From: Hugh Dickins <hughd@xxxxxxxxxx> Subject: mm/lru: revise the comments of lru_lock Since we changed the pgdat->lru_lock to lruvec->lru_lock, it's time to fix the incorrect comments in code. Also fixed some zone->lru_lock comment error from ancient time. etc. I struggled to understand the comment above move_pages_to_lru() (surely it never calls page_referenced()), and eventually realized that most of it had got separated from shrink_active_list(): move that comment back. Link: https://lkml.kernel.org/r/1604566549-62481-20-git-send-email-alex.shi@xxxxxxxxxxxxxxxxx Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> Acked-by: Vlastimil Babka <vbabka@xxxxxxx> Cc: Tejun Heo <tj@xxxxxxxxxx> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> Cc: Jann Horn <jannh@xxxxxxxxxx> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: Alexander Duyck <alexander.duyck@xxxxxxxxx> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> Cc: "Chen, Rong A" <rong.a.chen@xxxxxxxxx> Cc: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> Cc: "Huang, Ying" <ying.huang@xxxxxxxxx> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxx> Cc: Mika Penttilä <mika.penttila@xxxxxxxxxxxx> Cc: Minchan Kim <minchan@xxxxxxxxxx> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx> Cc: Wei Yang <richard.weiyang@xxxxxxxxx> Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- Documentation/admin-guide/cgroup-v1/memcg_test.rst | 15 ---- Documentation/admin-guide/cgroup-v1/memory.rst | 23 ++---- Documentation/trace/events-kmem.rst | 2 Documentation/vm/unevictable-lru.rst | 22 ++--- include/linux/mm_types.h | 2 include/linux/mmzone.h | 3 mm/filemap.c | 4 - mm/rmap.c | 4 - mm/vmscan.c | 41 ++++++----- 9 files changed, 51 insertions(+), 65 deletions(-) --- a/Documentation/admin-guide/cgroup-v1/memcg_test.rst~mm-lru-revise-the-comments-of-lru_lock +++ a/Documentation/admin-guide/cgroup-v1/memcg_test.rst @@ -133,18 +133,9 @@ Under below explanation, we assume CONFI 8. LRU ====== - Each memcg has its own private LRU. Now, its handling is under global - VM's control (means that it's handled under global pgdat->lru_lock). - Almost all routines around memcg's LRU is called by global LRU's - list management functions under pgdat->lru_lock. - - A special function is mem_cgroup_isolate_pages(). This scans - memcg's private LRU and call __isolate_lru_page() to extract a page - from LRU. - - (By __isolate_lru_page(), the page is removed from both of global and - private LRU.) - + Each memcg has its own vector of LRUs (inactive anon, active anon, + inactive file, active file, unevictable) of pages from each node, + each LRU handled under a single lru_lock for that memcg and node. 9. Typical Tests. ================= --- a/Documentation/admin-guide/cgroup-v1/memory.rst~mm-lru-revise-the-comments-of-lru_lock +++ a/Documentation/admin-guide/cgroup-v1/memory.rst @@ -287,20 +287,17 @@ When oom event notifier is registered, e 2.6 Locking ----------- - lock_page_cgroup()/unlock_page_cgroup() should not be called under - the i_pages lock. +Lock order is as follows: - Other lock order is following: - - PG_locked. - mm->page_table_lock - pgdat->lru_lock - lock_page_cgroup. - - In many cases, just lock_page_cgroup() is called. - - per-zone-per-cgroup LRU (cgroup's private LRU) is just guarded by - pgdat->lru_lock, it has no lock of its own. + Page lock (PG_locked bit of page->flags) + mm->page_table_lock or split pte_lock + lock_page_memcg (memcg->move_lock) + mapping->i_pages lock + lruvec->lru_lock. + +Per-node-per-memcgroup LRU (cgroup's private LRU) is guarded by +lruvec->lru_lock; PG_lru bit of page->flags is cleared before +isolating a page from its LRU under lruvec->lru_lock. 2.7 Kernel Memory Extension (CONFIG_MEMCG_KMEM) ----------------------------------------------- --- a/Documentation/trace/events-kmem.rst~mm-lru-revise-the-comments-of-lru_lock +++ a/Documentation/trace/events-kmem.rst @@ -69,7 +69,7 @@ When pages are freed in batch, the also Broadly speaking, pages are taken off the LRU lock in bulk and freed in batch with a page list. Significant amounts of activity here could indicate that the system is under memory pressure and can also indicate -contention on the zone->lru_lock. +contention on the lruvec->lru_lock. 4. Per-CPU Allocator Activity ============================= --- a/Documentation/vm/unevictable-lru.rst~mm-lru-revise-the-comments-of-lru_lock +++ a/Documentation/vm/unevictable-lru.rst @@ -33,7 +33,7 @@ reclaim in Linux. The problems have bee memory x86_64 systems. To illustrate this with an example, a