Message-ID: <aUGAf_NLbIt_sktF@bfoster>
Date: Tue, 16 Dec 2025 10:53:35 -0500
From: Brian Foster <bfoster@...hat.com>
To: Baokun Li <libaokun1@...wei.com>
Cc: linux-ext4@...r.kernel.org, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] ext4: fix dirtyclusters double decrement on fs shutdown

On Tue, Dec 16, 2025 at 12:01:41PM +0800, Baokun Li wrote:
> On 2025-12-15 23:28, Brian Foster wrote:
> > On Sat, Dec 13, 2025 at 09:46:23AM +0800, Baokun Li wrote:
> >> Hi Brian,
> >>
> >> Thanks for the patch.
> >>
> > Hi Baokun,
> >
> > Thanks for reviewing..
> >
> >> On 2025-12-12 23:47, Brian Foster wrote:
> >>> fstests test generic/388 occasionally reproduces a warning in
> >>> ext4_put_super() associated with the dirty clusters count:
> >>>
> >>>   WARNING: CPU: 7 PID: 76064 at fs/ext4/super.c:1324 ext4_put_super+0x48c/0x590 [ext4]
> >>>
> >>> Tracing the failure shows that the warning fires due to an
> >>> s_dirtyclusters_counter value of -1. IOW, this appears to be a
> >>> spurious decrement as opposed to some sort of leak. Further tracing
> >>> of the dirty cluster count deltas and an LLM scan of the resulting
> >>> output identified the cause as a double decrement in the error path
> >>> between ext4_mb_mark_diskspace_used() and the caller
> >>> ext4_mb_new_blocks().
> >>>
> >>> First, note that generic/388 is a shutdown vs. fsstress test and so
> >>> produces a random set of operations and shutdown injections. In the
> >>> problematic case, the shutdown triggers an error return from the
> >>> ext4_handle_dirty_metadata() call(s) made from
> >>> ext4_mb_mark_context(). The changed value is non-zero at this point,
> >>> so ext4_mb_mark_diskspace_used() does not exit after the error
> >>> bubbles up from ext4_mb_mark_context(). Instead, the former
> >>> decrements both cluster counters and returns the error up to
> >>> ext4_mb_new_blocks(). The latter falls into the !ar->len out path
> >>> which decrements the dirty clusters counter a second time, creating
> >>> the inconsistency.
> >>>
> >>> AFAICT the solution here is to exit immediately from
> >>> ext4_mb_mark_diskspace_used() on error, regardless of the changed
> >>> value. This leaves the caller responsible for clearing the block
> >>> reservation at the same level it is acquired. This also skips the
> >>> free clusters update, but the caller also calls into
> >>> ext4_discard_allocated_blocks() to free the blocks back into the
> >>> group. This survives an overnight loop test of generic/388 on an
> >>> otherwise reproducing system and survives a local regression run.
> >>>
> >>> Signed-off-by: Brian Foster <bfoster@...hat.com>
> >>> ---
> >>>
> >>> Hi all,
> >>>
> >>> I've thrown some testing at this and poked around the area enough that I
> >>> _think_ it is reasonably sane, but the error paths are hairy and I could
> >>> certainly be missing some details. I'm happy to try a different approach
> >>> if there are any thoughts around that.. thanks.
> >>>
> >>> Brian
> >>>
> >>>  fs/ext4/mballoc.c | 2 +-
> >>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> >>> index 56d50fd3310b..224abfd6a42b 100644
> >>> --- a/fs/ext4/mballoc.c
> >>> +++ b/fs/ext4/mballoc.c
> >>> @@ -4234,7 +4234,7 @@ ext4_mb_mark_diskspace_used(struct ext4_allocation_context *ac,
> >>>  				   ac->ac_b_ex.fe_start, ac->ac_b_ex.fe_len,
> >>>  				   flags, &changed);
> >>>  
> >>> -	if (err && changed == 0)
> >>> +	if (err)
> >>>  		return err;
> >>>  
> >>>  #ifdef AGGRESSIVE_CHECK
> >> I think we might need to swap that && for an ||.
> >>
> >> Basically, we should only proceed with the following logic if there's
> >> no error and the bitmap was actually changed. If we hit an error or
> >> if the section we intended to modify was already fully handled,
> >> we should just bail out early. Otherwise, the err could get quietly
> >> ignored unless we hit a duplicate allocation that happens to result in
> >> 'changed' being zero.
> >>
> > Hmm.. to make sure I understand, are you referring to an inconsistency
> > case where we allocated blocks that were already marked as such in the
> > group on disk..?
> Yes.
> > I'm a little uneasy about this because it seems to conflict with the
> > surrounding code. AFAICT the only way we can hit something like !err &&
> > !changed is via EXT4_MB_BITMAP_MARKED_CHECK, which causes
> > _mark_context() to check the bitmap for "already modified" bits up
> > front.
> >
> > If this scenario plays out, the caller has a BUG check just after the
> > return (also under the aggressive check macro). So ISTM that this sort
> > of (err || !changed) logic would bypass the aggressive checks and let
> > the fs carry on when it probably shouldn't. Hm?
> 
> Regarding ext4_mb_mark_context, if the passed ret_changed pointer is
> non-NULL, we initialize *ret_changed to 0. After updating the bitmap_bh,
> we then update *ret_changed with the actual number of blocks modified
> (changed).
> 
> Therefore, the original intention was for changed == 0 to signify that
> an error occurred in ext4_mb_mark_context() before ret_changed could be
> updated. However, as you pointed out, we also get changed == 0 when the
> target extent has already been fully marked as allocated within bitmap_bh.
> 
> Crucially, we only genuinely check the bitmap to modify changed when
> EXT4_MB_BITMAP_MARKED_CHECK is set (i.e., when AGGRESSIVE_CHECK is defined,
> or during fast commit or resize operations). Otherwise, changed is always
> set to the target length. This means that, in the general case, errors
> returned after the point where ret_changed is updated (e.g., the error
> from ext4_handle_dirty_metadata()) are usually ignored.
> 
> In summary:
> 
>  * (err && changed == 0) only concerns errors that occur before ret_changed
>    is updated.
>  * (err || changed == 0) concerns whether there was an error OR if any
>    modification actually took place.
> 
> If we only care about err, we could move the update of ret_changed inside
> ext4_mb_mark_context() to just before the successful return.
> 

