Re: [External] : [PATCH 04/38] xfs: fix block reservation for zoned RT extent remapping

[Mark Tinguely <mark.tinguely@oracle.com>]

On 8/18/26 7:12 PM, Dave Chinner wrote:
> xfs_zoned_end_io() uses XFS_EXTENTADD_SPACE_RES() for its block
> reservation, which only covers bmbt splits. However, the remap
> operation in xfs_zoned_map_extent() generates deferred rmap and
> refcount btree updates that also need blocks for btree splits
> when they are processed.
> 
> The code used XFS_TRANS_RES_FDBLKS as a workaround to recycle
> freed data blocks back into the transaction's block reservation.
> This is fragile — it depends on freed blocks being large enough
> to cover the metadata btree needs, and conflates data block
> recycling with metadata reservation.
> 
> Fix this by introducing XFS_RTEXTENTADD_SPACE_RES() which computes
> the correct reservation for any RT data extent modification: the
> bmbt split cost plus the rt rmap btree split cost plus the rt
> refcount btree cost. This covers all the deferred operations that
> are generated when an RT extent is mapped or unmapped.
> 
> Drop XFS_TRANS_RES_FDBLKS from xfs_zoned_end_io() since the
> reservation now correctly covers all btree costs.
> 
> Note: XFS_RTEXTENTADD_SPACE_RES() should eventually be folded
> into XFS_EXTENTADD_SPACE_RES() so that all callers operating on
> RT inodes automatically get the correct reservation. There are
> approximately 11 sites that use XFS_EXTENTADD_SPACE_RES either
> directly or via XFS_DIOSTRAT_SPACE_RES that have the same
> under-reservation issue for RT inodes.
> 
> Fixes: 4e4d52075577 ("xfs: add the zoned space allocator")
> Assisted-by: LLM
> Signed-off-by: Dave Chinner <dgc@kernel.org>
> ---


Cc: stable@vger.kernel.org # v6.14+ ?

Looks good

Reviewed-by: Mark Tinguely <mark.tinguely@oracle.com>


>   fs/xfs/libxfs/xfs_trans_space.h | 12 ++++++++++++
>   fs/xfs/xfs_zone_alloc.c         |  9 ++++++---
>   2 files changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_trans_space.h b/fs/xfs/libxfs/xfs_trans_space.h
> index d89b570aafcc..4cfc29ac764f 100644
> --- a/fs/xfs/libxfs/xfs_trans_space.h
> +++ b/fs/xfs/libxfs/xfs_trans_space.h
> @@ -55,6 +55,18 @@
>   	  XFS_MAX_CONTIG_EXTENTS_PER_BLOCK(mp)) * \
>   	  XFS_EXTENTADD_SPACE_RES(mp,w))
>   
> +/*
> + * Blocks needed to add or remove a realtime data extent: the bmbt split plus
> + * rt rmap btree and rt refcount btree updates deferred from the bmbt operation.
> + *
> + * TODO: this should be folded into XFS_EXTENTADD_SPACE_RES() so that all
> + * callers that operate on RT inodes automatically get the correct reservation.
> + */
> +#define XFS_RTEXTENTADD_SPACE_RES(mp)	\
> +	(XFS_EXTENTADD_SPACE_RES(mp, XFS_DATA_FORK) + \
> +	 (xfs_has_rmapbt(mp) ? XFS_RTRMAPADD_SPACE_RES(mp) : 0) + \
> +	 (xfs_has_reflink(mp) ? 2 * (mp)->m_rtrefc_maxlevels - 1 : 0))
> +
>   /* Blocks we might need to add "b" mappings & rmappings to a file. */
>   #define XFS_SWAP_RMAP_SPACE_RES(mp,b,w)\
>   	(XFS_NEXTENTADD_SPACE_RES((mp), (b), (w)) + \
> diff --git a/fs/xfs/xfs_zone_alloc.c b/fs/xfs/xfs_zone_alloc.c
> index 7d13fa7ab30a..61b483d24c88 100644
> --- a/fs/xfs/xfs_zone_alloc.c
> +++ b/fs/xfs/xfs_zone_alloc.c
> @@ -330,8 +330,11 @@ xfs_zoned_end_io(
>   		.br_startblock	= xfs_daddr_to_rtb(mp, daddr),
>   		.br_state	= XFS_EXT_NORM,
>   	};
> -	unsigned int		resblks =
> -		XFS_EXTENTADD_SPACE_RES(mp, XFS_DATA_FORK);
> +	/*
> +	 * A remap performs two independent BMBT modifications - an unmap
> +	 * of the old extent followed by a map of the new extent.
> +	 */
> +	unsigned int		resblks = 2 * XFS_RTEXTENTADD_SPACE_RES(mp);
>   	struct xfs_trans	*tp;
>   	int			error;
>   
> @@ -342,7 +345,7 @@ xfs_zoned_end_io(
>   		new.br_blockcount = end_fsb - new.br_startoff;
>   
>   		error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, resblks, 0,
> -				XFS_TRANS_RESERVE | XFS_TRANS_RES_FDBLKS, &tp);
> +				XFS_TRANS_RESERVE, &tp);
>   		if (error)
>   			return error;
>   		xfs_ilock(ip, XFS_ILOCK_EXCL);