mm: proc: smaps_rollup: do not stall write attempts on mmap_lock

smaps_rollup will try to grab mmap_lock and go through the whole vma list
until it finishes the iterating.  When encountering large processes, the
mmap_lock will be held for a longer time, which may block other write
requests like mmap and munmap from progressing smoothly.

There are upcoming mmap_lock optimizations like range-based locks, but the
lock applied to smaps_rollup would be the coarse type, which doesn't avoid
the occurrence of unpleasant contention.

To solve aforementioned issue, we add a check which detects whether anyone
wants to grab mmap_lock for write attempts.

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

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