Re: [PATCH V3 4/8] memcg: add per cgroup dirty pages accounting

[Greg Thelen]

On Tue, Dec 25 2012, Sha Zhengju wrote:

> From: Sha Zhengju <handai.szj@xxxxxxxxxx>
>
> This patch adds memcg routines to count dirty pages, which allows memory controller
> to maintain an accurate view of the amount of its dirty memory and can provide some
> info for users while cgroup's direct reclaim is working.
>
> After Kame's commit 89c06bd5(memcg: use new logic for page stat accounting), we can
> use 'struct page' flag to test page state instead of per page_cgroup flag. But memcg
> has a feature to move a page from a cgroup to another one and may have race between
> "move" and "page stat accounting". So in order to avoid the race we have designed a
> bigger lock:
>
>          mem_cgroup_begin_update_page_stat()
>          modify page information        -->(a)
>          mem_cgroup_update_page_stat()  -->(b)
>          mem_cgroup_end_update_page_stat()
> It requires (a) and (b)(dirty pages accounting) can stay close enough.
> In the previous two prepare patches, we have reworked the vfs set page dirty routines
> and now the interfaces are more explicit:
>         incrementing (2):
>                 __set_page_dirty
>                 __set_page_dirty_nobuffers
>         decrementing (2):
>                 clear_page_dirty_for_io
>                 cancel_dirty_page
>
> To prevent AB/BA deadlock mentioned by Greg Thelen in previous version
> (https://lkml.org/lkml/2012/7/30/227), we adjust the lock order:
> ->private_lock --> mapping->tree_lock --> memcg->move_lock.
> So we need to make mapping->tree_lock ahead of TestSetPageDirty in __set_page_dirty()
> and __set_page_dirty_nobuffers(). But in order to avoiding useless spinlock contention,
> a prepare PageDirty() checking is added.
>
>
> Signed-off-by: Sha Zhengju <handai.szj@xxxxxxxxxx>
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> Acked-by: Fengguang Wu <fengguang.wu@xxxxxxxxx>
> ---
>  fs/buffer.c                |   14 +++++++++++++-
>  include/linux/memcontrol.h |    1 +
>  mm/filemap.c               |   10 ++++++++++
>  mm/memcontrol.c            |   29 ++++++++++++++++++++++-------
>  mm/page-writeback.c        |   39 ++++++++++++++++++++++++++++++++-------
>  mm/truncate.c              |    6 ++++++
>  6 files changed, 84 insertions(+), 15 deletions(-)

