On Mon, Feb 22, 2021 at 10:27:45AM -0800, Darrick J. Wong wrote:
> On Mon, Feb 22, 2021 at 10:34:42AM -0500, Brian Foster wrote:
> > Freed extents are marked busy from the point the freeing transaction
> > commits until the associated CIL context is checkpointed to the log.
> > This prevents reuse and overwrite of recently freed blocks before
> > the changes are committed to disk, which can lead to corruption
> > after a crash. The exception to this rule is that metadata
> > allocation is allowed to reuse busy extents because metadata changes
> > are also logged.
> >
> > As of commit 97d3ac75e5e0 ("xfs: exact busy extent tracking"), XFS
> > has allowed modification or complete invalidation of outstanding
> > busy extents for metadata allocations. This implementation assumes
> > that use of the associated extent is imminent, which is not always
> > the case. For example, the trimmed extent might not satisfy the
> > minimum length of the allocation request, or the allocation
> > algorithm might be involved in a search for the optimal result based
> > on locality.
> >
> > generic/019 reproduces a corruption caused by this scenario. First,
> > a metadata block (usually a bmbt or symlink block) is freed from an
> > inode. A subsequent bmbt split on an unrelated inode attempts a near
> > mode allocation request that invalidates the busy block during the
> > search, but does not ultimately allocate it. Due to the busy state
> > invalidation, the block is no longer considered busy to subsequent
> > allocation. A direct I/O write request immediately allocates the
> > block and writes to it.
>
> I really hope there's a fstest case coming for this... :)
>
generic/019? :) I'm not sure of a good way to reproduce on demand given
the conditions required to reproduce.
> > Finally, the filesystem crashes while in a
> > state where the initial metadata block free had not committed to the
> > on-disk log. After recovery, the original metadata block is in its
> > original location as expected, but has been corrupted by the
> > aforementioned dio.
>
> Wheee!
>
> Looking at xfs_alloc_ag_vextent_exact, I guess the allocator will go
> find a freespace record, call xfs_extent_busy_trim (which could erase
> the busy extent entry), decide that it's not interested after all, and
> bail out without restoring the busy entry.
>
> Similarly, xfs_alloc_cur_check calls _busy_trim (same side effects) as
> we wander around the free space btrees looking for a good chunk of
> space... and doesn't restore the busy record if it decides to consider a
> different extent.
>
Yep. I was originally curious whether the more recent allocator rework
introduced this problem somehow, but AFAICT that just refactored the
relevant allocator code and this bug has been latent in the existing
code for quite some time. That's not hugely surprising given the rare
combination of conditions required to reproduce.
> So I guess this "speculatively remove busy records and forget to restore
> them" behavior opens the door to the write allocating blocks that aren't
> yet free and nonbusy, right? And the solution presented here is to
> avoid letting go of the busy record for the bmbt allocation, and if the
> btree split caller decides it really /must/ have that block for the bmbt
> it can force the log and try again, just like we do for a file data
> allocation?
>
Yes, pretty much. The metadata allocation that is allowed to safely
reuse busy extents ends up invalidating a set of blocks during a NEAR
mode search (i.e. bmbt allocation), but ends up only using one of those
blocks. A data allocation immediately comes along next, finds one of the
other invalidated blocks and writes to it. A crash/recovery leaves the
invalidated busy block in its original metadata location having already
been written to by the dio.
> Another solution could have been to restore the record if we decide not
> to go ahead with the allocation, but as we haven't yet committed to
> using the space, there's no sense in thrashing the busy records?
>
That was my original thought as well. Then after looking through the
code a bit I thought that something like allowing the allocator to
"track" a reusable, but still busy extent until allocation is imminent
might be a bit more straightforward of an implementation given the
layering between the allocator and busy extent tracking code. IOW, we'd
split the busy trim/available and busy invalidate logic into two steps
instead of doing it immediately in the busy trim path. That would allow
the allocator to consider the same set of reusable busy blocks but not
commit to any of them until the allocation search is complete.
However, either of those options require a bit of thought and rework
(and perhaps some value proposition justification for the complexity)
while the current trim reuse code is pretty much bolted on and broken.
Therefore, I think it's appropriate to fix the bug in one step and
follow up with a different implementation separately.
> Assuming I got all that right,
> Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx>
>
Thanks.
Brian
> --D
>
>
> > This demonstrates that it is fundamentally unsafe to modify busy
> > extent state for extents that are not guaranteed to be allocated.
> > This applies to pretty much all of the code paths that currently
> > trim busy extents for one reason or another. Therefore to address
> > this problem, drop the reuse mechanism from the busy extent trim
> > path. This code already knows how to return partial non-busy ranges
> > of the targeted free extent and higher level code tracks the busy
> > state of the allocation attempt. If a block allocation fails where
> > one or more candidate extents is busy, we force the log and retry
> > the allocation.
> >
> > Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
> > ---
> > fs/xfs/xfs_extent_busy.c | 14 --------------
> > 1 file changed, 14 deletions(-)
> >
> > diff --git a/fs/xfs/xfs_extent_busy.c b/fs/xfs/xfs_extent_busy.c
> > index 3991e59cfd18..ef17c1f6db32 100644
> > --- a/fs/xfs/xfs_extent_busy.c
> > +++ b/fs/xfs/xfs_extent_busy.c
> > @@ -344,7 +344,6 @@ xfs_extent_busy_trim(
> > ASSERT(*len > 0);
> >
> > spin_lock(&args->pag->pagb_lock);
> > -restart:
> > fbno = *bno;
> > flen = *len;
> > rbp = args->pag->pagb_tree.rb_node;
> > @@ -363,19 +362,6 @@ xfs_extent_busy_trim(
> > continue;
> > }
> >
> > - /*
> > - * If this is a metadata allocation, try to reuse the busy
> > - * extent instead of trimming the allocation.
> > - */
> > - if (!(args->datatype & XFS_ALLOC_USERDATA) &&
> > - !(busyp->flags & XFS_EXTENT_BUSY_DISCARDED)) {
> > - if (!xfs_extent_busy_update_extent(args->mp, args->pag,
> > - busyp, fbno, flen,
> > - false))
> > - goto restart;
> > - continue;
> > - }
> > -
> > if (bbno <= fbno) {
> > /* start overlap */
> >
> > --
> > 2.26.2
> >
>
