kernel - Improve VM fault performance for sequential access
* VM fault I/O pipelining was not working properly.
* Temporarily fix pipelining by introducing PG_RAM, A read-ahead mark
for vm_page_t, and adjust vm_fault to pass VM pages through to
getpages calls if PG_RAM is set, even if they are fully valid.
* Remove code in vnode_pager_generic_getpages() which shortcutted
the operation when the requested page was fully valid. This
prevented read-aheads from being issued.
* A more permanent solution is in the works (basically getting rid of
the whole VM read-ahead/read-behind array entirely, just passing
a single page through to vnode_pager_generic_getpages(), and
letting the filesystem handle the read-ahead in a more efficient
fashion.
Reported-by: "Mikhail T." <mi+thun@aldan.algebra.com>