diff options
Diffstat (limited to 'kernel/Documentation/vm/unevictable-lru.txt')
| -rw-r--r-- | kernel/Documentation/vm/unevictable-lru.txt | 128 |
1 files changed, 26 insertions, 102 deletions
diff --git a/kernel/Documentation/vm/unevictable-lru.txt b/kernel/Documentation/vm/unevictable-lru.txt index 3be0bfc47..fa3b52708 100644 --- a/kernel/Documentation/vm/unevictable-lru.txt +++ b/kernel/Documentation/vm/unevictable-lru.txt @@ -467,7 +467,13 @@ mmap(MAP_LOCKED) SYSTEM CALL HANDLING In addition the mlock()/mlockall() system calls, an application can request that a region of memory be mlocked supplying the MAP_LOCKED flag to the mmap() -call. Furthermore, any mmap() call or brk() call that expands the heap by a +call. There is one important and subtle difference here, though. mmap() + mlock() +will fail if the range cannot be faulted in (e.g. because mm_populate fails) +and returns with ENOMEM while mmap(MAP_LOCKED) will not fail. The mmaped +area will still have properties of the locked area - aka. pages will not get +swapped out - but major page faults to fault memory in might still happen. + +Furthermore, any mmap() call or brk() call that expands the heap by a task that has previously called mlockall() with the MCL_FUTURE flag will result in the newly mapped memory being mlocked. Before the unevictable/mlock changes, the kernel simply called make_pages_present() to allocate pages and @@ -525,83 +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. - - (*) try_to_unmap_anon() - - 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() - linear mappings - - 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 priority search tree. It also visits - each VMA in the page's mapping's non-linear list, if the list is - non-empty. - - 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. - - (*) try_to_unmap_file() - non-linear mappings - - If a page's mapping contains a non-empty non-linear mapping VMA list, then - try_to_un{map|lock}() must also visit each VMA in that list to determine - whether the page is mapped in a VM_LOCKED VMA. Again, the scan must visit - all VMAs in the non-linear list to ensure that the pages is not/should not - be mlocked. - - If a VM_LOCKED VMA is found in the list, the scan could terminate. - However, there is no easy way to determine whether the page is actually - mapped in a given VMA - either for unmapping or testing whether the - VM_LOCKED VMA actually pins the page. - - try_to_unmap_file() handles non-linear mappings by scanning a certain - number of pages - a "cluster" - in each non-linear VMA associated with the - page's mapping, for each file mapped page that vmscan tries to unmap. If - this happens to unmap the page we're trying to unmap, try_to_unmap() will - notice this on return (page_mapcount(page) will be 0) and return - SWAP_SUCCESS. Otherwise, it will return SWAP_AGAIN, causing vmscan to - recirculate this page. We take advantage of the cluster scan in - try_to_unmap_cluster() as follows: - - For each non-linear VMA, try_to_unmap_cluster() attempts to acquire the - mmap semaphore of the associated mm_struct for read without blocking. - - If this attempt is successful and the VMA is VM_LOCKED, - try_to_unmap_cluster() will retain the mmap semaphore for the scan; - otherwise it drops it here. - - Then, for each page in the cluster, if we're holding the mmap semaphore - for a locked VMA, try_to_unmap_cluster() calls mlock_vma_page() to - mlock the page. This call is a no-op if the page is already locked, - but will mlock any pages in the non-linear mapping that happen to be - unlocked. - - If one of the pages so mlocked is the page passed in to try_to_unmap(), - try_to_unmap_cluster() will return SWAP_MLOCK, rather than the default - SWAP_AGAIN. This will allow vmscan to cull the page, rather than - recirculating it on the inactive list. - - Again, if try_to_unmap_cluster() cannot acquire the VMA's mmap sem, it - returns SWAP_AGAIN, indicating that the page is mapped by a VM_LOCKED - VMA, but couldn't be mlocked. +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. + +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. + +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 @@ -617,29 +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 for anonymous pages and file -pages mapped in linear VMAs, 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. - -For file pages mapped into non-linear VMAs, the try_to_munlock() logic works -slightly differently. On encountering a VM_LOCKED non-linear VMA that might -map the page, try_to_munlock() returns SWAP_AGAIN without actually mlocking the -page. munlock_vma_page() will just leave the page unlocked and let vmscan deal -with it - the usual fallback position. +for VM_LOCKED VMAs. When such a VMA is found, as in the try_to_unmap() case, +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. @@ -667,11 +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. |