Re: [PATCH] xfs: account extra freespace btree splits for multiple allocations

Gao Xiang
2022-11-11T23:46:50Z

Hi Dave,

On Sat, Nov 12, 2022 at 07:39:05AM +1100, Dave Chinner wrote:
> On Wed, Nov 09, 2022 at 11:48:02AM +0800, Gao Xiang wrote:
> > There is a long standing issue which could cause fs shutdown due to
> > inode extent-to-btree conversion failure right after an extent
> > allocation in the same AG, which is absolutely unexpected due to the
> > proper minleft reservation in the previous allocation. Brian once
> > addressed one of the root cause [1], however, such symptom can still
> > occur after the commit is merged as reported [2], and our cloud
> > environment is also suffering from this issue.
> >
> > From the description of the commit [1], I found that Zirong has an
> > in-house stress test reproducer for this issue, therefore I asked him
> > to reproduce again and he confirmed that such issue can still be
> > reproduced on RHEL 9 in several days.
> >
> > Thanks to him, after adding some debugging code to dump the current
> > transaction log items, I think the root cause is as below:
> >
> > 1. xfs_bmapi_allocate() with the following condition:
> > freeblks: 18304 pagf_flcount: 6
> > reservation: 18276 need (min_free): 6
> > args->minleft: 1
> > available = freeblks + agflcount - reservation - need - minleft
> > = 18304 + min(6, 6) - 18276 - 6 - 1 = 27
> > The first allocation check itself is ok, and args->maxlen = 27
> > here
> >
> > At this time, AG 3 also has the following state:
> > 1st:64 last:69 cnt:6 longest:6395
> >
> > AGFL has the following state:
> > 64:547 65:167 66:1651 67:2040807 68:783 69:604
> >
> > 2. Tried to get 27 blocks from this AG, but in order to finish such
> > allocation, it had to need a new btree block for cntbt (so take
> > another free block from agfl). It can be seen with a new AGF
> > recorded in the transaction:
> > blkno 62914177, len 1, map_size 1
> > 00000000: 58 41 47 46 00 00 00 01 00 00 00 03 00 27 ff f0 XAGF.........'..
> > 00000010: 00 00 00 09 00 00 00 07 00 00 00 00 00 00 00 02 ................
> > 00000020: 00 00 00 02 00 00 00 00 00 00 00 41 00 00 00 45 ...........A...E
> > 00000030: 00 00 00 05 00 00 47 65 00 00 18 fb 00 00 00 09 ......Ge........
> > 00000040: 75 dc c1 b5 1a 45 40 2a 80 50 72 f0 59 6e 62 66 u....E@*.Pr.Ynbf
> >
> > It can be parsed as:
> > agf 3 flfirst: 65 (0x41) fllast: 69 (0x45) cnt: 5
> > freeblks 18277
> >
> > 3. agfl 64 (agbno 547, daddr 62918552) was then written as a cntbt
> > block, which can also be seen in a log item as below:
> > type#011= 0x123c
> > flags#011= 0x8
> > blkno 62918552, len 8, map_size 1
> > 00000000: 41 42 33 43 00 00 00 fd 00 1f 23 e4 ff ff ff ff AB3C......#.....
> > 00000010: 00 00 00 00 03 c0 0f 98 00 00 00 00 00 00 00 00 ................
> > 00000020: 75 dc c1 b5 1a 45 40 2a 80 50 72 f0 59 6e 62 66 u....E@*.Pr.Ynbf
> > ...
> >
> > 4. Finally, the following inode extent to btree allocation fails
> > as below:
> > kernel: ------------[ cut here ]------------
> > WARNING: CPU: 15 PID: 49290 at fs/xfs/libxfs/xfs_bmap.c:717 xfs_bmap_extents_to_btree+0xc51/0x1050 [xfs]
> > ...
> > XFS (sda2): agno 3 agflcount 5 freeblks 18277 reservation 18276 6
> >
> > since freeblks = 18304 - 27 = 18277, but with another agfl
> > block allocated (pagf_flcount from 6 to 5), the inequality will
> > not be satisfied:
> >
> > available = freeblks + agflcount - reservation - need - minleft
> > = 18277 + min(5, 6) - 18276 - 6 - 0 = 0 < 1
> >
> > Full current transaction log item dump can be fetched from [3].
> >
> > As a short-term solution, the following allocations (e.g. allocation
> > for inode extent-to-btree conversion) can be recorded in order to count
> > more blocks to reserve for safely freespace btree splits so that it
> > will shorten available and args->maxlen to
> > available = freeblks + agflcount - reservation - need - minleft
> > = 18304 + min(6, 6) - 18276 - 6*2 - 1 = 21
> > args->maxlen = 21
> > in the first allocation, and the following conversion should then
> > succeed. At least, it's easy to be backported and do hotfix.
> >
> > In the long term, args->total and args->minleft have be revisited
> > although it could cause more refactoring.
> >
> > [1] commit 1ca89fbc48e1 ("xfs: don't account extra agfl blocks as available")
> > https://lore.kernel.org/r/20190327145000.10756-1-bfoster@redhat.com
> > [2] https://lore.kernel.org/r/20220105071052.GD20464@templeofstupid.com
> > [3] https://lore.kernel.org/linux-xfs/Y2RevDyoeJZSpiat@B-P7TQMD6M-0146.local/2-dmesg.log.xz
> > Reported-by: Zirong Lang <zlang@redhat.com>
> > Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
> > ---
> > Previous discussion is at:
> > https://lore.kernel.org/linux-xfs/202211040048.FeUQMLE6-lkp@intel.com/T/#mfcfac181079ddaa5a22eecb74db56534fc4ff918
> >
> > fs/xfs/libxfs/xfs_alloc.c | 9 +++++++--
> > fs/xfs/libxfs/xfs_alloc.h | 1 +
> > fs/xfs/libxfs/xfs_bmap.c | 2 ++
> > 3 files changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> > index 6261599bb389..684c67310175 100644
> > --- a/fs/xfs/libxfs/xfs_alloc.c
> > +++ b/fs/xfs/libxfs/xfs_alloc.c
> > @@ -2630,7 +2630,12 @@ xfs_alloc_fix_freelist(
> > goto out_agbp_relse;
> > }
> >
> > - need = xfs_alloc_min_freelist(mp, pag);
> > + /*
> > + * Also need to fulfill freespace btree splits by reservaing more
> > + * blocks to perform multiple allocations from a single AG and
> > + * transaction if needed.
> > + */
> > + need = xfs_alloc_min_freelist(mp, pag) * (1 + args->postallocs);
> > if (!xfs_alloc_space_available(args, need, flags |
> > XFS_ALLOC_FLAG_CHECK))
> > goto out_agbp_relse;
> > @@ -2654,7 +2659,7 @@ xfs_alloc_fix_freelist(
> > xfs_agfl_reset(tp, agbp, pag);
> >
> > /* If there isn't enough total space or single-extent, reject it. */
> > - need = xfs_alloc_min_freelist(mp, pag);
> > + need = xfs_alloc_min_freelist(mp, pag) * (1 + args->postallocs);
> > if (!xfs_alloc_space_available(args, need, flags))
> > goto out_agbp_relse;
> >
> > diff --git a/fs/xfs/libxfs/xfs_alloc.h b/fs/xfs/libxfs/xfs_alloc.h
> > index 2c3f762dfb58..be7f15d6a40d 100644
> > --- a/fs/xfs/libxfs/xfs_alloc.h
> > +++ b/fs/xfs/libxfs/xfs_alloc.h
> > @@ -73,6 +73,7 @@ typedef struct xfs_alloc_arg {
> > int datatype; /* mask defining data type treatment */
> > char wasdel; /* set if allocation was prev delayed */
> > char wasfromfl; /* set if allocation is from freelist */
> > + bool postallocs; /* number of post-allocations */
> > struct xfs_owner_info oinfo; /* owner of blocks being allocated */
> > enum xfs_ag_resv_type resv; /* block reservation to use */
> > #ifdef DEBUG
> > diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> > index 49d0d4ea63fc..ed92c6a314b6 100644
> > --- a/fs/xfs/libxfs/xfs_bmap.c
> > +++ b/fs/xfs/libxfs/xfs_bmap.c
> > @@ -3497,6 +3497,7 @@ xfs_bmap_exact_minlen_extent_alloc(
> > args.alignment = 1;
> > args.minalignslop = 0;
> >
> > + args.postallocs = 1;
> > args.minleft = ap->minleft;
> > args.wasdel = ap->wasdel;
> > args.resv = XFS_AG_RESV_NONE;
> > @@ -3658,6 +3659,7 @@ xfs_bmap_btalloc(
> > args.alignment = 1;
> > args.minalignslop = 0;
> > }
> > + args.postallocs = 1;
> > args.minleft = ap->minleft;
> > args.wasdel = ap->wasdel;
> > args.resv = XFS_AG_RESV_NONE;
>
> That's not going to work. What happens when we do a full bno
> split? Or we do both a bno and a cnt split in the same allocation?

