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Date: Wed, 11 Jul 2012 17:32:18 +0800
From: Sha Zhengju <handai.szj@...il.com>
To: Greg Thelen <gthelen@...gle.com>
CC: linux-mm@...ck.org, cgroups@...r.kernel.org,
kamezawa.hiroyu@...fujitsu.com, yinghan@...gle.com,
akpm@...ux-foundation.org, mhocko@...e.cz,
linux-kernel@...r.kernel.org, Sha Zhengju <handai.szj@...bao.com>
Subject: Re: [PATCH 5/7] memcg: add per cgroup dirty pages accounting
On 07/10/2012 05:02 AM, Greg Thelen wrote:
> On Thu, Jun 28 2012, Sha Zhengju wrote:
>
>> From: Sha Zhengju<handai.szj@...bao.com>
>>
>> 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 group'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
>>
>>
>> Signed-off-by: Sha Zhengju<handai.szj@...bao.com>
>> ---
>> fs/buffer.c | 17 ++++++++++++++---
>> include/linux/memcontrol.h | 1 +
>> mm/filemap.c | 5 +++++
>> mm/memcontrol.c | 28 +++++++++++++++++++++-------
>> mm/page-writeback.c | 30 ++++++++++++++++++++++++------
>> mm/truncate.c | 6 ++++++
>> 6 files changed, 71 insertions(+), 16 deletions(-)
>>
>> diff --git a/fs/buffer.c b/fs/buffer.c
>> index 55522dd..d3714cc 100644
>> --- a/fs/buffer.c
>> +++ b/fs/buffer.c
>> @@ -613,11 +613,19 @@ EXPORT_SYMBOL(mark_buffer_dirty_inode);
>> int __set_page_dirty(struct page *page,
>> struct address_space *mapping, int warn)
>> {
>> + bool locked;
>> + unsigned long flags;
>> + int ret = 0;
> '= 0' and 'ret = 0' change (below) are redundant. My vote is to remove
> '= 0' here.
>
Nice catch. :-)
>> +
>> if (unlikely(!mapping))
>> return !TestSetPageDirty(page);
>>
>> - if (TestSetPageDirty(page))
>> - return 0;
>> + mem_cgroup_begin_update_page_stat(page,&locked,&flags);
>> +
>> + if (TestSetPageDirty(page)) {
>> + ret = 0;
>> + goto out;
>> + }
>>
>> spin_lock_irq(&mapping->tree_lock);
>> if (page->mapping) { /* Race with truncate? */
>> @@ -629,7 +637,10 @@ int __set_page_dirty(struct page *page,
>> spin_unlock_irq(&mapping->tree_lock);
>> __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
>>
>> - return 1;
>> + ret = 1;
>> +out:
>> + mem_cgroup_end_update_page_stat(page,&locked,&flags);
>> + return ret;
>> }
>>
>> /*
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index 20b0f2d..ad37b59 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -38,6 +38,7 @@ 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 1f19ec3..5159a49 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -140,6 +140,11 @@ void __delete_from_page_cache(struct page *page)
>> * having removed the page entirely.
>> */
>> if (PageDirty(page)&& mapping_cap_account_dirty(mapping)) {
>> + /*
>> + * Do not change page state, so no need to use mem_cgroup_
>> + * {begin, end}_update_page_stat to get lock.
>> + */
>> + mem_cgroup_dec_page_stat(page, MEM_CGROUP_STAT_FILE_DIRTY);
> I do not understand this comment. What serializes this function and
> mem_cgroup_move_account()?
>
The race is exist just because the two competitors share one
public variable and one reads it and the other writes it.
I thought if both sides(accounting and cgroup_move) do not
change page flag, then risks like doule-counting(see below)
will not happen.
CPU-A CPU-B
Set PG_dirty
(delay) move_lock_mem_cgroup()
if (PageDirty(page))
new_memcg->nr_dirty++
pc->mem_cgroup =
new_memcg;
move_unlock_mem_cgroup()
move_lock_mem_cgroup()
memcg = pc->mem_cgroup
new_memcg->nr_dirty++
But after second thoughts, it does have problem if without lock:
CPU-A CPU-B
if (PageDirty(page)) {
move_lock_mem_cgroup()
TestClearPageDirty(page))
memcg =
pc->mem_cgroup
new_memcg->nr_dirty --
move_unlock_mem_cgroup()
memcg = pc->mem_cgroup
new_memcg->nr_dirty--
}
It may occur race between clear_page_dirty() operation.
So this time I think we need the lock again...
Kame, what about your opinion...
