From: Hugh Dickins <hughd@google.com>
To: Huang Ying <ying.huang@intel.com>
Cc: 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>,
Hugh Dickins <hughd@google.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Michal Hocko <mhocko@suse.com>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
Matthew Wilcox <willy@infradead.org>,
Minchan Kim <minchan@kernel.org>
Subject: Re: [PATCH] mm,shmem: Fix a typo in shmem_swapin_page()
Date: Fri, 23 Jul 2021 13:23:07 -0700 (PDT) [thread overview]
Message-ID: <24187e5e-069-9f3f-cefe-39ac70783753@google.com> (raw)
In-Reply-To: <20210723080000.93953-1-ying.huang@intel.com>
On Fri, 23 Jul 2021, Huang Ying wrote:
> "-" is missing before "EINVAL".
>
> Fixes: 2efa33fc7f6e ("mm/shmem: fix shmem_swapin() race with swapoff")
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: Miaohe Lin <linmiaohe@huawei.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Minchan Kim <minchan@kernel.org>
> ---
> mm/shmem.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 9af4b2173fe9..e201a3ba12fa 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1708,7 +1708,7 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
> /* Prevent swapoff from happening to us. */
> si = get_swap_device(swap);
> if (!si) {
> - error = EINVAL;
> + error = -EINVAL;
> goto failed;
> }
> /* Look it up and read it in.. */
> --
> 2.30.2
Thanks for catching that; and as David says, it's worse than a typo.
But this is not the right fix:
2efa33fc7f6e ("mm/shmem: fix shmem_swapin() race with swapoff")
needs to be reverted.
It's been on my pile to look at for weeks: now I look at it and see
it's just a bad patch. Over-enthusiastic stablehands already rushed
it out, I was wary, and reverts are already in -rc for 5.13 and 5.10,
phew, but 5.12.19 EOL is stuck with it unfortunately, oh well.
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()).
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.
No need to hurry to a conclusion on 8fd2e0b505d1;
but 2efa33fc7f6e should definitely be reverted.
Thanks,
Hugh
next prev parent reply other threads:[~2021-07-23 20:23 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-23 8:00 [PATCH] mm,shmem: Fix a typo in shmem_swapin_page() Huang Ying
2021-07-23 11:10 ` David Hildenbrand
2021-07-23 20:23 ` Hugh Dickins [this message]
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
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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