}
#else
static inline void nfs_alloc_commit_data(struct nfs_direct_req *dreq)
{
        dreq->commit_data = NULL;
}

static void nfs_direct_write_complete(struct nfs_direct_req *dreq, struct inode *inode)
{
        nfs_end_data_update(inode);
        nfs_direct_free_writedata(dreq);
        nfs_zap_mapping(inode, inode->i_mapping);
        nfs_direct_complete(dreq);
}
#endif

static void nfs_direct_write_result(struct rpc_task *task, void *calldata)
{
        struct nfs_write_data *data = calldata;
        struct nfs_direct_req *dreq = (struct nfs_direct_req *) data->req;
        int status = task->tk_status;

        if (nfs_writeback_done(task, data) != 0)
                return;

        spin_lock(&dreq->lock);

        if (unlikely(dreq->error != 0))
                goto out_unlock;
        if (unlikely(status < 0)) {
                /* An error has occured, so we should not commit */
                dreq->flags = 0;
                dreq->error = status;
        }

        dreq->count += data->res.count;

        if (data->res.verf->committed != NFS_FILE_SYNC) {
                switch (dreq->flags) {
                        case 0:
                                memcpy(&dreq->verf, &data->verf, sizeof(dreq->verf));
                                dreq->flags = NFS_ODIRECT_DO_COMMIT;
                                break;
                        case NFS_ODIRECT_DO_COMMIT:
                                if (memcmp(&dreq->verf, &data->verf, sizeof(dreq->verf))) {
                                        dprintk("NFS: %5u write verify failed\n", task->tk_pid);
                                        dreq->flags = NFS_ODIRECT_RESCHED_WRITES;
                                }
                }
        }
out_unlock:
        spin_unlock(&dreq->lock);
}

/*
 * NB: Return the value of the first error return code.  Subsequent
 *     errors after the first one are ignored.
 */
static void nfs_direct_write_release(void *calldata)
{
        struct nfs_write_data *data = calldata;
        struct nfs_direct_req *dreq = (struct nfs_direct_req *) data->req;

        if (put_dreq(dreq))
                nfs_direct_write_complete(dreq, data->inode);
}

static const struct rpc_call_ops nfs_write_direct_ops = {
        .rpc_call_done = nfs_direct_write_result,
        .rpc_release = nfs_direct_write_release,
};

/*
 * For each wsize'd chunk of the user's buffer, dispatch an NFS WRITE
 * operation.  If nfs_writedata_alloc() or get_user_pages() fails,
 * bail and stop sending more writes.  Write length accounting is
 * handled automatically by nfs_direct_write_result().  Otherwise, if
 * no requests have been sent, just return an error.
 */
static ssize_t nfs_direct_write_schedule(struct nfs_direct_req *dreq, unsigned long user_addr, size_t count, loff_t pos, int sync)
{
        struct nfs_open_context *ctx = dreq->ctx;
        struct inode *inode = ctx->dentry->d_inode;
        size_t wsize = NFS_SERVER(inode)->wsize;
        unsigned int pgbase;
        int result;
        ssize_t started = 0;

        get_dreq(dreq);

        do {
                struct nfs_write_data *data;
                size_t bytes;

                pgbase = user_addr & ~PAGE_MASK;
                bytes = min(wsize,count);

                result = -ENOMEM;
                data = nfs_writedata_alloc(nfs_page_array_len(pgbase, bytes));
                if (unlikely(!data))
                        break;

                down_read(&current->mm->mmap_sem);
                result = get_user_pages(current, current->mm, user_addr,
                                        data->npages, 0, 0, data->pagevec, NULL);
                up_read(&current->mm->mmap_sem);
                if (result < 0) {
                        nfs_writedata_release(data);
                        break;
                }
                if ((unsigned)result < data->npages) {
                        nfs_direct_release_pages(data->pagevec, result);
                        nfs_writedata_release(data);
                        break;
                }

                get_dreq(dreq);

                list_move_tail(&data->pages, &dreq->rewrite_list);

