Re: [PATCH] mm,shmem: Fix a typo in shmem_swapin_page()
From: Hugh Dickins <hughd@google.com>
Date: 2021-07-23 20:23:27
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On Fri, 23 Jul 2021, Huang Ying wrote:
quoted hunk ↗ jump to hunk
"-" is missing before "EINVAL". Fixes: 2efa33fc7f6e ("mm/shmem: fix shmem_swapin() race with swapoff") Signed-off-by: "Huang, Ying" <redacted> 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 <redacted> 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