>> dec_zone_page_state(page, NR_FILE_DIRTY);
>> dec_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
>> }
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index ebed1ca..90e2946 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -82,6 +82,7 @@ static const char * const mem_cgroup_stat_names[] = {
>> "rss",
>> "mapped_file",
>> "swap",
>> + "dirty",
>> };
>>
>> enum mem_cgroup_events_index {
>> @@ -2538,6 +2539,18 @@ 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,
>> + enum mem_cgroup_stat_index idx)
>> +{
>> + /* Update stat data for mem_cgroup */
>> + preempt_disable();
>> + __this_cpu_dec(from->stat->count[idx]);
>> + __this_cpu_inc(to->stat->count[idx]);
>> + preempt_enable();
>> +}
>> +
>> /**
>> * mem_cgroup_move_account - move account of the page
>> * @page: the page
>> @@ -2583,13 +2596,14 @@ 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,
>> + MEM_CGROUP_STAT_FILE_MAPPED);
>> +
>> + if (PageDirty(page))
>> + mem_cgroup_move_account_page_stat(from, to,
>> + MEM_CGROUP_STAT_FILE_DIRTY);
>> +
>> mem_cgroup_charge_statistics(from, anon, -nr_pages);
>>
>> /* caller should have done css_get */
>> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
>> index e5363f3..e79a2f7 100644
>> --- a/mm/page-writeback.c
>> +++ b/mm/page-writeback.c
>> @@ -1962,6 +1962,7 @@ int __set_page_dirty_no_writeback(struct page *page)
>> void account_page_dirtied(struct page *page, struct address_space *mapping)
>> {
>> if (mapping_cap_account_dirty(mapping)) {
>> + mem_cgroup_inc_page_stat(page, MEM_CGROUP_STAT_FILE_DIRTY);
> It might be helpful to add a comment to account_page_dirtied()
> indicating that caller must hold mem_cgroup_begin_update_page_stat()
> lock. Extra credit for an new assertion added to
> mem_cgroup_update_page_stat() confirming the needed lock is held.
>
Got it! :-)
>> __inc_zone_page_state(page, NR_FILE_DIRTY);
>> __inc_zone_page_state(page, NR_DIRTIED);
>> __inc_bdi_stat(mapping->backing_dev_info, BDI_RECLAIMABLE);
>> @@ -2001,12 +2002,20 @@ EXPORT_SYMBOL(account_page_writeback);
>> */
>> int __set_page_dirty_nobuffers(struct page *page)
>> {
>> + bool locked;
>> + unsigned long flags;
>> + int ret = 0;
>> +
>> + mem_cgroup_begin_update_page_stat(page,&locked,&flags);
>> +
> Is there a strict lock ordering which says that
> mem_cgroup_begin_update_page_stat() must not be called while holding
> tree_lock? If yes, then maybe we should update the 'Lock ordering'
> comment in mm/filemap.c to describe the
> mem_cgroup_begin_update_page_stat() lock.
>
I think yes, otherwise it may cause deadlock. I'll update it later.
>> if (!TestSetPageDirty(page)) {
>> struct address_space *mapping = page_mapping(page);
>> struct address_space *mapping2;
>>
>> - if (!mapping)
>> - return 1;
>> + if (!mapping) {
>> + ret = 1;
>> + goto out;
>> + }
>
> The following seems even easier because it does not need your 'ret = 1'
> change below.
>
> + ret = 1;
> if (!mapping)
> - return 1;
> + goto out;
>
>
>>
>> spin_lock_irq(&mapping->tree_lock);
>> mapping2 = page_mapping(page);
>> @@ -2022,9 +2031,12 @@ int __set_page_dirty_nobuffers(struct page *page)
>> /* !PageAnon&& !swapper_space */
>> __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
>> }
>> - return 1;
>> + ret = 1;
> With the ret=1 change above, this can be changed to:
> - return 1;
>
Seems better.
>> }
>> - return 0;
>> +
>> +out:
>> + mem_cgroup_end_update_page_stat(page,&locked,&flags);
>> + return ret;
>> }
>> EXPORT_SYMBOL(__set_page_dirty_nobuffers);
>>
>> @@ -2139,6 +2151,9 @@ EXPORT_SYMBOL(set_page_dirty_lock);
>> int clear_page_dirty_for_io(struct page *page)
>> {
>> struct address_space *mapping = page_mapping(page);
>> + bool locked;
>> + unsigned long flags;
>> + int ret = 0;
>>
>> BUG_ON(!PageLocked(page));
>>
>> @@ -2180,13 +2195,16 @@ int clear_page_dirty_for_io(struct page *page)
>> * the desired exclusion. See mm/memory.c:do_wp_page()
>> * for more comments.
>> */
>> + mem_cgroup_begin_update_page_stat(page,&locked,&flags);
>> if (TestClearPageDirty(page)) {
>> + 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);
>> - return 1;
>> + ret = 1;
>> }
>> - return 0;
>> + mem_cgroup_end_update_page_stat(page,&locked,&flags);
>> + return ret;
>> }
>> return TestClearPageDirty(page);
>> }
>> diff --git a/mm/truncate.c b/mm/truncate.c
>> index 75801ac..052016a 100644
>> --- a/mm/truncate.c
>> +++ b/mm/truncate.c
>> @@ -73,9 +73,14 @@ static inline void truncate_partial_page(struct page *page, unsigned partial)
>> */
>> void cancel_dirty_page(struct page *page, unsigned int account_size)
>> {
>> + bool locked;
>> + unsigned long flags;
>> +
>> + mem_cgroup_begin_update_page_stat(page,&locked,&flags);
>> if (TestClearPageDirty(page)) {
>> struct address_space *mapping = page->mapping;
>> if (mapping&& 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);
>> @@ -83,6 +88,7 @@ void cancel_dirty_page(struct page *page, unsigned int account_size)
>> task_io_account_cancelled_write(account_size);
>> }
>> }
>> + mem_cgroup_end_update_page_stat(page,&locked,&flags);
>> }
>> EXPORT_SYMBOL(cancel_dirty_page);
--
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