I'm not sure if I got your point or not. I think it reserves another
full splits in the first allocation by doing:

need = xfs_alloc_min_freelist(mp, pag) * (1 + args->postallocs);

as I wrote above.

>
> Regardless, I don't see anything wrong with the allocation setup -
> it's telling the allocation code exactly what it needs for
> subsequent BMBT block allocations to succeed (i.e. args->minleft).

In the long term, I think the main point is that args->minleft doesn't
have the exact meaning. I don't know how many blocks should be counted
by args->minleft or other ways.

> The problem here is that the internal allocation code is failing to
> handle the corner case where space is just about gone correctly.
>
> As I pointed out previously - we have a huge amount of reserve space
> available in the AG here, so why not use some of the reserve space
> to get out of this temporary deficit corner case? We can argue that
> it's not really a deficit, either, because moving free blocks to the
> free list still accounts them as unused and free, so could still
> make up part of the unused reservation....
>
> i.e. is the problem here simply that we don't allow AGFL blocks to
> be considered part of the reserved free space?

I don't know how to simply reuse per-AG reservation blocks for this,
and I think it could another corner case for this if it designs without
careful thinking. For the moment, I'd like to have a quick fix for
our all old kernels, and if it's possible, we'd like to upstream it
first.

Thanks,
Gao Xiang

>
> -Dave.
> --
> Dave Chinner
> david@fromorbit.com