On Thu, Mar 17, 2022 at 02:21:01PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@xxxxxxxxxx>
>
> Currently, we use this undocumented macro to encode the minimum number
> of blocks needed to replenish a completely empty AGFL when an AG is
> nearly full. This has lead to confusion on the part of the maintainers,
> so let's document what the value actually means, and move it to
> xfs_alloc.c since it's not used outside of that module.
>
> Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
> ---
Reviewed-by: Brian Foster <bfoster@xxxxxxxxxx>
> fs/xfs/libxfs/xfs_alloc.c | 23 ++++++++++++++++++-----
> fs/xfs/libxfs/xfs_alloc.h | 1 -
> 2 files changed, 18 insertions(+), 6 deletions(-)
>
>
> diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> index 353e53b892e6..b0678e96ce61 100644
> --- a/fs/xfs/libxfs/xfs_alloc.c
> +++ b/fs/xfs/libxfs/xfs_alloc.c
> @@ -82,6 +82,19 @@ xfs_prealloc_blocks(
> }
>
> /*
> + * The number of blocks per AG that we withhold from xfs_mod_fdblocks to
> + * guarantee that we can refill the AGFL prior to allocating space in a nearly
> + * full AG. We require two blocks per free space btree because free space
> + * btrees shrink to a single block as the AG fills up, and any allocation can
> + * cause a btree split. The rmap btree uses a per-AG reservation to withhold
> + * space from xfs_mod_fdblocks, so we do not account for that here.
> + */
> +#define XFS_ALLOCBT_AGFL_RESERVE 4
> +
> +/*
> + * Compute the number of blocks that we set aside to guarantee the ability to
> + * refill the AGFL and handle a full bmap btree split.
> + *
> * 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
> @@ -93,14 +106,14 @@ xfs_prealloc_blocks(
> * 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.
> *
> - * We need to reserve 4 fsbs _per AG_ for the freelist and 4 more to handle a
> - * potential split of the file's bmap btree.
> + * For each AG, we need to reserve enough blocks to replenish a totally empty
> + * AGFL and 4 more to handle a potential split of the file's bmap btree.
> */
> unsigned int
> xfs_alloc_set_aside(
> struct xfs_mount *mp)
> {
> - return mp->m_sb.sb_agcount * (XFS_ALLOC_AGFL_RESERVE + 4);
> + return mp->m_sb.sb_agcount * (XFS_ALLOCBT_AGFL_RESERVE + 4);
> }
>
> /*
> @@ -124,12 +137,12 @@ xfs_alloc_ag_max_usable(
> unsigned int blocks;
>
> blocks = XFS_BB_TO_FSB(mp, XFS_FSS_TO_BB(mp, 4)); /* ag headers */
> - blocks += XFS_ALLOC_AGFL_RESERVE;
> + blocks += XFS_ALLOCBT_AGFL_RESERVE;
> blocks += 3; /* AGF, AGI btree root blocks */
> if (xfs_has_finobt(mp))
> blocks++; /* finobt root block */
> if (xfs_has_rmapbt(mp))
> - blocks++; /* rmap root block */
> + blocks++; /* rmap root block */
> if (xfs_has_reflink(mp))
> blocks++; /* refcount root block */
>
> diff --git a/fs/xfs/libxfs/xfs_alloc.h b/fs/xfs/libxfs/xfs_alloc.h
> index 1c14a0b1abea..d4c057b764f9 100644
> --- a/fs/xfs/libxfs/xfs_alloc.h
> +++ b/fs/xfs/libxfs/xfs_alloc.h
> @@ -88,7 +88,6 @@ typedef struct xfs_alloc_arg {
> #define XFS_ALLOC_NOBUSY (1 << 2)/* Busy extents not allowed */
>
> /* 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);
>
>
