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From: Hugh Dickins <firstname.lastname@example.org>
To: Matthew Wilcox <email@example.com>
Cc: "Huang, Ying" <firstname.lastname@example.org>,
Hugh Dickins <email@example.com>,
Andrew Morton <firstname.lastname@example.org>,
David Hildenbrand <email@example.com>,
Yang Shi <firstname.lastname@example.org>,
Miaohe Lin <email@example.com>,
Johannes Weiner <firstname.lastname@example.org>,
Michal Hocko <email@example.com>,
Joonsoo Kim <firstname.lastname@example.org>,
Minchan Kim <email@example.com>
Subject: Re: [PATCH] mm,shmem: Fix a typo in shmem_swapin_page()
Date: Tue, 3 Aug 2021 22:34:52 -0700 (PDT) [thread overview]
Message-ID: <firstname.lastname@example.org> (raw)
On Tue, 3 Aug 2021, Matthew Wilcox wrote:
> On Tue, Aug 03, 2021 at 04:14:38PM +0800, Huang, Ying wrote:
> > Matthew Wilcox <email@example.com> writes:
> > > 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.
> So do something to provoke it. Widen the window. Put an msleep(1000)
> between *pagep = NULL and the call to get_swap_device(). That's assuming
> that the swapon/swapoff loop that I proposed doesn't work. Did you
> try it?
I've been doing that swapon/swapoff loop for years, while running kernel
builds on tmpfs going out to swap; for better or worse on baremetal not VM.
You're right that few will ever need that level of reliability; but it
has caught problems from time to time, and I do insist on fixing them.
I'm not as insistent as you on wanting a reproducer; and we all take pride
sometimes in fixing ever more inconceivable bugs. I'm not against that,
but it's easy to end up with a fix more dangerous than what it claims to
fix, rather like with random newbie cleanups.
I've never seen the swapoff race claimed by Miaohe, and don't expect to;
but he's probably right, given the current code. I just dislike adding
unnecessary complexity, and siting it in the wrong place (mm/shmem.c).
Yang, is it possible that 5.1 commit 8fd2e0b505d1 ("mm: swap: check if
swap backing device is congested or not") was actually developed and
measured on 4.1 or earlier, which still had blk_set_queue_congested()?
I cannot explain its usefulness nowadays, on congested HDD anyway:
Matthew is right that NFS and a few others may still be setting
congested flags, but they're not what that commit was proposed for.
If it is still useful, then I contend (but Huang Ying will disagree)
that the get_swap_device() and put_swap_device() should be around
8fd2e0b505d1's inode_read_congested() block in swap_cluster_readahead(),
not encroaching into mm/shmem.c.
But if that block is not useful, then it should simply be removed (later).
next prev parent reply other threads:[~2021-08-04 5:35 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
2021-08-03 12:01 ` Matthew Wilcox
2021-08-04 5:34 ` Hugh Dickins [this message]
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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