mm: wp lock page before deciding cow

An application may rely on get_user_pages() to give it pages writable from
userspace and shared with a driver, GUP breaking COW if necessary.  It may
mprotect() the pages' writability, off and on, from time to time.

Normally this works fine (so long as the app does not fork); but just
occasionally, under memory pressure, a readonly pte in a newly writable
area is COWed unnecessarily, breaking the link with the driver: because
do_wp_page() does trylock_page, and falls back to COW whenever that fails.

For reliable behaviour in the unshared case, when the trylock_page fails,
now unlock pagetable, lock page and relock pagetable, before deciding
whether Copy-On-Write is really necessary.

Reported-by: Zhou Yingchao
Signed-off-by: Hugh Dickins <[email protected]>
Cc: Lee Schermerhorn <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Nick Piggin <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Robin Holt <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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