Re: [PATCH 031/119] xfs: rmap btree requires more reserved free space

Brian Foster
2016-07-08T13:21:59Z

On Thu, Jun 16, 2016 at 06:21:11PM -0700, Darrick J. Wong wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> The rmap btree is allocated from the AGFL, which means we have to
> ensure ENOSPC is reported to userspace before we run out of free
> space in each AG. The last allocation in an AG can cause a full
> height rmap btree split, and that means we have to reserve at least
> this many blocks *in each AG* to be placed on the AGFL at ENOSPC.
> Update the various space calculation functiosn to handle this.

functions

>
> Also, because the macros are now executing conditional code and are called quite
> frequently, convert them to functions that initialise varaibles in the struct
> xfs_mount, use the new variables everywhere and document the calculations
> better.
>
> v2: If rmapbt is disabled, it is incorrect to require 1 extra AGFL block
> for the rmapbt (due to the + 1); the entire clause needs to be gated
> on the feature flag.
>
> v3: Use m_rmap_maxlevels to determine min_free.
>
> [darrick.wong@oracle.com: don't reserve blocks if !rmap]
> [dchinner@redhat.com: update m_ag_max_usable after growfs]
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> Reviewed-by: Dave Chinner <dchinner@redhat.com>
> Signed-off-by: Dave Chinner <david@fromorbit.com>
> ---
> fs/xfs/libxfs/xfs_alloc.c | 71 +++++++++++++++++++++++++++++++++++++++++++++
> fs/xfs/libxfs/xfs_alloc.h | 41 +++-----------------------
> fs/xfs/libxfs/xfs_bmap.c | 2 +
> fs/xfs/libxfs/xfs_sb.c | 2 +
> fs/xfs/xfs_discard.c | 2 +
> fs/xfs/xfs_fsops.c | 5 ++-
> fs/xfs/xfs_log_recover.c | 1 +
> fs/xfs/xfs_mount.c | 2 +
> fs/xfs/xfs_mount.h | 2 +
> fs/xfs/xfs_super.c | 2 +
> 10 files changed, 88 insertions(+), 42 deletions(-)
>
>
> diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> index 570ca17..4c8ffd4 100644
> --- a/fs/xfs/libxfs/xfs_alloc.c
> +++ b/fs/xfs/libxfs/xfs_alloc.c
> @@ -63,6 +63,72 @@ xfs_prealloc_blocks(
> }
>
> /*
> + * In order to avoid ENOSPC-related deadlock caused by out-of-order locking of
> + * AGF buffer (PV 947395), we place constraints on the relationship among
> + * actual allocations for data blocks, freelist blocks, and potential file data
> + * bmap btree blocks. However, these restrictions may result in no actual space
> + * allocated for a delayed extent, for example, a data block in a certain AG is
> + * allocated but there is no additional block for the additional bmap btree
> + * block due to a split of the bmap btree of the file. The result of this may
> + * lead to an infinite loop when the file gets flushed to disk and all delayed
> + * extents need to be actually allocated. To get around this, we explicitly set
> + * aside a few blocks which will not be reserved in delayed allocation.
> + *
> + * The minimum number of needed freelist blocks is 4 fsbs _per AG_ when we are
> + * not using rmap btrees a potential split of file's bmap btree requires 1 fsb,
> + * so we set the number of set-aside blocks to 4 + 4*agcount when not using
> + * rmap btrees.
> + *

That's a bit wordy.

> + * When rmap btrees are active, we have to consider that using the last block
> + * in the AG can cause a full height rmap btree split and we need enough blocks
> + * on the AGFL to be able to handle this. That means we have, in addition to
> + * the above consideration, another (2 * mp->m_rmap_levels) - 1 blocks required
> + * to be available to the free list.

I'm probably missing something, but why does a full tree split require 2
blocks per-level (minus 1)? Wouldn't that involve an allocated block per
level (and possibly a new root block)?

Otherwise, the rest looks good to me.