non-NUMA x86_64 platform with 128GB of -main memory will have over 32 million 4k pages in a single zone. When a large +main memory will have over 32 million 4k pages in a single node. When a large fraction of these pages are not evictable for any reason [see below], vmscan will spend a lot of time scanning the LRU lists looking for the small fraction of pages that are evictable. This can result in a situation where all CPUs are @@ -55,7 +55,7 @@ unevictable, either by definition or by The Unevictable Page List ------------------------- -The Unevictable LRU infrastructure consists of an additional, per-zone, LRU list +The Unevictable LRU infrastructure consists of an additional, per-node, LRU list called the "unevictable" list and an associated page flag, PG_unevictable, to indicate that the page is being managed on the unevictable list. @@ -84,15 +84,9 @@ The unevictable list does not differenti swap-backed pages. This differentiation is only important while the pages are, in fact, evictable. -The unevictable list benefits from the "arrayification" of the per-zone LRU +The unevictable list benefits from the "arrayification" of the per-node LRU lists and statistics originally proposed and posted by Christoph Lameter. -The unevictable list does not use the LRU pagevec mechanism. Rather, -unevictable pages are placed directly on the page's zone's unevictable list -under the zone lru_lock. This allows us to prevent the stranding of pages on -the unevictable list when one task has the page isolated from the LRU and other -tasks are changing the "evictability" state of the page. - Memory Control Group Interaction -------------------------------- @@ -101,8 +95,8 @@ The unevictable LRU facility interacts w memory controller; see Documentation/admin-guide/cgroup-v1/memory.rst] by extending the lru_list enum. -The memory controller data structure automatically gets a per-zone unevictable -list as a result of the "arrayification" of the per-zone LRU lists (one per +The memory controller data structure automatically gets a per-node unevictable +list as a result of the "arrayification" of the per-node LRU lists (one per lru_list enum element). The memory controller tracks the movement of pages to and from the unevictable list. @@ -196,7 +190,7 @@ for the sake of expediency, to leave a u active/inactive LRU lists for vmscan to deal with. vmscan checks for such pages in all of the shrink_{active|inactive|page}_list() functions and will "cull" such pages that it encounters: that is, it diverts those pages to the -unevictable list for the zone being scanned. +unevictable list for the node being scanned. There may be situations where a page is mapped into a VM_LOCKED VMA, but the page is not marked as PG_mlocked. Such pages will make it all the way to @@ -328,7 +322,7 @@ If the page was NOT already mlocked, mlo page from the LRU, as it is likely on the appropriate active or inactive list at that time. If the isolate_lru_page() succeeds, mlock_vma_page() will put back the page - by calling putback_lru_page() - which will notice that the page -is now mlocked and divert the page to the zone's unevictable list. If +is now mlocked and divert the page to the node's unevictable list. If mlock_vma_page() is unable to isolate the page from the LRU, vmscan will handle it later if and when it attempts to reclaim the page. @@ -603,7 +597,7 @@ Some examples of these unevictable pages unevictable list in mlock_vma_page(). shrink_inactive_list() also diverts any unevictable pages that it finds on the -inactive lists to the appropriate zone's unevictable list. +inactive lists to the appropriate node's unevictable list. shrink_inactive_list() should only see SHM_LOCK'd pages that became SHM_LOCK'd after shrink_active_list() had moved them to the inactive list, or pages mapped --- a/include/linux/mm_types.h~mm-lru-revise-the-comments-of-lru_lock +++ a/include/linux/mm_types.h @@ -79,7 +79,7 @@ struct page { struct { /* Page cache and anonymous pages */ /** * @lru: Pageout list, eg. active_list protected by - * pgdat->lru_lock. Sometimes used as a generic list + * lruvec->lru_lock. Sometimes used as a generic list * by the page owner. */ struct list_head lru; --- a/include/linux/mmzone.h~mm-lru-revise-the-comments-of-lru_lock +++ a/include/linux/mmzone.h @@ -113,8 +113,7 @@ static inline bool free_area_empty(struc struct pglist_data; /* - * zone->lock and the zone lru_lock are two of the hottest locks in the kernel. - * So add a wild amount of padding here to ensure that they fall into separate + * Add a