Yeah, I think the _mark_context() logic is reasonably straightforward
from the code. What is less clear is why the allocation path only cares
about errors prior to ret_changed being set.

It looks to me that this is just wrong. I suspect either due to
copy/paste error in the mark_diskspace_used() path at some point in the
past, or an attempt to filter out the BUG case from an obvious case
where changed will be 0 on certain error returns.

I don't think the mark_context() logic alone tells the full story here.
I think what's relevant is the high level error handling of the
!delalloc allocation path:

1. ext4_mb_new_blocks() reserves blocks and attempts physical allocation
in memory. On success, it calls into ext4_mb_mark_diskspace_used() to
update on-disk structures.

2a. If ext4_mb_mark_diskspace_used() is successful, it decrements the
freeclusters counter and releases res from the dirtyclusers counter
(i.e. transfer the block from dirty to used). ext4_mb_new_blocks()
basically just returns the result.

2b. If ext4_mb_mark_diskspace_used() fails, ext4_mb_new_blocks()
receives the error. In the error exit path, it frees the blocks back
into the incore structures and releases the reservation it acquired in
step 1.  

However the bug this patch is trying to fix is that
ext4_mb_mark_diskspace_used() runs the counter updates regardless of
error in some cases. If the on-disk update fails, _new_blocks() will
release its reservation and return the blocks, so _mark_diskspace_used()
shouldn't account that block from the dirty and free counters.

What isn't quite clear to me is how this is expected to deal with the
modified buffer. This particular case is a shutdown and journal abort,
so I suspect the buffer can't write back.

> >> By the way, I spotted two other spots with similar error logic:
> >> ext4_mb_clear_bb() and ext4_group_add_blocks().
> >>
> > Yeah, I saw those as well but didn't think they needed changing. My high
> > level understanding of the alloc case is that ext4_mb_new_blocks()
> > acquires res (!delalloc), allocs blocks out of in-core structures, then
> > calls down into _mark_diskspace_used() to update/journal on-disk
> > structures with the pending alloc. If the latter fails, we release res
> > and feed blocks back into the in-core structures. So IOW, if we return
> > directly from _mark_diskspace_used() the counters/state end up
> > consistent afaict.
> >
> > For the ext4_free_blocks() case, we call _mark_context() and if it fails
> > with changed != 0 (and don't otherwise BUG), we still go ahead and free
> > the blocks in the e4b and return the error. It does look like the
> > discard code can clobber the error though, so perhaps that should be
> > fixed. But otherwise it's not clear to me why we might want to exit
> > early there. Am I missing something else?
> 
> The core issue is that they risk ignoring certain errors, which can
> result in inconsistency.
> 

Hmm.. I'm not sure it's that simple in the free path. It looks like
things are ordered differently there. We modify the on-disk struct and
if it changes something, then even it fails we go ahead and proceed with
the in-core updates, and then return the error. Modulo the discard logic
thing, the error is then passed into the standard error handling code,
so it isn't really ignored.

Though again I'm not quite sure what the expected result is here in the
case where it's the ext4_handle_dirty_metadata() call that fails. Is
this a guaranteed shutdown/abort situation? Perhaps Jan or somebody
familiar with the journaling code could chime in on this..? Thanks.

> >
> >> Since this issue popped up in the last couple of years, we should
> >> probably add a Fixes: tag to make backporting easier.
> >>
> > Do you have a target patch in mind? I made a pass through historical
> > changes and it looked like this was a longer standing issue through
> > various bits of refactoring..
> >
> No, I haven't, but I suspect it was introduced when
> (err && changed == 0) was added.
> 

Making another pass.. that code is introduced in commit 2f94711b098b
("ext4: call ext4_mb_mark_context in ext4_mb_mark_diskspace_used"), but
that just refactors things and doesn't appear to introduce the problem.
It looks like it still double decrements due to how the code is ordered.

Going back further.. commit 0087d9fb3f29 ("ext4: Fix
s_dirty_blocks_counter if block allocation failed with nodelalloc") adds
the release in ext4_mb_new_blocks(), so that appears to be where the
logic wart was first introduced.

That said, that one goes back to 2009 and the current fix wouldn't
apply. TBH given how old the bug is and that it's basically just a
warning in a shutdown case, I'm not sure it's really worth the risk of
backporting at all.. I'll defer to the community on that however and can
at least mention the commit in the changelog if we don't ultimately tag
it.

Brian

> 
> Cheers,
> Baokun
> 

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