Re: [PATCH 1/2] xfs: reserve quota for dir expansion when linking/unlinking files

[Dave Chinner <david@fromorbit.com>]

On Thu, Mar 10, 2022 at 01:53:33PM -0800, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> XFS does not reserve quota for directory expansion when linking or
> unlinking children from a directory.  This means that we don't reject
> the expansion with EDQUOT when we're at or near a hard limit, which
> means that unprivileged userspace can use link()/unlink() to exceed
> quota.
> 
> The fix for this is nuanced -- link operations don't always expand the
> directory, and we allow a link to proceed with no space reservation if
> we don't need to add a block to the directory to handle the addition.
> Unlink operations generally do not expand the directory (you'd have to
> free a block and then cause a btree split) and we can defer the
> directory block freeing if there is no space reservation.
> 
> Moreover, there is a further bug in that we do not trigger the blockgc
> workers to try to clear space when we're out of quota.
> 
> To fix both cases, create a new xfs_trans_alloc_dir function that
> allocates the transaction, locks and joins the inodes, and reserves
> quota for the directory.  If there isn't sufficient space or quota,
> we'll switch the caller to reservationless mode.  This should prevent
> quota usage overruns with the least restriction in functionality.
> 
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
>  fs/xfs/xfs_inode.c |   46 ++++++++++------------------
>  fs/xfs/xfs_trans.c |   86 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  fs/xfs/xfs_trans.h |    3 ++
>  3 files changed, 106 insertions(+), 29 deletions(-)

Looks good.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
-- 
Dave Chinner
david@fromorbit.com