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Message-ID: <874jlx1vtr.fsf@suse.de>
Date: Fri, 21 Jul 2023 10:02:24 +0100
From: Luís Henriques <lhenriques@...e.de>
To: Filipe Manana <fdmanana@...nel.org>
Cc: Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
David Sterba <dsterba@...e.com>,
Johannes Thumshirn <Johannes.Thumshirn@....com>,
linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] btrfs: propagate error from function
unpin_extent_cache()
Filipe Manana <fdmanana@...nel.org> writes:
> On Thu, Jul 20, 2023 at 5:05 PM Luís Henriques <lhenriques@...e.de> wrote:
>>
>> Function unpin_extent_cache() doesn't propagate an error if the call to
>> lookup_extent_mapping() fails. This patch adds an error return (EINVAL)
>> and simply logs it in the only caller.
>>
>> Signed-off-by: Luís Henriques <lhenriques@...e.de>
>> ---
>> Hi!
>>
>> As per David and Johannes reviews, I'm now proposing a different approach.
>> Note that I kept the WARN_ON() instead of replacing it by an ASSERT(). In
>> fact, I considered removing the WARN_ON() completely and simply return the
>> error if em->start != start. But I guess it may useful for debug.
>>
>> Changes since v1:
>> Instead of changing unpin_extent_cache() into a void function, make it
>> propage an error code instead.
>>
>> fs/btrfs/extent_map.c | 4 +++-
>> fs/btrfs/inode.c | 8 ++++++--
>> 2 files changed, 9 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c
>> index 0cdb3e86f29b..f4e7956edc05 100644
>> --- a/fs/btrfs/extent_map.c
>> +++ b/fs/btrfs/extent_map.c
>> @@ -304,8 +304,10 @@ int unpin_extent_cache(struct extent_map_tree *tree, u64 start, u64 len,
>>
>> WARN_ON(!em || em->start != start);
>>
>> - if (!em)
>> + if (!em) {
>> + ret = -EINVAL;
>> goto out;
>> + }
>>
>> em->generation = gen;
>> clear_bit(EXTENT_FLAG_PINNED, &em->flags);
>> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
>> index dbbb67293e34..21eb66fcc0df 100644
>> --- a/fs/btrfs/inode.c
>> +++ b/fs/btrfs/inode.c
>> @@ -3273,8 +3273,12 @@ int btrfs_finish_one_ordered(struct btrfs_ordered_extent *ordered_extent)
>> ordered_extent->disk_num_bytes);
>> }
>> }
>> - unpin_extent_cache(&inode->extent_tree, ordered_extent->file_offset,
>> - ordered_extent->num_bytes, trans->transid);
>> +
>> + /* Proceed even if we fail to unpin extent from cache */
>> + if (unpin_extent_cache(&inode->extent_tree, ordered_extent->file_offset,
>> + ordered_extent->num_bytes, trans->transid) < 0)
>> + btrfs_warn(fs_info, "failed to unpin extent from cache");
>
> Well, this is not very useful. It doesn't provide any more useful
> information than what we get from the WARN_ON() at
> unpin_extent_cache(), making the patch not useful.
>
> This warning has actually happened a few times when running fstests
> that exercise relocation (not sure if it's gone and accidently fixed
> by something recently).
Oh! In that case replacing the WARN_ON() by an ASSERT() would have
definitely be a bad idea.
> But to make this more useful, I would place the message at
> unpin_extent_cache() with useful information such as:
>
> - inode number
> - id of the root the inode belongs to
> - the file offset (the start argument) and extent length (or end offset)
> - why the warning triggered: we didn't find the extent map or we found
> one with a different start offset
> - if we found an unexpected extent map, dump its flags (so we can see
> if it happens only with compressed or prealloc extents for e.g.) and
> other details (length/end offset for e.g.)
Thanks, Filipe. This all makes sense. And in this case I guess I should
go back to the initial approach of changing the function to a void
function, but adding all this useful information.
Cheers,
--
Luís
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