Brian

> + */
> +unsigned int
> +xfs_alloc_set_aside(
> + struct xfs_mount *mp)
> +{
> + unsigned int blocks;
> +
> + blocks = 4 + (mp->m_sb.sb_agcount * XFS_ALLOC_AGFL_RESERVE);
> + if (!xfs_sb_version_hasrmapbt(&mp->m_sb))
> + return blocks;
> + return blocks + (mp->m_sb.sb_agcount * (2 * mp->m_rmap_maxlevels) - 1);
> +}
> +
> +/*
> + * When deciding how much space to allocate out of an AG, we limit the
> + * allocation maximum size to the size the AG. However, we cannot use all the
> + * blocks in the AG - some are permanently used by metadata. These
> + * blocks are generally:
> + * - the AG superblock, AGF, AGI and AGFL
> + * - the AGF (bno and cnt) and AGI btree root blocks, and optionally
> + * the AGI free inode and rmap btree root blocks.
> + * - blocks on the AGFL according to xfs_alloc_set_aside() limits
> + *
> + * The AG headers are sector sized, so the amount of space they take up is
> + * dependent on filesystem geometry. The others are all single blocks.
> + */
> +unsigned int
> +xfs_alloc_ag_max_usable(struct xfs_mount *mp)
> +{
> + unsigned int blocks;
> +
> + blocks = XFS_BB_TO_FSB(mp, XFS_FSS_TO_BB(mp, 4)); /* ag headers */
> + blocks += XFS_ALLOC_AGFL_RESERVE;
> + blocks += 3; /* AGF, AGI btree root blocks */
> + if (xfs_sb_version_hasfinobt(&mp->m_sb))
> + blocks++; /* finobt root block */
> + if (xfs_sb_version_hasrmapbt(&mp->m_sb)) {
> + /* rmap root block + full tree split on full AG */
> + blocks += 1 + (2 * mp->m_ag_maxlevels) - 1;
> + }
> +
> + return mp->m_sb.sb_agblocks - blocks;
> +}
> +
> +/*
> * Lookup the record equal to [bno, len] in the btree given by cur.
> */
> STATIC int /* error */
> @@ -1904,6 +1970,11 @@ xfs_alloc_min_freelist(
> /* space needed by-size freespace btree */
> min_free += min_t(unsigned int, pag->pagf_levels[XFS_BTNUM_CNTi] + 1,
> mp->m_ag_maxlevels);
> + /* space needed reverse mapping used space btree */
> + if (xfs_sb_version_hasrmapbt(&mp->m_sb))
> + min_free += min_t(unsigned int,
> + pag->pagf_levels[XFS_BTNUM_RMAPi] + 1,
> + mp->m_rmap_maxlevels);
>
> return min_free;
> }
> diff --git a/fs/xfs/libxfs/xfs_alloc.h b/fs/xfs/libxfs/xfs_alloc.h
> index 0721a48..7b6c66b 100644
> --- a/fs/xfs/libxfs/xfs_alloc.h
> +++ b/fs/xfs/libxfs/xfs_alloc.h
> @@ -56,42 +56,6 @@ typedef unsigned int xfs_alloctype_t;
> #define XFS_ALLOC_FLAG_FREEING 0x00000002 /* indicate caller is freeing extents*/
>
> /*
> - * In order to avoid ENOSPC-related deadlock caused by
> - * out-of-order locking of AGF buffer (PV 947395), we place
> - * constraints on the relationship among actual allocations for
> - * data blocks, freelist blocks, and potential file data bmap
> - * btree blocks. However, these restrictions may result in no
> - * actual space allocated for a delayed extent, for example, a data
> - * block in a certain AG is allocated but there is no additional
> - * block for the additional bmap btree block due to a split of the
> - * bmap btree of the file. The result of this may lead to an
> - * infinite loop in xfssyncd when the file gets flushed to disk and
