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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: 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>
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@ -461,6 +461,27 @@ unevictable LRU is enabled, the work of compaction is mostly handled by
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the page migration code and the same work flow as described in MIGRATING
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the page migration code and the same work flow as described in MIGRATING
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MLOCKED PAGES will apply.
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MLOCKED PAGES will apply.
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MLOCKING TRANSPARENT HUGE PAGES
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-------------------------------
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A transparent huge page is represented by a single entry on an LRU list.
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Therefore, we can only make unevictable an entire compound page, not
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individual subpages.
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If a user tries to mlock() part of a huge page, we want the rest of the
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page to be reclaimable.
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We cannot just split the page on partial mlock() as split_huge_page() can
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fail and new intermittent failure mode for the syscall is undesirable.
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We handle this by keeping PTE-mapped huge pages on normal LRU lists: the
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PMD on border of VM_LOCKED VMA will be split into PTE table.
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This way the huge page is accessible for vmscan. Under memory pressure the
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page will be split, subpages which belong to VM_LOCKED VMAs will be moved
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to unevictable LRU and the rest can be reclaimed.
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See also comment in follow_trans_huge_pmd().
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mmap(MAP_LOCKED) SYSTEM CALL HANDLING
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mmap(MAP_LOCKED) SYSTEM CALL HANDLING
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-------------------------------------
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-------------------------------------
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