mm: gup persist for write permission
commit878b63ac889df706d01048f2c110e322ad2f996d
authorHugh Dickins <hugh@veritas.com>
Tue, 6 Jan 2009 22:39:32 +0000 (6 14:39 -0800)
committerLinus Torvalds <torvalds@linux-foundation.org>
Tue, 6 Jan 2009 23:59:03 +0000 (6 15:59 -0800)
treee0bd69b890fda939fc190675d977dd0fd75e0427
parent2da02997e08d3efe8174c7a47696e6f7cbe69ba9
mm: gup persist for write permission

do_wp_page()'s VM_FAULT_WRITE return value tells __get_user_pages() that
COW has been done if necessary, though it may be leaving the pte without
write permission - for the odd case of forced writing to a readonly vma
for ptrace.  At present GUP then retries the follow_page() without asking
for write permission, to escape an endless loop when forced.

But an application may be relying on GUP to guarantee a writable page
which won't be COWed again when written from userspace, whereas a race
here might leave a readonly pte in place?  Change the VM_FAULT_WRITE
handling to ask follow_page() for write permission again, except in that
odd case of forced writing to a readonly vma.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Robin Holt <holt@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
mm/memory.c