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thp, mlock: update unevictable-lru.txt

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Add description of THP handling into unevictable-lru.txt.

Link: http://lkml.kernel.org/r/1466021202-61880-7-git-send-email-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Kirill A. Shutemov 2016-07-26 15:25:15 -07:00 committed by Linus Torvalds
parent 1f52e67e5e
commit 6fb8ddfc45
1 changed files with 21 additions and 0 deletions
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Documentation/vm/unevictable-lru.txt
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@ -461,6 +461,27 @@ unevictable LRU is enabled, the work of compaction is mostly handled by
the page migration code and the same work flow as described in MIGRATING
MLOCKED PAGES will apply.
MLOCKING TRANSPARENT HUGE PAGES
-------------------------------
A transparent huge page is represented by a single entry on an LRU list.
Therefore, we can only make unevictable an entire compound page, not
individual subpages.
If a user tries to mlock() part of a huge page, we want the rest of the
page to be reclaimable.
We cannot just split the page on partial mlock() as split_huge_page() can
fail and new intermittent failure mode for the syscall is undesirable.
We handle this by keeping PTE-mapped huge pages on normal LRU lists: the
PMD on border of VM_LOCKED VMA will be split into PTE table.
This way the huge page is accessible for vmscan. Under memory pressure the
page will be split, subpages which belong to VM_LOCKED VMAs will be moved
to unevictable LRU and the rest can be reclaimed.
See also comment in follow_trans_huge_pmd().
mmap(MAP_LOCKED) SYSTEM CALL HANDLING
-------------------------------------