[pve-devel] [PATCH zfsonlinux] cherry-pick fix for overgrown dnode cache

[Stoiko Ivanov <s.ivanov@proxmox.com>]

the following patch seems applicable and might fix an issue observed
in our enterprise support a while ago. containers run in their own
cgroups, thus were probably not scanned by the kernel shrinker - this
resulted in Dnode cache numbers of 300+% reported in arc_summary.

FWICT the issue was introduced in ZFS 2.2.7
(commit 5f73630e9cbea5efa23d16809f06e0d08523b241 see:
https://github.com/openzfs/zfs/issues/17052#issuecomment-3065907783)
but I assume that the increase of zfs_arc_max by default makes it
trigger OOMs far easier.

The discussion of the PR was quite instructive:
https://github.com/openzfs/zfs/pull/17542

minimally tested on a pair of trixie VMs (building + running
replication of a couple of containers)

Suggested-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
---
 .../0010-enforce-arc_dnode_limit.patch        | 216 ++++++++++++++++++
 debian/patches/series                         |   1 +
 2 files changed, 217 insertions(+)
 create mode 100644 debian/patches/0010-enforce-arc_dnode_limit.patch

diff --git a/debian/patches/0010-enforce-arc_dnode_limit.patch b/debian/patches/0010-enforce-arc_dnode_limit.patch
new file mode 100644
index 000000000..38814bfab
--- /dev/null
+++ b/debian/patches/0010-enforce-arc_dnode_limit.patch
@@ -0,0 +1,216 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: shodanshok <g.danti@assyoma.it>
+Date: Mon, 21 Jul 2025 19:32:01 +0200
+Subject: [PATCH] enforce arc_dnode_limit
+
+Linux kernel shrinker in the context of null/root memcg does not scan
+dentry and inode caches added by a task running in non-root memcg. For
+ZFS this means that dnode cache routinely overflows, evicting valuable
+meta/data and putting additional memory pressure on the system.
+
+This patch restores zfs_prune_aliases as fallback when the kernel
+shrinker does nothing, enabling zfs to actually free dnodes. Moreover,
+it (indirectly) calls arc_evict when dnode_size > dnode_limit.
+
+Reviewed-by: Rob Norris <robn@despairlabs.com>
+Reviewed-by: Alexander Motin <mav@FreeBSD.org>
+Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
+Signed-off-by: Gionatan Danti <g.danti@assyoma.it>
+Closes #17487
+Closes #17542
+(cherry picked from commit a7a144e655850b4160943e4ba315eb9a5dc2b2fe)
+Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
+---
+ include/sys/arc_impl.h           |  2 +-
+ module/os/linux/zfs/zfs_vfsops.c | 65 ++++++++++++++++++++++++++++++++
+ module/zfs/arc.c                 | 22 ++++++-----
+ 3 files changed, 78 insertions(+), 11 deletions(-)
+
+diff --git a/include/sys/arc_impl.h b/include/sys/arc_impl.h
+index 1b30389107c55d558ecc4a21f7471bc03e4155a4..b55d5da3378c0608c62b0004848baeca925eda4a 100644
+--- a/include/sys/arc_impl.h
++++ b/include/sys/arc_impl.h
+@@ -954,7 +954,7 @@ typedef struct arc_sums {
+ 	wmsum_t arcstat_data_size;
+ 	wmsum_t arcstat_metadata_size;
+ 	wmsum_t arcstat_dbuf_size;
+-	wmsum_t arcstat_dnode_size;
++	aggsum_t arcstat_dnode_size;
+ 	wmsum_t arcstat_bonus_size;
+ 	wmsum_t arcstat_l2_hits;
+ 	wmsum_t arcstat_l2_misses;
+diff --git a/module/os/linux/zfs/zfs_vfsops.c b/module/os/linux/zfs/zfs_vfsops.c
+index 56af4fe0a4648fa6a94ebc3e0bdb88fc95836aa6..7961549e637bd080f74e8a4fde9560f7bd86f179 100644
+--- a/module/os/linux/zfs/zfs_vfsops.c
++++ b/module/os/linux/zfs/zfs_vfsops.c
+@@ -1176,6 +1176,63 @@ zfs_root(zfsvfs_t *zfsvfs, struct inode **ipp)
+ 	return (error);
+ }
+ 
++/*
++ * Dentry and inode caches referenced by a task in non-root memcg are
++ * not going to be scanned by the kernel-provided shrinker. So, if
++ * kernel prunes nothing, fall back to this manual walk to free dnodes.
