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xfs: correct the parent pointer space reservation comment

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The comment on xfs_parent_calc_space_res() claims parent pointers are
"always the first attr in an attr tree".  They are not: a parent pointer
is recorded per dirent, so by the Nth hardlink the attr fork is already
in leaf or node format.  The reservation is still correct, because
XFS_DAENTER_SPACE_RES() covers a split at every level of a maximum-depth
attr dabtree whatever format the fork is in, but anyone auditing a
shortfall here is led by the comment to look for a bug that is not
there.

Rewrite the comment to state what actually bounds the result, and record
why the double split allowance and the extent-add term differ from
xfs_attr_calc_size().

Signed-off-by: Javier Tia <floss@jetm.me>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>
This commit is contained in:
Javier Tia 2026-08-10 17:06:16 -06:00 committed by Carlos Maiolino
parent d058f24b16
commit 6c0fc3cb4e
1 changed files with 17 additions and 2 deletions
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19
fs/xfs/libxfs/xfs_trans_space.c
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@ -22,8 +22,23 @@ xfs_parent_calc_space_res(
unsigned int namelen)
{
/*
* Parent pointers are always the first attr in an attr tree, and never
* larger than a block
* A parent pointer is recorded per dirent, so an inode with N links
* carries N of them and the attr fork can already be in leaf or node
* format when one is added. That does not affect the reservation:
* XFS_DAENTER_SPACE_RES covers a split at every level of a
* maximum-depth attr dabtree, whatever format the fork is in now.
*
* The name is a dirent name and the value is a struct xfs_parent_rec,
* so the leaf entry is always local and never exceeds 272 bytes.
* Parent pointers require V5, hence a 1k minimum block size, so the
* entry always stays under half a block and this needs none of the
* double split allowance that xfs_attr_calc_size() makes.
*
* The second term hands a byte count to a macro whose parameter counts
* mappings, so it asks for more extent-add allowance than the single
* mapping a parent pointer adds - how much more depends on the block
* size. It over-reserves either way, which is why it is left alone:
* correcting the unit would shrink a reservation that is only generous.
*/
return XFS_DAENTER_SPACE_RES(mp, XFS_ATTR_FORK) +
XFS_NEXTENTADD_SPACE_RES(mp, namelen, XFS_ATTR_FORK);