Re: [PATCH] ext4: Fix bug on in ext4_es_cache_extent as ext4_split_extent_at failed

[yebin <yebin10@huawei.com>]

On 2021/4/9 9:47, Theodore Ts'o wrote:
> On Wed, Apr 07, 2021 at 09:41:57AM +0800, yebin wrote:
>>>> If call ext4_ext_insert_extent failed but new extent already inserted, we just
>>>> update "ex->ee_len = orig_ex.ee_len", this will lead to extent overlap, then
>>>> cause bug on when cache extent.
>>> How did this happen in the first place?  It sounds like if the extent
>>> was already inserted, that would be casue there was an on-disk file
>>> system corruption, no?
>>>
>>> In that case, shouldn't we call ext4_error() to declare the file
>>> system has an inconsistency, so it can be fixed by fsck?
>> We inject IO fault when runing  fsstress,  JBD detect IO error then trigger
>> JBD abort.  At the same time,
>> if ext4_ext_insert_extent already insert new exntent then call
>> ext4_ext_dirty to dirty metadata , but
>> JBD already aborted ,  ext4_ext_dirty will return error.
>> In ext4_ext_dirty function call  ext4_ext_check_inode check extent if ok, if
>> not, trigger BUG_ON and
>> also print extent detail information.
> In this particular case, skipping the "ex->ee_len = orig_ex.ee_len"
> may avoid the BUG_ON.  But it's not clear that this is always the
> right thing to do.  The fundamental question is what should we do we
> run into an error while we are in the middle of making changes to
> on-disk and in-memory data structures?
>
> In the ideal world, we should undo the changes that we were in the
> middle of making before we return an error.  That way, the semantics
> are very clear; on success, the function has made the requested change
> to the file system.  If the function returns an error, then no changes
> should be made.
>
> That was the reasoning behind resetting ex->ee_len to orig_ex.ee_len
> in the fix_extent_len inside ext4_split_extent_at().  Unofrtunately,
> ext4_ext_insert_extent() does *not* always follow this convention, and
> that's because it would be extremely difficult for it to do so --- the
> mutations that it makes can be quite complex, including potentially
> increasing the height of the extent tree.
>
> However, I don't think your fix is by any means the ideal one, because
> the much more common way that ext4_ext_insert_extent() is when it
> needs to insert a new leaf node, or need to increase the height of the
> extent tree --- and in it returns an ENOSPC failure.  In that case, it
> won't have made any changes changes in the extent tree, and so having
> ext4_split_extent_at() undo the change to ex->ee_len is the right
> thing to do.
>
> Having blocks get leaked when there is an ENOSPC failure, requiring
> fsck to be run --- and without giving the user any warning that this
> has happened is *not* a good way to fail.  So I don't think the
> proposed patch is the right way to go.
>
> A better way to go would be to teach ext4_ext_insert_extent() so if
> there is a late failure, that it unwinds the leaf node back to its
> original state (at least from a semantic value).  Since the extent
> leaf node could have been split, and/or adjacent extent entries may
> have been merged, what it would need to do is to remember the starting
> block number and length, and make whatever changes are necessaries to
> the extent entries in that leaf node corresponding to that starting
> block number and length.
>
> If you don't want to do that, then a "do no harm" fix would be
> something like this:
>
> 	...
> 	} else if (err == -EROFS) {
> 		return err;
> 	} else if (err)
> 		goto fix_extent_len;
Thanks  for your advice. I will send v2 patch.
>
> So in the journal abort case, when err is set to EROFS, we don't try
> to reset the length, since in theory the file system is read-only
> already anyway.  However, in the ENOSPC case, we won't end up silently
> leaking blocks that will be lost until the user somehow decides to run
> fsck.
>
> There are still times when this doesn't get things completely right
> (e.g., what if we get a late ENOMEM error versus an early ENOMEM
> failure), where the only real fix is to make ext4_ext_insert_extent()
> obey the convention that if it returns an error, it must not result in
> any user-visible state change.
>
> Cheers,
>
> 						- Ted
> .