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From: Hugh Dickins <hughd@google.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Alex Shi <alex.shi@linux.alibaba.com>,
	cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, akpm@linux-foundation.org,
	mgorman@techsingularity.net, tj@kernel.org, hughd@google.com,
	khlebnikov@yandex-team.ru, daniel.m.jordan@oracle.com,
	yang.shi@linux.alibaba.com, willy@infradead.org,
	shakeelb@google.com, Michal Hocko <mhocko@kernel.org>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	Roman Gushchin <guro@fb.com>, Chris Down <chris@chrisdown.name>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vlastimil Babka <vbabka@suse.cz>, Qian Cai <cai@lca.pw>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Jerome Glisse <jglisse@redhat.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	David Rientjes <rientjes@google.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	swkhack <swkhack@gmail.com>,
	"Potyra, Stefan" <Stefan.Potyra@elektrobit.com>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Colin Ian King <colin.king@canonical.com>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
	Peng Fan <peng.fan@nxp.com>, Nikolay Borisov <nborisov@suse.com>,
	Ira Weiny <ira.weiny@intel.com>,
	Kirill Tkhai <ktkhai@virtuozzo.com>,
	Yafang Shao <laoar.shao@gmail.com>
Subject: Re: [PATCH v4 3/9] mm/lru: replace pgdat lru_lock with lruvec lock
Date: Fri, 22 Nov 2019 16:58:34 -0800 (PST)	[thread overview]
Message-ID: <alpine.LSU.2.11.1911221616580.1144@eggly.anvils> (raw)
In-Reply-To: <20191122161652.GA489821@cmpxchg.org>

On Fri, 22 Nov 2019, Johannes Weiner wrote:
> 
> But that leaves me with one more worry: compaction. We locked out
> charge moving now, so between that and knowing that the page is alive,
> we have page->mem_cgroup stable. But compaction doesn't know whether
> the page is alive - it comes from a pfn and finds out using PageLRU.
> 
> In the current code, pgdat->lru_lock remains the same before and after
> the page is charged to a cgroup, so once compaction has that locked
> and it observes PageLRU, it can go ahead and isolate the page.
> 
> But lruvec->lru_lock changes during charging, and then compaction may
> hold the wrong lock during isolation:
> 
> compaction:				generic_file_buffered_read:
> 
> 					page_cache_alloc()
> 
> !PageBuddy()
> 
> lock_page_lruvec(page)
>   lruvec = mem_cgroup_page_lruvec()
>   spin_lock(&lruvec->lru_lock)
>   if lruvec != mem_cgroup_page_lruvec()
>     goto again
> 
> 					add_to_page_cache_lru()
> 					  mem_cgroup_commit_charge()
> 					    page->mem_cgroup = foo
> 					  lru_cache_add()
> 					    __pagevec_lru_add()
> 					      SetPageLRU()
> 
> if PageLRU(page):
>   __isolate_lru_page()
> 
> I don't see what prevents the lruvec from changing under compaction,
> neither in your patches nor in Hugh's. Maybe I'm missing something?

Speaking for my patches only: I'm humbled, I think you have caught me,
I cannot find any argument against the race you suggest here.

The race with mem_cgroup_move_account(), which Konstantin pointed out
in 2012's https://lore.kernel.org/lkml/4F433418.3010401@openvz.org/
but I later misunderstood, and came to think I needed no patch against,
until this week coming to perceive the same race in isolate_lru_page():
that one is easily and satisfactorily fixed by holding lruvec lock in
mem_cgroup_move_account() - embarrassing, but not too serious.

Your race here (again, lruvec lock taken then PageLRU observed, but
page->mem_cgroup changed in between) really questions my whole scheme:
I am not going to propose a solution now, I'll have to go back and
recheck my assumptions all over.  Certainly isolate_migratepage_block()
has a harder job than any other, but I need to re-review it all.

Maybe we got it right back in the days of PageCgroupUsed, and then I
paid too little attention when rebasing to your welcome simplifications.
I don't think any of us want to bring back PageCgroupUsed! And maybe we
could get it right by always holding lruvec lock in commit_charge(),
lrucare or not; but that's a much hotter path, and not a change I'd
expect anyone to embrace.

I'll go away and re-examine it all; probably start by verifying that
your race actually happens in practice, though we never observed it.

Heavy-hearted thanks, Hannes!
Hugh

  reply	other threads:[~2019-11-23  0:58 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-19 12:23 [PATCH v4 0/9] per lruvec lru_lock for memcg Alex Shi
2019-11-19 12:23 ` [PATCH v4 1/9] mm/swap: fix uninitialized compiler warning Alex Shi
2019-11-19 15:41   ` Johannes Weiner
2019-11-20 11:42     ` Alex Shi
2019-11-19 12:23 ` [PATCH v4 2/9] mm/huge_memory: " Alex Shi
2019-11-19 15:42   ` Johannes Weiner
2019-11-19 12:23 ` [PATCH v4 3/9] mm/lru: replace pgdat lru_lock with lruvec lock Alex Shi
2019-11-19 15:57   ` Matthew Wilcox
2019-11-19 16:44     ` Johannes Weiner
2019-11-20 11:50       ` Alex Shi
2019-11-19 16:04   ` Johannes Weiner
2019-11-20 11:41     ` Alex Shi
2019-11-21 22:06       ` Johannes Weiner
2019-11-22  2:36         ` Alex Shi
2019-11-22 16:16           ` Johannes Weiner
2019-11-23  0:58             ` Hugh Dickins [this message]
2019-11-24 15:19             ` Alex Shi
2019-11-25  9:26             ` Alex Shi
2019-11-25 17:27               ` Shakeel Butt
2019-11-19 16:49   ` Shakeel Butt
2019-11-19 12:23 ` [PATCH v4 4/9] mm/mlock: only change the lru_lock iff page's lruvec is different Alex Shi
2019-11-19 12:23 ` [PATCH v4 5/9] mm/swap: " Alex Shi
2019-11-19 12:23 ` [PATCH v4 6/9] mm/vmscan: " Alex Shi
2019-11-19 12:23 ` [PATCH v4 7/9] mm/pgdat: remove pgdat lru_lock Alex Shi
2019-11-19 12:23 ` [PATCH v4 8/9] mm/lru: likely enhancement Alex Shi
2019-11-19 12:23 ` [PATCH v4 9/9] mm/lru: revise the comments of lru_lock Alex Shi
2019-11-19 16:19   ` Johannes Weiner
2019-11-20 11:48     ` Alex Shi
2019-11-24 15:49 ` [PATCH v4 0/9] per lruvec lru_lock for memcg Konstantin Khlebnikov

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