Add description of THP handling into unevictable-lru.txt. Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> --- Documentation/vm/unevictable-lru.txt | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) diff --git a/Documentation/vm/unevictable-lru.txt b/Documentation/vm/unevictable-lru.txt index fa3b527086fa..c47566fc49cb 100644 --- a/Documentation/vm/unevictable-lru.txt +++ b/Documentation/vm/unevictable-lru.txt @@ -461,6 +461,25 @@ 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 +------------------------------- + +Transparent huge page is represented by single entry on a lru list and +therefore we can only make unevictable entire compound page, not +individual subpages. + +We allow a part of transparent huge page to be mapped PTEs (i.e. after +mremap() or mprotect()) and VMA with PTE-mapped huge page can be mlocked. + +That means we need to be careful, not making entire huge page unevicable +if user mlock() only part of it. + +We handle this by forbidding mlocking PTE-mapped huge pages. This way we +keep the huge page accessible for vmscan. Under memory pressure the page +will be split, subpages from VM_LOCKED VMAs moved to unevictable lru and +the rest can be evicted. + +See also comment in follow_trans_huge_pmd(). mmap(MAP_LOCKED) SYSTEM CALL HANDLING ------------------------------------- -- 2.7.0 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>