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[PATCHv9-rebased2 06/37] thp, mlock: update unevictable-lru.txt

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

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 Documentation/vm/unevictable-lru.txt | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

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
 -------------------------------------
-- 
2.8.1

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[PATCHv9-rebased2 00/37] THP-enabled tmpfs/shmem using compound pages "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 05/37] khugepaged: recheck pmd after mmap_sem re-acquired "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
    Re: [PATCHv9-rebased2 05/37] khugepaged: recheck pmd after mmap_sem  re-acquired Ebru Akagunduz <ebru.akagunduz@gmail.com> - 2016-06-16 13:50 +0200
      Re: [PATCHv9-rebased2 05/37] khugepaged: recheck pmd after mmap_sem  re-acquired "Kirill A. Shutemov" <kirill@shutemov.name> - 2016-06-16 14:00 +0200
  [PATCHv9-rebased2 12/37] thp, vmstats: add counters for huge file pages "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 17/37] thp: prepare change_huge_pmd() for file thp "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 09/37] mm: postpone page table allocation until we have page to map "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 18/37] thp: run vma_adjust_trans_huge() outside i_mmap_rwsem "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 21/37] vmscan: split file huge pages before paging them out "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 13/37] thp: support file pages in zap_huge_pmd() "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 29/37] shmem: add huge pages support "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 06/37] thp, mlock: update unevictable-lru.txt "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 19/37] thp: file pages support for split_huge_page() "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 28/37] shmem: get_unmapped_area align huge page "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 07/37] mm: do not pass mm_struct into handle_mm_fault "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 34/37] khugepaged: add support of collapse for tmpfs/shmem pages "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 14/37] thp: handle file pages in split_huge_pmd() "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 11/37] mm: introduce do_set_pmd() "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 26/37] mm, rmap: account shmem thp pages "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 25/37] truncate: handle file thp "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 24/37] filemap: prepare find and delete operations for huge pages "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 22/37] page-flags: relax policy for PG_mappedtodisk and PG_reclaim "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 27/37] shmem: prepare huge= mount option and sysfs knob "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 10/37] rmap: support file thp "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:20 +0200
  [PATCHv9-rebased2 04/37]  mm-thp-make-swapin-readahead-under-down_read-of-mmap_sem-fix-2-fix "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> - 2016-06-15 22:30 +0200

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