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

[Gao Xiang <hsiangkao@linux.alibaba.com>]

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;
-- 
2.24.4