> - * all delayed extents need to be actually allocated. To get around
> - * this, we explicitly set aside a few blocks which will not be
> - * reserved in delayed allocation. Considering the minimum number of
> - * needed freelist blocks is 4 fsbs _per AG_, a potential split of file's bmap
> - * btree requires 1 fsb, so we set the number of set-aside blocks
> - * to 4 + 4*agcount.
> - */
> -#define XFS_ALLOC_SET_ASIDE(mp) (4 + ((mp)->m_sb.sb_agcount * 4))
> -
> -/*
> - * When deciding how much space to allocate out of an AG, we limit the
> - * allocation maximum size to the size the AG. However, we cannot use all the
> - * blocks in the AG - some are permanently used by metadata. These
> - * blocks are generally:
> - * - the AG superblock, AGF, AGI and AGFL
> - * - the AGF (bno and cnt) and AGI btree root blocks
> - * - 4 blocks on the AGFL according to XFS_ALLOC_SET_ASIDE() limits
> - *
> - * The AG headers are sector sized, so the amount of space they take up is
> - * dependent on filesystem geometry. The others are all single blocks.
> - */
> -#define XFS_ALLOC_AG_MAX_USABLE(mp) \
> - ((mp)->m_sb.sb_agblocks - XFS_BB_TO_FSB(mp, XFS_FSS_TO_BB(mp, 4)) - 7)
> -
> -
> -/*
> * Argument structure for xfs_alloc routines.
> * This is turned into a structure to avoid having 20 arguments passed
> * down several levels of the stack.
> @@ -133,6 +97,11 @@ typedef struct xfs_alloc_arg {
> #define XFS_ALLOC_INITIAL_USER_DATA (1 << 1)/* special case start of file */
> #define XFS_ALLOC_USERDATA_ZERO (1 << 2)/* zero extent on allocation */
>
> +/* freespace limit calculations */
> +#define XFS_ALLOC_AGFL_RESERVE 4
> +unsigned int xfs_alloc_set_aside(struct xfs_mount *mp);
> +unsigned int xfs_alloc_ag_max_usable(struct xfs_mount *mp);
> +
> xfs_extlen_t xfs_alloc_longest_free_extent(struct xfs_mount *mp,
> struct xfs_perag *pag, xfs_extlen_t need);
> unsigned int xfs_alloc_min_freelist(struct xfs_mount *mp,
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 2c28f2a..61c0231 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3672,7 +3672,7 @@ xfs_bmap_btalloc(
> args.fsbno = ap->blkno;
>
> /* Trim the allocation back to the maximum an AG can fit. */
> - args.maxlen = MIN(ap->length, XFS_ALLOC_AG_MAX_USABLE(mp));
> + args.maxlen = MIN(ap->length, mp->m_ag_max_usable);
> args.firstblock = *ap->firstblock;
> blen = 0;
> if (nullfb) {
> diff --git a/fs/xfs/libxfs/xfs_sb.c b/fs/xfs/libxfs/xfs_sb.c
> index f86226b..59c9f59 100644
> --- a/fs/xfs/libxfs/xfs_sb.c
> +++ b/fs/xfs/libxfs/xfs_sb.c
> @@ -749,6 +749,8 @@ xfs_sb_mount_common(
> mp->m_ialloc_min_blks = sbp->sb_spino_align;
> else
> mp->m_ialloc_min_blks = mp->m_ialloc_blks;
> + mp->m_alloc_set_aside = xfs_alloc_set_aside(mp);
> + mp->m_ag_max_usable = xfs_alloc_ag_max_usable(mp);
> }
>
> /*
> diff --git a/fs/xfs/xfs_discard.c b/fs/xfs/xfs_discard.c
> index 272c3f8..4ff499a 100644
> --- a/fs/xfs/xfs_discard.c
> +++ b/fs/xfs/xfs_discard.c
> @@ -179,7 +179,7 @@ xfs_ioc_trim(
> * matter as trimming blocks is an advisory interface.
> */
> if (range.start >= XFS_FSB_TO_B(mp, mp->m_sb.sb_dblocks) ||
