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From: "Huang, Ying" <ying.huang@intel.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Hugh Dickins <hughd@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Hildenbrand <david@redhat.com>,
	Yang Shi <shy828301@gmail.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Miaohe Lin <linmiaohe@huawei.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@suse.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Minchan Kim <minchan@kernel.org>
Subject: Re: [PATCH] mm,shmem: Fix a typo in shmem_swapin_page()
Date: Tue, 03 Aug 2021 16:14:38 +0800	[thread overview]
Message-ID: <8735rr54i9.fsf@yhuang6-desk2.ccr.corp.intel.com> (raw)
In-Reply-To: <YPs6cQo7iG1JcOn8@casper.infradead.org> (Matthew Wilcox's message of "Fri, 23 Jul 2021 22:53:53 +0100")

Matthew Wilcox <willy@infradead.org> writes:

> On Fri, Jul 23, 2021 at 01:23:07PM -0700, Hugh Dickins wrote:
>> I was wary because, if the (never observed) race to be fixed is in
>> swap_cluster_readahead(), why was shmem_swapin_page() being patched?
>> Not explained in its commit message, probably a misunderstanding of
>> how mm/shmem.c already manages races (and prefers not to be involved
>> in swap_info_struct stuff).
>> 
>> But why do I now say it's bad?  Because even if you correct the EINVAL
>> to -EINVAL, that's an unexpected error: -EEXIST is common, -ENOMEM is
>> not surprising, -ENOSPC can need consideration, but -EIO and anything
>> else just end up as SIGBUS when faulting (or as error from syscall).
>> So, 2efa33fc7f6e converts a race with swapoff to SIGBUS: not good,
>> and I think much more likely than the race to be fixed (since
>> swapoff's percpu_ref_kill() rightly comes before synchronize_rcu()).
>
> Yes, I think a lot more thought was needed here.  And I would have
> preferred to start with a reproducer instead of "hey, this could
> happen".  Maybe something like booting a 1GB VM, adding two 2GB swap
> partitions, swapon(partition A); run a 2GB memhog and then
>
> loop:
> 	swapon(part B);
> 	swapoff(part A);
> 	swapon(part A);
> 	swapoff(part B);
>
> to make this happen.
>
> but if it does happen, why would returning EINVAL be the right thing
> to do?  We've swapped it out.  It must be on swap somewhere, or we've
> really messed up.  So I could see there being a race where we get
> preempted between looking up the swap entry and calling get_swap_device().
> But if that does happen, then the page gets brought in, and potentially
> reswapped to the other swap device.
>
> So returning -EEXIST here would actually work.  That forces a re-lookup
> in the page cache, so we'll get the new swap entry that tells us which
> swap device the page is now on.

Yes.  -EEXIST is the right error code.  We use that in
shmem_swapin_page() to deal with race condition.

> But I REALLY REALLY REALLY want a reproducer.  Right now, I have a hard
> time believing this, or any of the other races can really happen.

I think the race is only theoretical too.  Firstly, swapoff is a rare
operations in practice; secondly, the race window is really small.

Best Regards,
Huang, Ying

>> 2efa33fc7f6e was intending to fix a race introduced by two-year-old
>> 8fd2e0b505d1 ("mm: swap: check if swap backing device is congested
>> or not"), which added a call to inode_read_congested().  Certainly
>> relying on si->swap_file->f_mapping->host there was new territory:
>> whether actually racy I'm not sure offhand - I've forgotten whether
>> synchronize_rcu() waits for preempted tasks or not.
>> 
>> But if it is racy, then I wonder if the right fix might be to revert
>> 8fd2e0b505d1 too. Convincing numbers were offered for it, but I'm
>> puzzled: because Matthew has in the past noted that the block layer
>> broke and further broke bdi congestion tracking (I don't know the
>> relevant release numbers), so I don't understand how checking
>> inode_read_congested() is actually useful there nowadays.
>
> It might be useful for NFS?  I don't think congestion is broken there
> (except how does the NFS client have any idea whether the server is
> congested or not?)

  reply	other threads:[~2021-08-03  8:14 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-23  8:00 Huang Ying
2021-07-23 11:10 ` David Hildenbrand
2021-07-23 20:23 ` Hugh Dickins
2021-07-23 21:53   ` Matthew Wilcox
2021-08-03  8:14     ` Huang, Ying [this message]
2021-08-03 12:01       ` Matthew Wilcox
2021-08-04  5:34         ` Hugh Dickins
2021-08-04  9:04           ` Huang, Ying
2021-08-05 23:08           ` Yang Shi
2021-08-06  6:01             ` Hugh Dickins
2021-08-06 20:37               ` Yang Shi
2021-08-09 21:26                 ` Yang Shi
2021-08-09 23:43                   ` Huang, Ying
2021-08-10  1:11                     ` Yang Shi
2021-08-04  9:01         ` Huang, Ying
2021-07-28 13:03   ` huang ying
2021-08-03  8:06     ` Huang, Ying
2021-08-03 11:59       ` Matthew Wilcox
2021-08-04  4:28       ` Hugh Dickins
2021-08-04  6:42     ` Hugh Dickins
2021-08-04  8:59       ` Huang, Ying

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