On Tue, Feb 23, 2021 at 07:10:17AM -0500, Brian Foster wrote:

On Mon, Feb 22, 2021 at 10:34:08AM -0800, Darrick J. Wong wrote:

quoted

On Mon, Feb 22, 2021 at 10:21:08AM -0500, Brian Foster wrote:

quoted

The blocks used for allocation btrees (bnobt and countbt) are
technically considered free space. This is because as free space is
used, allocbt blocks are removed and naturally become available for
traditional allocation. However, this means that a significant
portion of free space may consist of in-use btree blocks if free
space is severely fragmented.

On large filesystems with large perag reservations, this can lead to
a rare but nasty condition where a significant amount of physical
free space is available, but the majority of actual usable blocks
consist of in-use allocbt blocks. We have a record of a (~12TB, 32
AG) filesystem with multiple AGs in a state with ~2.5GB or so free
blocks tracked across ~300 total allocbt blocks, but effectively at
100% full because the the free space is entirely consumed by
refcountbt perag reservation.

Hmm.  I'm still a bit confused by what's going on here.  Free space is
heavily fragmented and the bnobt/cntbt are very large, but what actually
triggers the (bug? customer escalation?) report?  Did the fs go offline?
Did fsync() suddenly start emitting errors?

(Sorry, I'm going to go wandering stream-of-consciousness now...)

quoted

Do the finobt/rmapbt per-ag reservations present similar challenges?

The refcountbt per-ag reservation code computes the difference between
the max btree size and the actual btree size, and subtracts that from
fdblocks.  The rmapbt reservation subtracts the entire reservation from
fdblocks because the rmapbt also lives in "free" space.  The finobt
reservation follows the rmapbt formula, so none of the three btrees
should be directly causing whatever's going on.

So I think the answer to my question is a tentative "no, because those
btrees already subtracted themselves from fdblocks" while I continue to
try to work out what's going on here...

[narrator: No, he's wrong, let's watch him figure out why.]

I think any large perag reservation can contribute to an elevated
btreeblks count if free space is fragmented enough. That said, it's not
clear to me that an elevated btreeblks count is enough to cause this
problem. So far I've not been able to reproduce, but have only been able
to play around with a filesystem that was already put into this state by
a workload that seemingly made heavy/fragmented use of COW and
persistently beat against -ENOSPC over time. For that reason I suspect
COW remaps dipping into the reserve block pool may also be a
contributing factor, but it's hard to say for sure. Given the
circumstances, I opted to keep this isolated to reflink at least for the
time being.

We could certainly enable the enforcement for any filesystem that
requires perag reservation if we preferred to do that out of caution.
The accounting logic is trivial and already unconditional. The one
caveat to be aware of is that the enforcement is not effective until
m_btree_blks is fully populated, which doesn't occur until each AGF has
been read for the first time. This is not a problem for reflink and
rmapbt because both of those features read the AGF at mount time to
calculate the perag res.

The finobt perag res calculation only reads the AGIs atm, so we'd either
want to also read the AGFs there as well (which is perhaps not such a
big deal if we're already reading the AGIs), or perhaps just leave
things as is, enforce by default, and hope (and document) that the
counter is initialized in the cases where it's most important. I don't
have a strong opinion on any of those particular options. Thoughts?

Brian

quoted

--D

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Such a large perag reservation is by design on large filesystems.
The problem is that because the free space is so fragmented, this AG
contributes the 300 or so allocbt blocks to the global counters as
free space.

How big is the refcount btree?

As an example scenario (with exaggerated/made up numbers), pretend our
filesystem has a single AG with 100 blocks.  The refcount btree consumes
5 blocks, and the refcountbt reservation is 11 blocks.  Say there are 15
free blocks in the AG -- 6 individual blocks pointed to by the free
space btrees, 4 blocks used by the free space btrees, and 5 blocks in
the AGFL.  (Pretend that we actually need 2 blocks per tree to store 6
records.)

The 5 blocks actually in use by the refcount btree were subtracted from
fdblocks ages ago when we expanded the btree shape.

The (11-5)==6 blocks remaining in the per-ag reservation were subtracted
from fdblocks at mount time, so this AG contributes (15-6)==9 blocks to
the incore fdblocks counter.

Unfortunately, those 9 "free" blocks aren't all free -- we're using four
freesp btree blocks to hold in reserve the 6 blocks for the refcount
btree.  Until we rearrange the free space btree or grow the refcount
btree to consume the rest of its reservation, we're never going to get
those four blocks back...

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If this pattern repeats across enough AGs, the
filesystem lands in a state where global block reservation can
outrun physical block availability. For example, a streaming
buffered write on the affected filesystem continues to allow delayed
allocation beyond the point where writeback starts to fail due to
physical block allocation failures. The expected behavior is for the
delalloc block reservation to fail gracefully with -ENOSPC before
physical block allocation failure is a possibility.

...and then (continuing the scenario) a 9-block delalloc write comes
along and sees 9 free blocks, so it creates a reservation for 9 blocks.
When writeback kicks off it'll try to allocate those 9 blocks, but only
receives the 5 blocks that are on the AGFL.  Next it... runs around in
circles wondering where the other four blocks went?  Or maybe it just
ENOSPCs, but that's bad because delalloc was supposed to prevent ENOSPC
on writeback.  Either way it's bad news.

quoted

quoted

To address this problem, introduce a counter to track the sum of the
allocbt block counters already tracked in the AGF. Use the new

Ahh, ok.  I think I grok what this patch does now.  XFS (prior to per-ag
space reservations) counts all the space used by the free space btrees
as free space, which means that a delalloc reservation expects to be
able to consume both the free space in the tree as well as the tree
itself.

