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Commit 6fb8ddfc authored by Kirill A. Shutemov's avatar Kirill A. Shutemov Committed by Linus Torvalds
Browse files

thp, mlock: update unevictable-lru.txt 

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: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 1f52e67e

Documentation/vm/unevictable-lru.txt

+21 −0
Original line number Diff line number Diff line
@@ -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

-------------------------------------