                data->req = (struct nfs_page *) dreq;
                data->inode = inode;
                data->cred = ctx->cred;
                data->args.fh = NFS_FH(inode);
                data->args.context = ctx;
                data->args.offset = pos;
                data->args.pgbase = pgbase;
                data->args.pages = data->pagevec;
                data->args.count = bytes;
                data->res.fattr = &data->fattr;
                data->res.count = bytes;
                data->res.verf = &data->verf;

                rpc_init_task(&data->task, NFS_CLIENT(inode), RPC_TASK_ASYNC,
                                &nfs_write_direct_ops, data);
                NFS_PROTO(inode)->write_setup(data, sync);

                data->task.tk_priority = RPC_PRIORITY_NORMAL;
                data->task.tk_cookie = (unsigned long) inode;

                rpc_execute(&data->task);

                dprintk("NFS: %5u initiated direct write call "
                        "(req %s/%Ld, %zu bytes @ offset %Lu)\n",
                                data->task.tk_pid,
                                inode->i_sb->s_id,
                                (long long)NFS_FILEID(inode),
                                bytes,
                                (unsigned long long)data->args.offset);

                started += bytes;
                user_addr += bytes;
                pos += bytes;

                /* FIXME: Remove this useless math from the final patch */
                pgbase += bytes;
                pgbase &= ~PAGE_MASK;
                BUG_ON(pgbase != (user_addr & ~PAGE_MASK));

                count -= bytes;
        } while (count != 0);

        if (put_dreq(dreq))
                nfs_direct_write_complete(dreq, inode);

        if (started)
                return 0;
        return result < 0 ? (ssize_t) result : -EFAULT;
}

static ssize_t nfs_direct_write(struct kiocb *iocb, unsigned long user_addr, size_t count, loff_t pos)
{
        ssize_t result = 0;
        sigset_t oldset;
        struct inode *inode = iocb->ki_filp->f_mapping->host;
        struct rpc_clnt *clnt = NFS_CLIENT(inode);
        struct nfs_direct_req *dreq;
        size_t wsize = NFS_SERVER(inode)->wsize;
        int sync = 0;

        dreq = nfs_direct_req_alloc();
        if (!dreq)
                return -ENOMEM;
        nfs_alloc_commit_data(dreq);

        if (dreq->commit_data == NULL || count < wsize)
                sync = FLUSH_STABLE;

        dreq->inode = inode;
        dreq->ctx = get_nfs_open_context((struct nfs_open_context *)iocb->ki_filp->private_data);
        if (!is_sync_kiocb(iocb))
                dreq->iocb = iocb;

        nfs_add_stats(inode, NFSIOS_DIRECTWRITTENBYTES, count);

        nfs_begin_data_update(inode);

        rpc_clnt_sigmask(clnt, &oldset);
        result = nfs_direct_write_schedule(dreq, user_addr, count, pos, sync);
        if (!result)
                result = nfs_direct_wait(dreq);
        rpc_clnt_sigunmask(clnt, &oldset);
        nfs_direct_req_release(dreq);

        return result;
}

/**
 * nfs_file_direct_read - file direct read operation for NFS files
 * @iocb: target I/O control block
 * @iov: vector of user buffers into which to read data
 * @nr_segs: size of iov vector
 * @pos: byte offset in file where reading starts
 *
 * We use this function for direct reads instead of calling
 * generic_file_aio_read() in order to avoid gfar's check to see if
 * the request starts before the end of the file.  For that check
 * to work, we must generate a GETATTR before each direct read, and
 * even then there is a window between the GETATTR and the subsequent
 * READ where the file size could change.  Our preference is simply
 * to do all reads the application wants, and the server will take
 * care of managing the end of file boundary.
 *
 * This function also eliminates unnecessarily updating the file's
 * atime locally, as the NFS server sets the file's atime, and this
 * client must read the updated atime from the server back into its
 * cache.
 */
ssize_t nfs_file_direct_read(struct kiocb *iocb, const struct iovec *iov,
                                unsigned long nr_segs, loff_t pos)
{
        ssize_t retval = -EINVAL;
        struct file *file = iocb->ki_filp;
        struct address_space *mapping = file->f_mapping;
        /* XXX: temporary */
        const char __user *buf = iov[0].iov_base;
        size_t count = iov[0].iov_len;

