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 a counter to track the sum of the allocbt block counters already tracked in the AGF. Use the new 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 <bfoster@xxxxxxxxxx> --- v2: - Use an atomic counter instead of a percpu counter. v1: https://lore.kernel.org/linux-xfs/20210217132339.651020-1-bfoster@xxxxxxxxxx/ 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); 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
