diff options
Diffstat (limited to 'Documentation/vm')
| -rw-r--r-- | Documentation/vm/unevictable-lru.txt | 61 |
1 files changed, 16 insertions, 45 deletions
diff --git a/Documentation/vm/unevictable-lru.txt b/Documentation/vm/unevictable-lru.txt index e983ee4e2758..fa3b527086fa 100644 --- a/Documentation/vm/unevictable-lru.txt +++ b/Documentation/vm/unevictable-lru.txt @@ -531,37 +531,20 @@ map. try_to_unmap() is always called, by either vmscan for reclaim or for page migration, with the argument page locked and isolated from the LRU. Separate -functions handle anonymous and mapped file pages, as these types of pages have -different reverse map mechanisms. +functions handle anonymous and mapped file and KSM pages, as these types of +pages have different reverse map lookup mechanisms, with different locking. +In each case, whether rmap_walk_anon() or rmap_walk_file() or rmap_walk_ksm(), +it will call try_to_unmap_one() for every VMA which might contain the page. - (*) try_to_unmap_anon() +When trying to reclaim, if try_to_unmap_one() finds the page in a VM_LOCKED +VMA, it will then mlock the page via mlock_vma_page() instead of unmapping it, +and return SWAP_MLOCK to indicate that the page is unevictable: and the scan +stops there. - To unmap anonymous pages, each VMA in the list anchored in the anon_vma - must be visited - at least until a VM_LOCKED VMA is encountered. If the - page is being unmapped for migration, VM_LOCKED VMAs do not stop the - process because mlocked pages are migratable. However, for reclaim, if - the page is mapped into a VM_LOCKED VMA, the scan stops. - - try_to_unmap_anon() attempts to acquire in read mode the mmap semaphore of - the mm_struct to which the VMA belongs. If this is successful, it will - mlock the page via mlock_vma_page() - we wouldn't have gotten to - try_to_unmap_anon() if the page were already mlocked - and will return - SWAP_MLOCK, indicating that the page is unevictable. - - If the mmap semaphore cannot be acquired, we are not sure whether the page - is really unevictable or not. In this case, try_to_unmap_anon() will - return SWAP_AGAIN. - - (*) try_to_unmap_file() - - Unmapping of a mapped file page works the same as for anonymous mappings, - except that the scan visits all VMAs that map the page's index/page offset - in the page's mapping's reverse map interval search tree. - - As for anonymous pages, on encountering a VM_LOCKED VMA for a mapped file - page, try_to_unmap_file() will attempt to acquire the associated - mm_struct's mmap semaphore to mlock the page, returning SWAP_MLOCK if this - is successful, and SWAP_AGAIN, if not. +mlock_vma_page() is called while holding the page table's lock (in addition +to the page lock, and the rmap lock): to serialize against concurrent mlock or +munlock or munmap system calls, mm teardown (munlock_vma_pages_all), reclaim, +holepunching, and truncation of file pages and their anonymous COWed pages. try_to_munlock() REVERSE MAP SCAN @@ -577,22 +560,15 @@ all PTEs from the page. For this purpose, the unevictable/mlock infrastructure introduced a variant of try_to_unmap() called try_to_munlock(). try_to_munlock() calls the same functions as try_to_unmap() for anonymous and -mapped file pages with an additional argument specifying unlock versus unmap +mapped file and KSM pages with a flag argument specifying unlock versus unmap processing. Again, these functions walk the respective reverse maps looking for VM_LOCKED VMAs. When such a VMA is found, as in the try_to_unmap() case, -the functions attempt to acquire the associated mmap semaphore, mlock the page -via mlock_vma_page() and return SWAP_MLOCK. This effectively undoes the -pre-clearing of the page's PG_mlocked done by munlock_vma_page. - -If try_to_unmap() is unable to acquire a VM_LOCKED VMA's associated mmap -semaphore, it will return SWAP_AGAIN. This will allow shrink_page_list() to -recycle the page on the inactive list and hope that it has better luck with the -page next time. +the functions mlock the page via mlock_vma_page() and return SWAP_MLOCK. This +undoes the pre-clearing of the page's PG_mlocked done by munlock_vma_page. Note that try_to_munlock()'s reverse map walk must visit every VMA in a page's reverse map to determine that a page is NOT mapped into any VM_LOCKED VMA. -However, the scan can terminate when it encounters a VM_LOCKED VMA and can -successfully acquire the VMA's mmap semaphore for read and mlock the page. +However, the scan can terminate when it encounters a VM_LOCKED VMA. Although try_to_munlock() might be called a great many times when munlocking a large region or tearing down a large address space that has been mlocked via mlockall(), overall this is a fairly rare event. @@ -620,11 +596,6 @@ Some examples of these unevictable pages on the LRU lists are: (3) mlocked pages that could not be isolated from the LRU and moved to the unevictable list in mlock_vma_page(). - (4) Pages mapped into multiple VM_LOCKED VMAs, but try_to_munlock() couldn't - acquire the VMA's mmap semaphore to test the flags and set PageMlocked. - munlock_vma_page() was forced to let the page back on to the normal LRU - list for vmscan to handle. - shrink_inactive_list() also diverts any unevictable pages that it finds on the inactive lists to the appropriate zone's unevictable list. |