linux-kernel - Re: [PATCH] mm,shmem: Fix a typo in shmem_swapin

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Date:   Fri, 23 Jul 2021 13:23:07 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Huang Ying <ying.huang@...el.com>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Yang Shi <shy828301@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Miaohe Lin <linmiaohe@...wei.com>,
        Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH] mm,shmem: Fix a typo in shmem_swapin_page()

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@...el.com>
> Cc: Miaohe Lin <linmiaohe@...wei.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Minchan Kim <minchan@...nel.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