On Thu, Aug 20, 2020 at 10:25:12AM -0700, Darrick J. Wong wrote:
> On Thu, Aug 20, 2020 at 01:07:34PM -0400, Brian Foster wrote:
> > The inode chunk allocation transaction reserves inobt_maxlevels-1
> > blocks to accommodate a full split of the inode btree. A full split
> > requires an allocation for every existing level and a new root
> > block, which means inobt_maxlevels is the worst case block
> > requirement for a transaction that inserts to the inobt. This can
> > lead to a transaction block reservation overrun when tmpfile
> > creation allocates an inode chunk and expands the inobt to its
> > maximum depth. This problem has been observed in conjunction with
> > overlayfs, which makes frequent use of tmpfiles internally.
> >
> > The existing reservation code goes back as far as the Linux git repo
> > history (v2.6.12). It was likely never observed as a problem because
> > the traditional file/directory creation transactions also include
> > worst case block reservation for directory modifications, which most
> > likely is able to make up for a single block deficiency in the inode
> > allocation portion of the calculation. tmpfile support is relatively
> > more recent (v3.15), less heavily used, and only includes the inode
> > allocation block reservation as tmpfiles aren't linked into the
> > directory tree on creation.
> >
> > Fix up the inode alloc block reservation macro and a couple of the
> > block allocator minleft parameters that enforce an allocation to
> > leave enough free blocks in the AG for a full inobt split.
>
> Looks all fine to me, but... does a similar logic apply to the other
> maxlevels uses in the kernel?
>
> fs/xfs/libxfs/xfs_trans_resv.c:73: blocks = num_ops * 2 * (2 * mp->m_ag_maxlevels - 1);
> fs/xfs/libxfs/xfs_trans_resv.c:75: blocks += max(num_ops * (2 * mp->m_rmap_maxlevels - 1),
> fs/xfs/libxfs/xfs_trans_resv.c:78: blocks += num_ops * (2 * mp->m_refc_maxlevels - 1);
>
> Can we end up in the same kind of situation with those other trees
> {bno,cnt,rmap,refc} where we have a maxlevels-1 tall tree and split each
> level all the way to the top?
>
Hmm.. it seems so at first glance, but I'm not sure I follow the
calculations in that function. If we factor out the obvious
num_ops/num_trees components, the comment refers to the following
generic formula:
((2 blocks/level * max depth) - 1)
I take it that since this is a log reservation calculation, the two
block/level multiplier is there because we have to move records between
two blocks for each level that splits. Is there a reason the -1 is
applied after that multiplier (as opposed to subtracting 1 from the max
depth first)? I'm wondering if that's intentional and it reflects that
the root level is only one block...
Brian
> > Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
>
> For this bit,
> Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
>
> --D
>
> > ---
> > fs/xfs/libxfs/xfs_ialloc.c | 4 ++--
> > fs/xfs/libxfs/xfs_trans_space.h | 2 +-
> > 2 files changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/xfs/libxfs/xfs_ialloc.c b/fs/xfs/libxfs/xfs_ialloc.c
> > index f742a96a2fe1..a6b37db55169 100644
> > --- a/fs/xfs/libxfs/xfs_ialloc.c
> > +++ b/fs/xfs/libxfs/xfs_ialloc.c
> > @@ -688,7 +688,7 @@ xfs_ialloc_ag_alloc(
> > args.minalignslop = igeo->cluster_align - 1;
> >
> > /* Allow space for the inode btree to split. */
> > - args.minleft = igeo->inobt_maxlevels - 1;
> > + args.minleft = igeo->inobt_maxlevels;
> > if ((error = xfs_alloc_vextent(&args)))
> > return error;
> >
> > @@ -736,7 +736,7 @@ xfs_ialloc_ag_alloc(
> > /*
> > * Allow space for the inode btree to split.
> > */
> > - args.minleft = igeo->inobt_maxlevels - 1;
> > + args.minleft = igeo->inobt_maxlevels;
> > if ((error = xfs_alloc_vextent(&args)))
> > return error;
> > }
> > diff --git a/fs/xfs/libxfs/xfs_trans_space.h b/fs/xfs/libxfs/xfs_trans_space.h
> > index c6df01a2a158..7ad3659c5d2a 100644
> > --- a/fs/xfs/libxfs/xfs_trans_space.h
> > +++ b/fs/xfs/libxfs/xfs_trans_space.h
> > @@ -58,7 +58,7 @@
> > #define XFS_IALLOC_SPACE_RES(mp) \
> > (M_IGEO(mp)->ialloc_blks + \
> > ((xfs_sb_version_hasfinobt(&mp->m_sb) ? 2 : 1) * \
> > - (M_IGEO(mp)->inobt_maxlevels - 1)))
> > + M_IGEO(mp)->inobt_maxlevels))
> >
> > /*
> > * Space reservation values for various transactions.
> > --
> > 2.25.4
> >
>
