mm, thp: make swapin readahead under down_read of mmap_sem

mm, thp: make swapin readahead under down_read of mmap_sem

Currently khugepaged makes swapin readahead under down_write.  This
patch supplies to make swapin readahead under down_read instead of
down_write.

The patch was tested with a test program that allocates 800MB of memory,
writes to it, and then sleeps.  The system was forced to swap out all.
Afterwards, the test program touches the area by writing, it skips a
page in each 20 pages of the area.

[[email protected]: update comment to match new code]
[[email protected]: passing 'vma' to hugepage_vma_revlidate() is useless]
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/1466021202-61880-3-git-send-email-kirill.shutemov@linux.intel.com
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/1466021202-61880-2-git-send-email-kirill.shutemov@linux.intel.com
Signed-off-by: Ebru Akagunduz <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Minchan Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>