        dprintk("nfs: direct read(%s/%s, %lu@%Ld)\n",
                file->f_path.dentry->d_parent->d_name.name,
                file->f_path.dentry->d_name.name,
                (unsigned long) count, (long long) pos);

        if (nr_segs != 1)
                return -EINVAL;

        if (count < 0)
                goto out;
        retval = -EFAULT;
        if (!access_ok(VERIFY_WRITE, buf, count))
                goto out;
        retval = 0;
        if (!count)
                goto out;

        retval = nfs_sync_mapping(mapping);
        if (retval)
                goto out;

        retval = nfs_direct_read(iocb, (unsigned long) buf, count, pos);
        if (retval > 0)
                iocb->ki_pos = pos + retval;

out:
        return retval;
}

/**
 * nfs_file_direct_write - file direct write operation for NFS files
 * @iocb: target I/O control block
 * @iov: vector of user buffers from which to write data
 * @nr_segs: size of iov vector
 * @pos: byte offset in file where writing starts
 *
 * We use this function for direct writes instead of calling
 * generic_file_aio_write() in order to avoid taking the inode
 * semaphore and updating the i_size.  The NFS server will set
 * the new i_size and this client must read the updated size
 * back into its cache.  We let the server do generic write
 * parameter checking and report problems.
 *
 * We also avoid an unnecessary invocation of generic_osync_inode(),
 * as it is fairly meaningless to sync the metadata of an NFS file.
 *
 * We eliminate local atime updates, see direct read above.
 *
 * We avoid unnecessary page cache invalidations for normal cached
 * readers of this file.
 *
 * Note that O_APPEND is not supported for NFS direct writes, as there
 * is no atomic O_APPEND write facility in the NFS protocol.
 */
ssize_t nfs_file_direct_write(struct kiocb *iocb, const struct iovec *iov,
                                unsigned long nr_segs, loff_t pos)
{
        ssize_t retval;
        struct file *file = iocb->ki_filp;
        struct address_space *mapping = file->f_mapping;
        /* XXX: temporary */
        const char __user *buf = iov[0].iov_base;
        size_t count = iov[0].iov_len;

        dprintk("nfs: direct write(%s/%s, %lu@%Ld)\n",
                file->f_path.dentry->d_parent->d_name.name,
                file->f_path.dentry->d_name.name,
                (unsigned long) count, (long long) pos);

        if (nr_segs != 1)
                return -EINVAL;

        retval = generic_write_checks(file, &pos, &count, 0);
        if (retval)
                goto out;

        retval = -EINVAL;
        if ((ssize_t) count < 0)
                goto out;
        retval = 0;
        if (!count)
                goto out;

        retval = -EFAULT;
        if (!access_ok(VERIFY_READ, buf, count))
                goto out;

        retval = nfs_sync_mapping(mapping);
        if (retval)
                goto out;

        retval = nfs_direct_write(iocb, (unsigned long) buf, count, pos);

        if (retval > 0)
                iocb->ki_pos = pos + retval;

out:
        return retval;
}

/**
 * nfs_init_directcache - create a slab cache for nfs_direct_req structures
 *
 */
int __init nfs_init_directcache(void)
{
        nfs_direct_cachep = kmem_cache_create("nfs_direct_cache",
                                                sizeof(struct nfs_direct_req),
                                                0, (SLAB_RECLAIM_ACCOUNT|
                                                        SLAB_MEM_SPREAD),
                                                NULL, NULL);
        if (nfs_direct_cachep == NULL)
                return -ENOMEM;

        return 0;
}

/**
 * nfs_destroy_directcache - destroy the slab cache for nfs_direct_req structures
 *
 */
void nfs_destroy_directcache(void)
{
        kmem_cache_destroy(nfs_direct_cachep);
}