[PATCH v3 2/2] xfs: set aside allocation btree blocks from block reservation

Brian Foster
2021-03-18T16:18:10Z

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.

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. 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.

To address this problem, introduce an in-core counter to track the
sum of all allocbt blocks in use by the filesystem. Use the new
counter to set these blocks aside at reservation time and thus
ensure they cannot be reserved until truly available. Since this is
only necessary when perag reservations are active and the counter
requires a read of each AGF to fully populate, only enforce on perag
res enabled filesystems. This allows initialization of the counter
at ->pagf_init time because the perag reservation init code reads
each AGF at mount time.

Signed-off-by: Brian Foster <bfoster@redhat.com>
---
fs/xfs/libxfs/xfs_alloc.c | 12 ++++++++++++
fs/xfs/libxfs/xfs_alloc_btree.c | 2 ++
fs/xfs/xfs_mount.c | 18 +++++++++++++++++-
fs/xfs/xfs_mount.h | 6 ++++++
4 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
index 0c623d3c1036..9fa378d2724e 100644
--- a/fs/xfs/libxfs/xfs_alloc.c
+++ b/fs/xfs/libxfs/xfs_alloc.c
@@ -3036,6 +3036,7 @@ xfs_alloc_read_agf(
struct xfs_agf *agf; /* ag freelist header */
struct xfs_perag *pag; /* per allocation group data */
int error;
+ uint32_t allocbt_blks;

trace_xfs_alloc_read_agf(mp, agno);

@@ -3066,6 +3067,17 @@ xfs_alloc_read_agf(
pag->pagf_refcount_level = be32_to_cpu(agf->agf_refcount_level);
pag->pagf_init = 1;
pag->pagf_agflreset = xfs_agfl_needs_reset(mp, agf);
+
+ /*
+ * Update the global in-core allocbt block counter. Filter
+ * rmapbt blocks from the on-disk counter because those are
+ * managed by perag reservation.
+ */
+ if (pag->pagf_btreeblks > be32_to_cpu(agf->agf_rmap_blocks)) {
+ allocbt_blks = pag->pagf_btreeblks -
+ be32_to_cpu(agf->agf_rmap_blocks);
+ atomic64_add(allocbt_blks, &mp->m_allocbt_blks);
+ }
}
#ifdef DEBUG
else if (!XFS_FORCED_SHUTDOWN(mp)) {
diff --git a/fs/xfs/libxfs/xfs_alloc_btree.c b/fs/xfs/libxfs/xfs_alloc_btree.c
index 8e01231b308e..9f5a45f7baed 100644
--- a/fs/xfs/libxfs/xfs_alloc_btree.c
+++ b/fs/xfs/libxfs/xfs_alloc_btree.c
@@ -71,6 +71,7 @@ xfs_allocbt_alloc_block(
return 0;
}

+ atomic64_inc(&cur->bc_mp->m_allocbt_blks);
xfs_extent_busy_reuse(cur->bc_mp, cur->bc_ag.agno, bno, 1, false);

xfs_trans_agbtree_delta(cur->bc_tp, 1);
@@ -95,6 +96,7 @@ xfs_allocbt_free_block(
if (error)
return error;

+ atomic64_dec(&cur->bc_mp->m_allocbt_blks);
xfs_extent_busy_insert(cur->bc_tp, be32_to_cpu(agf->agf_seqno), bno, 1,
XFS_EXTENT_BUSY_SKIP_DISCARD);
xfs_trans_agbtree_delta(cur->bc_tp, -1);
diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index 1c97b155a8ee..29f97e909607 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -1176,6 +1176,7 @@ xfs_mod_fdblocks(
int64_t lcounter;
long long res_used;
s32 batch;
+ uint64_t set_aside;

if (delta > 0) {
/*
@@ -1215,8 +1216,23 @@ xfs_mod_fdblocks(
else
batch = XFS_FDBLOCKS_BATCH;

+ /*
+ * Set aside allocbt blocks because these blocks are tracked as free
+ * space but not available for allocation. Technically this means that a
+ * single reservation cannot consume all remaining free space, but the
+ * ratio of allocbt blocks to usable free blocks should be rather small.
+ * The tradeoff without this is that filesystems that maintain high
+ * perag block reservations can over reserve physical block availability
+ * and fail physical allocation, which leads to much more serious
+ * problems (i.e. transaction abort, pagecache discards, etc.) than
+ * slightly premature -ENOSPC.
+ */
+ set_aside = mp->m_alloc_set_aside;
+ if (mp->m_has_agresv)
+ set_aside += atomic64_read(&mp->m_allocbt_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 489d9b2c53d9..041f437dc117 100644
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -171,6 +171,12 @@ typedef struct xfs_mount {
* extents or anything related to the rt device.
*/
struct percpu_counter m_delalloc_blks;
+ /*
+ * Global count of allocation btree blocks in use across all AGs. Only
+ * used when perag reservation is enabled. Helps prevent block
+ * reservation from attempting to reserve allocation btree blocks.
+ */
+ atomic64_t m_allocbt_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