tmpfs: preserve explicitly disabled quota limits

An administrator’s decision to disable a user’s or group’s quota
limits can be silently lost when tmpfs restores the mount defaults
after releasing and reacquiring the quota record. This can cause
later file creation or writes to fail with EDQUOT, even though the
administrator has explicitly disabled quota limits for the user or
group.

do_set_dqblk() sets DQ_FAKE_B when all block and inode limits are zero.
This means quota enforcement is disabled for that user or group.
shmem_is_empty_dquot() treats the flag as a reason to discard the entry
from the mount's quota tree. The next shmem_acquire_dquot() finds no
saved entry and recreates it with the mount's default hard limits.

If the cached dquot is reused, DQ_FAKE_B remains set: Q_GETQUOTA reports
the restored defaults, but enforcement is still disabled. Once that
dquot has been reclaimed, a new dquot enforces the restored defaults.

Remove the DQ_FAKE_B test. Only discard entries with zero usage, zero
soft limits and hard limits equal to the mount defaults. Explicitly
disabled limits are then preserved whenever the mount defaults are
nonzero.

Explicitly disabled limits now retain a quota-tree entry, just like other
limits that differ from the mount defaults.

Rename shmem_is_empty_dquot() to shmem_is_default_dquot() to describe
the check for default values.

Fix generated by an LLM and sent by Josef Bacik in private.

Fixes: de4c0e7ca8b5 ("shmem: Add default quota limit mount options")
Cc: stable@vger.kernel.org
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
[ david: code cleanups; rewrite description ]
Link: https://patch.msgid.link/20261029-fixes_shmem-v4-3-392fc715d8f6@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

0 files changed, 0 insertions, 0 deletions