Re: [PATCH v2] btrfs: propagate error from function unpin_extent_cache()

[Luís Henriques]

Filipe Manana <fdmanana@xxxxxxxxxx> writes:

> On Thu, Jul 20, 2023 at 5:05 PM Luís Henriques <lhenriques@xxxxxxx> 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@xxxxxxx>
>> ---
>> 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