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authorKirill A. Shutemov <kirill.shutemov@linux.intel.com>2016-07-26 15:25:15 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2016-07-26 16:19:19 -0700
commit6fb8ddfc455ca82a3ce674f54298cd20f27ca518 (patch)
treee5bf5726da0e5e9d41ee7034969e7219be602fba /Documentation/vm
parent1f52e67e5e7f48424059f8168e3a65bddc80ad97 (diff)
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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>
Diffstat (limited to 'Documentation/vm')
-rw-r--r--Documentation/vm/unevictable-lru.txt21
1 files changed, 21 insertions, 0 deletions
diff --git a/Documentation/vm/unevictable-lru.txt b/Documentation/vm/unevictable-lru.txt
index fa3b527086fa..0026a8d33fc0 100644
--- a/Documentation/vm/unevictable-lru.txt
+++ b/Documentation/vm/unevictable-lru.txt
@@ -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
-------------------------------------