__nilfs_clear_page_dirty() clears PageDirty, does it need modification
for this patch series?

> diff --git a/fs/buffer.c b/fs/buffer.c
> index 762168a..53402d2 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -612,19 +612,31 @@ EXPORT_SYMBOL(mark_buffer_dirty_inode);
>  int __set_page_dirty(struct page *page,
>  		struct address_space *mapping, int warn)
>  {
> +	bool locked;
> +	unsigned long flags;
> +
>  	if (unlikely(!mapping))
>  		return !TestSetPageDirty(page);
>  
> -	if (TestSetPageDirty(page))
> +	if (PageDirty(page))
>  		return 0;
>  
>  	spin_lock_irq(&mapping->tree_lock);
> +	mem_cgroup_begin_update_page_stat(page, &locked, &flags);
> +
> +	if (TestSetPageDirty(page)) {
> +		mem_cgroup_end_update_page_stat(page, &locked, &flags);
> +		spin_unlock_irq(&mapping->tree_lock);
> +		return 0;
> +	}
> +
>  	if (page->mapping) {	/* Race with truncate? */
>  		WARN_ON_ONCE(warn && !PageUptodate(page));
>  		account_page_dirtied(page, mapping);
>  		radix_tree_tag_set(&mapping->page_tree,
>  				page_index(page), PAGECACHE_TAG_DIRTY);
>  	}
> +	mem_cgroup_end_update_page_stat(page, &locked, &flags);
>  	spin_unlock_irq(&mapping->tree_lock);
>  	__mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
>  
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 5421b8a..2685d8a 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -44,6 +44,7 @@ enum mem_cgroup_stat_index {
>  	MEM_CGROUP_STAT_RSS,	   /* # of pages charged as anon rss */
>  	MEM_CGROUP_STAT_FILE_MAPPED,  /* # of pages charged as file rss */
>  	MEM_CGROUP_STAT_SWAP, /* # of pages, swapped out */
> +	MEM_CGROUP_STAT_FILE_DIRTY,  /* # of dirty pages in page cache */
>  	MEM_CGROUP_STAT_NSTATS,
>  };
>  
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 83efee7..b589be5 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -62,6 +62,11 @@
>   *      ->swap_lock		(exclusive_swap_page, others)
>   *        ->mapping->tree_lock
>   *
> + *    ->private_lock		(__set_page_dirty_buffers)
> + *      ->mapping->tree_lock
> + *        ->memcg->move_lock	(mem_cgroup_begin_update_page_stat->
> + *							move_lock_mem_cgroup)
> + *
>   *  ->i_mutex
>   *    ->i_mmap_mutex		(truncate->unmap_mapping_range)
>   *
> @@ -112,6 +117,8 @@
>  void __delete_from_page_cache(struct page *page)
>  {
>  	struct address_space *mapping = page->mapping;
> +	bool locked;
> +	unsigned long flags;
>  
>  	/*
>  	 * if we're uptodate, flush out into the cleancache, otherwise
> @@ -139,10 +146,13 @@ void __delete_from_page_cache(struct page *page)
>  	 * Fix it up by doing a final dirty accounting check after
>  	 * having removed the page entirely.
>  	 */
> +	mem_cgroup_begin_update_page_stat(page, &locked, &flags);
>  	if (PageDirty(page) && mapping_cap_account_dirty(mapping)) {
> +		mem_cgroup_dec_page_stat(page, MEM_CGROUP_STAT_FILE_DIRTY);
>  		dec_zone_page_state(page, NR_FILE_DIRTY);
>  		dec_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
>  	}
> +	mem_cgroup_end_update_page_stat(page, &locked, &flags);
>  }
>  
>  /**
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d450c04..c884640 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -95,6 +95,7 @@ static const char * const mem_cgroup_stat_names[] = {
>  	"rss",
>  	"mapped_file",
>  	"swap",
> +	"dirty",
>  };
>  
>  enum mem_cgroup_events_index {
> @@ -3609,6 +3610,19 @@ void mem_cgroup_split_huge_fixup(struct page *head)
>  }
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  
> +static inline
> +void mem_cgroup_move_account_page_stat(struct mem_cgroup *from,
> +					struct mem_cgroup *to,
> +					unsigned int nr_pages,
> +					enum mem_cgroup_stat_index idx)
> +{
> +	/* Update stat data for mem_cgroup */
> +	preempt_disable();
> +	__this_cpu_add(from->stat->count[idx], -nr_pages);

What you do think about adding a WARN_ON_ONCE() here to check for
underflow?  A check might help catch:
a) unresolved races between move accounting vs setting/clearing
   dirtying.
b) future modifications that mess with PageDirty/Writeback flags without
   considering memcg.

> +	__this_cpu_add(to->stat->count[idx], nr_pages);
> +	preempt_enable();
> +}
> +
>  /**
>   * mem_cgroup_move_account - move account of the page
>   * @page: the page
> @@ -3654,13 +3668,14 @@ static int mem_cgroup_move_account(struct page *page,
>  
>  	move_lock_mem_cgroup(from, &flags);
>  
> -	if (!anon && page_mapped(page)) {
> -		/* Update mapped_file data for mem_cgroup */
> -		preempt_disable();
> -		__this_cpu_dec(from->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> -		__this_cpu_inc(to->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> -		preempt_enable();
> -	}
> +	if (!anon && page_mapped(page))
> +		mem_cgroup_move_account_page_stat(from, to, nr_pages,
> +			MEM_CGROUP_STAT_FILE_MAPPED);
> +
> +	if (PageDirty(page))

Is (!anon && PageDirty(page)) better?  If dirty anon pages are moved
between memcg M1 and M2 I think that we'd mistakenly underflow M1 if it
was not previously accounting for the dirty anon page.
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