Now we use the free space btrees to hold space for future metadata
expansions, so the expectation above no longer holds.  Hence subtracting
agf_btreeblks from fdblocks at mount time.

With that in mind, the finobt and rmapbt reservations are no different
from the refcountbt reservation -- the unused-but-reserved space for all
three reservations are held in the free space btrees, so they all need
this now.  The answer to my question is "yes, they need it too".

With that in mind, should xfs_ag_resv_init be responsible for
subtracting agf_btreeblocks from fdblocks if there are any reservations?

quoted

quoted

counter to set these blocks aside at reservation time and thus
ensure they cannot be allocated until truly available. Since this is
only necessary when large reflink perag reservations are in place
and the counter requires a read of each AGF to fully populate, only
enforce on reflink enabled filesystems. This allows initialization
of the counter at ->pagf_init time because the refcountbt perag
reservation init code reads each AGF at mount time.

Signed-off-by: Brian Foster <redacted>
---

v2:
- Use an atomic counter instead of a percpu counter.
v1: https://lore.kernel.org/linux-xfs/20210217132339.651020-1-bfoster@redhat.com/ (local)

 fs/xfs/libxfs/xfs_alloc.c |  3 +++
 fs/xfs/xfs_mount.c        | 15 ++++++++++++++-
 fs/xfs/xfs_mount.h        |  6 ++++++
 3 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
index 0c623d3c1036..fb3d36cad173 100644
--- a/fs/xfs/libxfs/xfs_alloc.c
+++ b/fs/xfs/libxfs/xfs_alloc.c

@@ -2746,6 +2746,7 @@ xfs_alloc_get_freelist(
 	if (btreeblk) {
 		be32_add_cpu(&agf->agf_btreeblks, 1);
 		pag->pagf_btreeblks++;
+		atomic64_inc(&mp->m_btree_blks);
 		logflags |= XFS_AGF_BTREEBLKS;
 	}
 

@@ -2853,6 +2854,7 @@ xfs_alloc_put_freelist(
 	if (btreeblk) {
 		be32_add_cpu(&agf->agf_btreeblks, -1);
 		pag->pagf_btreeblks--;
+		atomic64_dec(&mp->m_btree_blks);
 		logflags |= XFS_AGF_BTREEBLKS;
 	}
 

@@ -3055,6 +3057,7 @@ xfs_alloc_read_agf(
 	if (!pag->pagf_init) {
 		pag->pagf_freeblks = be32_to_cpu(agf->agf_freeblks);
 		pag->pagf_btreeblks = be32_to_cpu(agf->agf_btreeblks);
+		atomic64_add(pag->pagf_btreeblks, &mp->m_btree_blks);

Come to think of it, this isn't quite correct -- rmapbt blocks are
counted in agf_btreeblks and separately in agf_rmap_blocks.  At mount
time, the rmapbt per-ag reservation subtracts the entire rmapbt
reservation from fdblocks (not just agf_rmap_blocks), so this
effectively double-charges fdblocks for the rmap btree.

--D

quoted

quoted

 		pag->pagf_flcount = be32_to_cpu(agf->agf_flcount);
 		pag->pagf_longest = be32_to_cpu(agf->agf_longest);
 		pag->pagf_levels[XFS_BTNUM_BNOi] =

diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index 52370d0a3f43..16482e02da01 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c

@@ -1178,6 +1178,7 @@ xfs_mod_fdblocks(
 	int64_t			lcounter;
 	long long		res_used;
 	s32			batch;
+	uint64_t		set_aside = mp->m_alloc_set_aside;
 
 	if (delta > 0) {
 		/*

@@ -1217,8 +1218,20 @@ xfs_mod_fdblocks(
 	else
 		batch = XFS_FDBLOCKS_BATCH;
 
+	/*
+	 * Set aside allocbt blocks on reflink filesystems because COW remaps
+	 * can dip into the reserved block pool. This is problematic if free
+	 * space is fragmented and m_fdblocks tracks a significant number of
+	 * allocbt blocks. Note this also ensures the counter is accurate before
+	 * the filesystem is active because perag reservation reads all AGFs at
+	 * mount time. The only call prior to that is to populate the reserve
+	 * pool itself.
+	 */
+	if (xfs_sb_version_hasreflink(&mp->m_sb))
+		set_aside += atomic64_read(&mp->m_btree_blks);
+
 	percpu_counter_add_batch(&mp->m_fdblocks, delta, batch);
-	if (__percpu_counter_compare(&mp->m_fdblocks, mp->m_alloc_set_aside,
+	if (__percpu_counter_compare(&mp->m_fdblocks, set_aside,
 				     XFS_FDBLOCKS_BATCH) >= 0) {
 		/* we had space! */
 		return 0;

diff --git a/fs/xfs/xfs_mount.h b/fs/xfs/xfs_mount.h
index 659ad95fe3e0..70e1dd6b538a 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h

@@ -170,6 +170,12 @@ typedef struct xfs_mount {
 	 * extents or anything related to the rt device.
 	 */
 	struct percpu_counter	m_delalloc_blks;
+	/*
+	 * Optional count of btree blocks in use across all AGs. Only used when
+	 * reflink is enabled. Helps prevent block reservation from attempting
+	 * to reserve allocation btree blocks.
+	 */
+	atomic64_t		m_btree_blks;
 
 	struct radix_tree_root	m_perag_tree;	/* per-ag accounting info */
 	spinlock_t		m_perag_lock;	/* lock for m_perag_tree */

-- 
2.26.2