mmap locking API: add mmap_lock_is_contended()

Patch series "Try to release mmap_lock temporarily in smaps_rollup", v4.

Recently, we have observed some janky issues caused by unpleasantly long
contention on mmap_lock which is held by smaps_rollup when probing large
processes.  To address the problem, we let smaps_rollup detect if anyone
wants to acquire mmap_lock for write attempts.  If yes, just release the
lock temporarily to ease the contention.

smaps_rollup is a procfs interface which allows users to summarize the
process's memory usage without the overhead of seq_* calls.  Android uses
it to sample the memory usage of various processes to balance its memory
pool sizes.  If no one wants to take the lock for write requests,
smaps_rollup with this patch will behave like the original one.

Although there are on-going mmap_lock optimizations like range-based
locks, the lock applied to smaps_rollup would be the coarse one, which is
hard to avoid the occurrence of aforementioned issues.  So the detection
and temporary release for write attempts on mmap_lock in smaps_rollup is
still necessary.

This patch (of 3):

Add new API to query if someone wants to acquire mmap_lock for write
attempts.

Using this instead of rwsem_is_contended makes it more tolerant of future
changes to the lock type.

Signed-off-by: Chinwen Chang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Steven Price <[email protected]>
Acked-by: Michel Lespinasse <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Daniel Jordan <[email protected]>
Cc: Daniel Kiss <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Jimmy Assarsson <[email protected]>
Cc: Laurent Dufour <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: Matthias Brugger <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

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