Re: [PATCH] ext4: if zeroout fails fall back to splitting the extent node

On Wed 24-11-21 17:01:12, yangerkun wrote:

On 2021/11/23 17:27, Jan Kara wrote:

quoted

Hello,

On Sun 26-09-21 19:35:01, yangerkun wrote:

quoted

Rethink about this problem. Should we consider other place which call
ext4_issue_zeroout? Maybe it can trigger the problem too(in theory, not
really happened)...

How about include follow patch which not only transfer ENOSPC to EIO. But
also stop to overwrite the error return by ext4_ext_insert_extent in
ext4_split_extent_at.

Besides, 308c57ccf431 ("ext4: if zeroout fails fall back to splitting the
extent node") can work together with this patch.

I've got back to this. The ext4_ext_zeroout() calls in
ext4_split_extent_at() seem to be there as fallback when insertion of a new
extent fails due to ENOSPC / EDQUOT. If even ext4_ext_zeroout(), then I
think returning an error as the code does now is correct and we don't have
much other option. Also we are really running out of disk space so I think
returning ENOSPC is fine. What exact scenario are you afraid of?

I am afraid about the EDQUOT from ext4_ext_insert_extent may be overwrite by
ext4_ext_zeroout with ENOSPC. And this may lead to dead loop since
ext4_writepages will retry once get ENOSPC? Maybe I am wrong...

OK, so passing back original error instead of the error from
ext4_ext_zeroout() makes sense. But I don't think doing much more is needed
- firstly, ENOSPC or EDQUOT should not happen in ext4_split_extent_at()
called from ext4_writepages() because we should have reserved enough
space for extent splits when writing data. So hitting that is already
unexpected. Committing transaction holding blocks that are expected to be
free is the most likely reason for us seeing ENOSPC and returning EIO in
that case would be bug. Secondly, returning EIO instead of ENOSPC is IMO a
bit confusing for upper layers and makes it harder to analyze where the
real problem is...

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR