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
next prev parent 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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=alpine.LSU.2.11.1911221616580.1144@eggly.anvils \
--to=hughd@google.com \
--cc=Stefan.Potyra@elektrobit.com \
--cc=aarcange@redhat.com \
--cc=akpm@linux-foundation.org \
--cc=alex.shi@linux.alibaba.com \
--cc=aneesh.kumar@linux.ibm.com \
--cc=aryabinin@virtuozzo.com \
--cc=cai@lca.pw \
--cc=cgroups@vger.kernel.org \
--cc=chris@chrisdown.name \
--cc=colin.king@canonical.com \
--cc=daniel.m.jordan@oracle.com \
--cc=guro@fb.com \
--cc=hannes@cmpxchg.org \
--cc=ira.weiny@intel.com \
--cc=jgg@ziepe.ca \
--cc=jglisse@redhat.com \
--cc=khlebnikov@yandex-team.ru \
--cc=kirill.shutemov@linux.intel.com \
--cc=ktkhai@virtuozzo.com \
--cc=laoar.shao@gmail.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mchehab+samsung@kernel.org \
--cc=mgorman@techsingularity.net \
--cc=mhocko@kernel.org \
--cc=nborisov@suse.com \
--cc=peng.fan@nxp.com \
--cc=rientjes@google.com \
--cc=rppt@linux.vnet.ibm.com \
--cc=sfr@canb.auug.org.au \
--cc=shakeelb@google.com \
--cc=swkhack@gmail.com \
--cc=tglx@linutronix.de \
--cc=tj@kernel.org \
--cc=vbabka@suse.cz \
--cc=vdavydov.dev@gmail.com \
--cc=willy@infradead.org \
--cc=yang.shi@linux.alibaba.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).