> - range.minlen > XFS_FSB_TO_B(mp, XFS_ALLOC_AG_MAX_USABLE(mp)) ||
> + range.minlen > XFS_FSB_TO_B(mp, mp->m_ag_max_usable) ||
> range.len < mp->m_sb.sb_blocksize)
> return -EINVAL;
>
> diff --git a/fs/xfs/xfs_fsops.c b/fs/xfs/xfs_fsops.c
> index 8a85e49..3772f6c 100644
> --- a/fs/xfs/xfs_fsops.c
> +++ b/fs/xfs/xfs_fsops.c
> @@ -583,6 +583,7 @@ xfs_growfs_data_private(
> } else
> mp->m_maxicount = 0;
> xfs_set_low_space_thresholds(mp);
> + mp->m_alloc_set_aside = xfs_alloc_set_aside(mp);
>
> /* update secondary superblocks. */
> for (agno = 1; agno < nagcount; agno++) {
> @@ -720,7 +721,7 @@ xfs_fs_counts(
> cnt->allocino = percpu_counter_read_positive(&mp->m_icount);
> cnt->freeino = percpu_counter_read_positive(&mp->m_ifree);
> cnt->freedata = percpu_counter_read_positive(&mp->m_fdblocks) -
> - XFS_ALLOC_SET_ASIDE(mp);
> + mp->m_alloc_set_aside;
>
> spin_lock(&mp->m_sb_lock);
> cnt->freertx = mp->m_sb.sb_frextents;
> @@ -793,7 +794,7 @@ retry:
> __int64_t free;
>
> free = percpu_counter_sum(&mp->m_fdblocks) -
> - XFS_ALLOC_SET_ASIDE(mp);
> + mp->m_alloc_set_aside;
> if (!free)
> goto out; /* ENOSPC and fdblks_delta = 0 */
>
> diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
> index 0c41bd2..b33187b 100644
> --- a/fs/xfs/xfs_log_recover.c
> +++ b/fs/xfs/xfs_log_recover.c
> @@ -5027,6 +5027,7 @@ xlog_do_recover(
> xfs_warn(mp, "Failed post-recovery per-ag init: %d", error);
> return error;
> }
> + mp->m_alloc_set_aside = xfs_alloc_set_aside(mp);
>
> xlog_recover_check_summary(log);
>
> diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> index 8af1c88..879f3ef 100644
> --- a/fs/xfs/xfs_mount.c
> +++ b/fs/xfs/xfs_mount.c
> @@ -1219,7 +1219,7 @@ xfs_mod_fdblocks(
> batch = XFS_FDBLOCKS_BATCH;
>
> __percpu_counter_add(&mp->m_fdblocks, delta, batch);
> - if (__percpu_counter_compare(&mp->m_fdblocks, XFS_ALLOC_SET_ASIDE(mp),
> + if (__percpu_counter_compare(&mp->m_fdblocks, mp->m_alloc_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 0ed0f29..b36676c 100644
> --- a/fs/xfs/xfs_mount.h
> +++ b/fs/xfs/xfs_mount.h
> @@ -123,6 +123,8 @@ typedef struct xfs_mount {
> uint m_in_maxlevels; /* max inobt btree levels. */
> uint m_rmap_maxlevels; /* max rmap btree levels */
> xfs_extlen_t m_ag_prealloc_blocks; /* reserved ag blocks */
> + uint m_alloc_set_aside; /* space we can't use */
> + uint m_ag_max_usable; /* max space per AG */
> struct radix_tree_root m_perag_tree; /* per-ag accounting info */
> spinlock_t m_perag_lock; /* lock for m_perag_tree */
> struct mutex m_growlock; /* growfs mutex */
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index bf63f6d..1575849 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -1076,7 +1076,7 @@ xfs_fs_statfs(
> statp->f_blocks = sbp->sb_dblocks - lsize;
> spin_unlock(&mp->m_sb_lock);
>
> - statp->f_bfree = fdblocks - XFS_ALLOC_SET_ASIDE(mp);
> + statp->f_bfree = fdblocks - mp->m_alloc_set_aside;
> statp->f_bavail = statp->f_bfree;
>
> fakeinos = statp->f_bfree << sbp->sb_inopblog;
>
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