wild amount of padding here to ensure datas fall into separate * cachelines. There are very few zone structures in the machine, so space * consumption is not a concern here. */ --- a/mm/filemap.c~mm-lru-revise-the-comments-of-lru_lock +++ a/mm/filemap.c @@ -102,8 +102,8 @@ * ->swap_lock (try_to_unmap_one) * ->private_lock (try_to_unmap_one) * ->i_pages lock (try_to_unmap_one) - * ->pgdat->lru_lock (follow_page->mark_page_accessed) - * ->pgdat->lru_lock (check_pte_range->isolate_lru_page) + * ->lruvec->lru_lock (follow_page->mark_page_accessed) + * ->lruvec->lru_lock (check_pte_range->isolate_lru_page) * ->private_lock (page_remove_rmap->set_page_dirty) * ->i_pages lock (page_remove_rmap->set_page_dirty) * bdi.wb->list_lock (page_remove_rmap->set_page_dirty) --- a/mm/rmap.c~mm-lru-revise-the-comments-of-lru_lock +++ a/mm/rmap.c @@ -28,12 +28,12 @@ * hugetlb_fault_mutex (hugetlbfs specific page fault mutex) * anon_vma->rwsem * mm->page_table_lock or pte_lock - * pgdat->lru_lock (in mark_page_accessed, isolate_lru_page) * swap_lock (in swap_duplicate, swap_info_get) * mmlist_lock (in mmput, drain_mmlist and others) * mapping->private_lock (in __set_page_dirty_buffers) - * mem_cgroup_{begin,end}_page_stat (memcg->move_lock) + * lock_page_memcg move_lock (in __set_page_dirty_buffers) * i_pages lock (widely used) + * lruvec->lru_lock (in lock_page_lruvec_irq) * inode->i_lock (in set_page_dirty's __mark_inode_dirty) * bdi.wb->list_lock (in set_page_dirty's __mark_inode_dirty) * sb_lock (within inode_lock in fs/fs-writeback.c) --- a/mm/vmscan.c~mm-lru-revise-the-comments-of-lru_lock +++ a/mm/vmscan.c @@ -1613,14 +1613,16 @@ static __always_inline void update_lru_s } /** - * pgdat->lru_lock is heavily contended. Some of the functions that + * Isolating page from the lruvec to fill in @dst list by nr_to_scan times. + * + * lruvec->lru_lock is heavily contended. Some of the functions that * shrink the lists perform better by taking out a batch of pages * and working on them outside the LRU lock. * * For pagecache intensive workloads, this function is the hottest * spot in the kernel (apart from copy_*_user functions). * - * Appropriate locks must be held before calling this function. + * Lru_lock must be held before calling this function. * * @nr_to_scan: The number of eligible pages to look through on the list. * @lruvec: The LRU vector to pull pages from. @@ -1814,25 +1816,11 @@ static int too_many_isolated(struct pgli } /* - * This moves pages from @list to corresponding LRU list. - * - * We move them the other way if the page is referenced by one or more - * processes, from rmap. - * - * If the pages are mostly unmapped, the processing is fast and it is - * appropriate to hold zone_lru_lock across the whole operation. But if - * the pages are mapped, the processing is slow (page_referenced()) so we - * should drop zone_lru_lock around each page. It's impossible to balance - * this, so instead we remove the pages from the LRU while processing them. - * It is safe to rely on PG_active against the non-LRU pages in here because - * nobody will play with that bit on a non-LRU page. - * - * The downside is that we have to touch page->_refcount against each page. - * But we had to alter page->flags anyway. + * move_pages_to_lru() moves pages from private @list to appropriate LRU list. + * On return, @list is reused as a list of pages to be freed by the caller. * * Returns the number of pages moved to the given lruvec. */ - static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec, struct list_head *list) { @@ -2010,6 +1998,23 @@ shrink_inactive_list(unsigned long nr_to return nr_reclaimed; } +/* + * shrink_active_list() moves pages from the active LRU to the inactive LRU. + * + * We move them the other way if the page is referenced by one or more + * processes. + * + * If the pages are mostly unmapped, the processing is fast and it is + * appropriate to hold lru_lock across the whole operation. But if + * the pages are mapped, the processing is slow (page_referenced()), so + * we should drop lru_lock around each page. It's impossible to balance + * this, so instead we remove the pages from the LRU while processing them. + * It is safe to rely on PG_active against the non-LRU pages in here because + * nobody will play with that bit on a non-LRU page. + * + * The downside is that we have to touch page->_refcount against each page. + * But we had to alter page->flags anyway. + */ static void shrink_active_list(unsigned long nr_to_scan, struct lruvec *lruvec, struct scan_control *sc, _
