[PATCH 5.10 31/37] xfs: remove infinite loop when reserving free block pool

Greg Kroah-Hartman
2022-09-02T13:03:16Z

From: Darrick J. Wong <djwong@kernel.org>

commit 15f04fdc75aaaa1cccb0b8b3af1be290e118a7bc upstream.

[Added wrapper xfs_fdblocks_unavailable() for 5.10.y backport]

Infinite loops in kernel code are scary. Calls to xfs_reserve_blocks
should be rare (people should just use the defaults!) so we really don't
need to try so hard. Simplify the logic here by removing the infinite
loop.

Cc: Brian Foster <bfoster@redhat.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/xfs/xfs_fsops.c | 52 +++++++++++++++++++++-------------------------------
fs/xfs/xfs_mount.h | 8 ++++++++
2 files changed, 29 insertions(+), 31 deletions(-)

--- a/fs/xfs/xfs_fsops.c
+++ b/fs/xfs/xfs_fsops.c
@@ -376,46 +376,36 @@ xfs_reserve_blocks(
* If the request is larger than the current reservation, reserve the
* blocks before we update the reserve counters. Sample m_fdblocks and
* perform a partial reservation if the request exceeds free space.
+ *
+ * The code below estimates how many blocks it can request from
+ * fdblocks to stash in the reserve pool. This is a classic TOCTOU
+ * race since fdblocks updates are not always coordinated via
+ * m_sb_lock.
*/
- error = -ENOSPC;
- do {
- free = percpu_counter_sum(&mp->m_fdblocks) -
- mp->m_alloc_set_aside;
- if (free <= 0)
- break;
-
- delta = request - mp->m_resblks;
- lcounter = free - delta;
- if (lcounter < 0)
- /* We can't satisfy the request, just get what we can */
- fdblks_delta = free;
- else
- fdblks_delta = delta;
-
+ free = percpu_counter_sum(&mp->m_fdblocks) -
+ xfs_fdblocks_unavailable(mp);
+ delta = request - mp->m_resblks;
+ if (delta > 0 && free > 0) {
/*
* We'll either succeed in getting space from the free block
- * count or we'll get an ENOSPC. If we get a ENOSPC, it means
- * things changed while we were calculating fdblks_delta and so
- * we should try again to see if there is anything left to
- * reserve.
- *
- * Don't set the reserved flag here - we don't want to reserve
- * the extra reserve blocks from the reserve.....
+ * count or we'll get an ENOSPC. Don't set the reserved flag
+ * here - we don't want to reserve the extra reserve blocks
+ * from the reserve.
*/
+ fdblks_delta = min(free, delta);
spin_unlock(&mp->m_sb_lock);
error = xfs_mod_fdblocks(mp, -fdblks_delta, 0);
spin_lock(&mp->m_sb_lock);
- } while (error == -ENOSPC);

- /*
- * Update the reserve counters if blocks have been successfully
- * allocated.
- */
- if (!error && fdblks_delta) {
- mp->m_resblks += fdblks_delta;
- mp->m_resblks_avail += fdblks_delta;
+ /*
+ * Update the reserve counters if blocks have been successfully
+ * allocated.
+ */
+ if (!error) {
+ mp->m_resblks += fdblks_delta;
+ mp->m_resblks_avail += fdblks_delta;
+ }
}
-
out:
if (outval) {
outval->resblks = mp->m_resblks;
--- a/fs/xfs/xfs_mount.h
+++ b/fs/xfs/xfs_mount.h
@@ -406,6 +406,14 @@ extern int xfs_initialize_perag(xfs_moun
xfs_agnumber_t *maxagi);
extern void xfs_unmountfs(xfs_mount_t *);

+/* Accessor added for 5.10.y backport */
+static inline uint64_t
+xfs_fdblocks_unavailable(
+ struct xfs_mount *mp)
+{
+ return mp->m_alloc_set_aside;
+}
+
extern int xfs_mod_fdblocks(struct xfs_mount *mp, int64_t delta,
bool reserved);
extern int xfs_mod_frextents(struct xfs_mount *mp, int64_t delta);