Re: [PATCH v18 23/32] mm/lru: revise the comments of lru_lock

[Hugh Dickins <hughd@google.com>]

On Mon, 24 Aug 2020, Alex Shi wrote:

> From: Hugh Dickins <hughd@google.com>
> 
> 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.
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>

I'm not the right person to be Acking this one; but when I scanned
through, I did notice some wording had been added that I want to
change. I should just send you a new version, but not tonight.

> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
> Cc: Jann Horn <jannh@google.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: cgroups@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-mm@kvack.org
> ---
>  Documentation/admin-guide/cgroup-v1/memcg_test.rst | 15 +++------------
>  Documentation/admin-guide/cgroup-v1/memory.rst     | 21 +++++++++------------
>  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/memcontrol.c                                    |  2 +-
>  mm/rmap.c                                          |  4 ++--
>  mm/vmscan.c                                        | 12 ++++++++----
>  10 files changed, 36 insertions(+), 51 deletions(-)
> 
> diff --git a/Documentation/admin-guide/cgroup-v1/memcg_test.rst b/Documentation/admin-guide/cgroup-v1/memcg_test.rst
> index 3f7115e07b5d..0b9f91589d3d 100644
> --- a/Documentation/admin-guide/cgroup-v1/memcg_test.rst
> +++ b/Documentation/admin-guide/cgroup-v1/memcg_test.rst
> @@ -133,18 +133,9 @@ Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y.
>  
>  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.
>  =================
> diff --git a/Documentation/admin-guide/cgroup-v1/memory.rst b/Documentation/admin-guide/cgroup-v1/memory.rst
> index 12757e63b26c..24450696579f 100644
> --- a/Documentation/admin-guide/cgroup-v1/memory.rst
> +++ b/Documentation/admin-guide/cgroup-v1/memory.rst
> @@ -285,20 +285,17 @@ When oom event notifier is registered, event will be delivered.
>  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:
> +  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.
>  
> -   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.
> +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)
>  -----------------------------------------------
> diff --git a/Documentation/trace/events-kmem.rst b/Documentation/trace/events-kmem.rst
> index 555484110e36..68fa75247488 100644
> --- a/Documentation/trace/events-kmem.rst
> +++ b/Documentation/trace/events-kmem.rst
> @@ -69,7 +69,7 @@ When pages are freed in batch, the also mm_page_free_batched is triggered.
>  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
>  =============================
> diff --git a/Documentation/vm/unevictable-lru.rst b/Documentation/vm/unevictable-lru.rst
> index 17d0861b0f1d..0e1490524f53 100644
> --- a/Documentation/vm/unevictable-lru.rst
> +++ b/Documentation/vm/unevictable-lru.rst
> @@ -33,7 +33,7 @@ reclaim in Linux.  The problems have been observed at customer sites on large
>  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 circumstance, in the future.
>  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 differentiate between file-backed and anonymous,
>  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 with the memory control group [aka
>  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 unevictable page on one of the regular
>  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, mlock_vma_page() attempts to isolate the
>  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 on the LRU lists are:
>       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
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 496c3ff97cce..c3f1e76720af 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -78,7 +78,7 @@ 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;
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 27a1513a43fc..f0596e634863 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -113,8 +113,7 @@ static inline bool free_area_empty(struct free_area *area, int migratetype)
>  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.
>   */
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 1aaea26556cc..6f8d58fb16db 100644
> --- a/mm/filemap.c
> +++ b/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)
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5b95529e64a4..454b3f205d1b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3279,7 +3279,7 @@ void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size)
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  
>  /*
> - * Because tail pages are not marked as "used", set it. We're under
> + * Because tail pages are not marked as "used", set it. Don't need
>   * lruvec->lru_lock and migration entries setup in all page mappings.
>   */
>  void mem_cgroup_split_huge_fixup(struct page *head)
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 83cc459edc40..259c323e06ea 100644
> --- a/mm/rmap.c
> +++ b/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)
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 04ef94190530..601fbcb994fb 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1614,14 +1614,16 @@ static __always_inline void update_lru_sizes(struct lruvec *lruvec,
>  }
>  
>  /**
> - * 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.
> @@ -1820,14 +1822,16 @@ static int too_many_isolated(struct pglist_data *pgdat, int file,
>  
>  /*
>   * This moves pages from @list to corresponding LRU list.
> + * The pages from @list is out of any lruvec, and in the end list reuses as
> + * pages_to_free 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
> + * 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 zone_lru_lock around each page.  It's impossible to balance
> + * 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.
> -- 
> 1.8.3.1
> 
>