++ * To avoid scanning the same znodes multiple times they are always rotated
++ * to the end of the z_all_znodes list. New znodes are inserted at the
++ * end of the list so we're always scanning the oldest znodes first.
++ */
++static int
++zfs_prune_aliases(zfsvfs_t *zfsvfs, unsigned long nr_to_scan)
++{
++	znode_t **zp_array, *zp;
++	int max_array = MIN(nr_to_scan, PAGE_SIZE * 8 / sizeof (znode_t *));
++	int objects = 0;
++	int i = 0, j = 0;
++
++	zp_array = vmem_zalloc(max_array * sizeof (znode_t *), KM_SLEEP);
++
++	mutex_enter(&zfsvfs->z_znodes_lock);
++	while ((zp = list_head(&zfsvfs->z_all_znodes)) != NULL) {
++
++		if ((i++ > nr_to_scan) || (j >= max_array))
++			break;
++
++		ASSERT(list_link_active(&zp->z_link_node));
++		list_remove(&zfsvfs->z_all_znodes, zp);
++		list_insert_tail(&zfsvfs->z_all_znodes, zp);
++
++		/* Skip active znodes and .zfs entries */
++		if (MUTEX_HELD(&zp->z_lock) || zp->z_is_ctldir)
++			continue;
++
++		if (igrab(ZTOI(zp)) == NULL)
++			continue;
++
++		zp_array[j] = zp;
++		j++;
++	}
++	mutex_exit(&zfsvfs->z_znodes_lock);
++
++	for (i = 0; i < j; i++) {
++		zp = zp_array[i];
++
++		ASSERT3P(zp, !=, NULL);
++		d_prune_aliases(ZTOI(zp));
++
++		if (atomic_read(&ZTOI(zp)->i_count) == 1)
++			objects++;
++
++		zrele(zp);
++	}
++
++	vmem_free(zp_array, max_array * sizeof (znode_t *));
++
++	return (objects);
++}
++
+ /*
+  * The ARC has requested that the filesystem drop entries from the dentry
+  * and inode caches.  This can occur when the ARC needs to free meta data
+@@ -1227,6 +1284,14 @@ zfs_prune(struct super_block *sb, unsigned long nr_to_scan, int *objects)
+ 	*objects = (*shrinker->scan_objects)(shrinker, &sc);
+ #endif
+ 
++	/*
++	 * Fall back to zfs_prune_aliases if kernel's shrinker did nothing
++	 * due to dentry and inode caches being referenced by a task running
++	 * in non-root memcg.
++	 */
++	if (*objects == 0)
++		*objects = zfs_prune_aliases(zfsvfs, nr_to_scan);
++
+ 	zfs_exit(zfsvfs, FTAG);
+ 
+ 	dprintf_ds(zfsvfs->z_os->os_dsl_dataset,
+diff --git a/module/zfs/arc.c b/module/zfs/arc.c
+index 75be2b02a7e5c6ed3f97377c05c062b9199c3dd3..520171eae37b8b3a7eae8991d4a8b5dc71846db9 100644
+--- a/module/zfs/arc.c
++++ b/module/zfs/arc.c
+@@ -2631,7 +2631,7 @@ arc_space_consume(uint64_t space, arc_space_type_t type)
+ 		ARCSTAT_INCR(arcstat_bonus_size, space);
+ 		break;
+ 	case ARC_SPACE_DNODE:
+-		ARCSTAT_INCR(arcstat_dnode_size, space);
++		aggsum_add(&arc_sums.arcstat_dnode_size, space);
+ 		break;
+ 	case ARC_SPACE_DBUF:
+ 		ARCSTAT_INCR(arcstat_dbuf_size, space);
+@@ -2677,7 +2677,7 @@ arc_space_return(uint64_t space, arc_space_type_t type)
+ 		ARCSTAT_INCR(arcstat_bonus_size, -space);
+ 		break;
+ 	case ARC_SPACE_DNODE:
+-		ARCSTAT_INCR(arcstat_dnode_size, -space);
++		aggsum_add(&arc_sums.arcstat_dnode_size, -space);
+ 		break;
+ 	case ARC_SPACE_DBUF:
+ 		ARCSTAT_INCR(arcstat_dbuf_size, -space);
+@@ -4490,7 +4490,7 @@ arc_evict(void)
+ 	 * target is not evictable or if they go over arc_dnode_limit.
+ 	 */
+ 	int64_t prune = 0;
+-	int64_t dn = wmsum_value(&arc_sums.arcstat_dnode_size);
++	int64_t dn = aggsum_value(&arc_sums.arcstat_dnode_size);
+ 	int64_t nem = zfs_refcount_count(&arc_mru->arcs_size[ARC_BUFC_METADATA])
+ 	    + zfs_refcount_count(&arc_mfu->arcs_size[ARC_BUFC_METADATA])
+ 	    - zfs_refcount_count(&arc_mru->arcs_esize[ARC_BUFC_METADATA])
+@@ -5082,11 +5082,13 @@ arc_is_overflowing(boolean_t lax, boolean_t use_reserve)
+ 	 * in the ARC. In practice, that's in the tens of MB, which is low
+ 	 * enough to be safe.
+ 	 */
+-	int64_t over = aggsum_lower_bound(&arc_sums.arcstat_size) - arc_c -
++	int64_t arc_over = aggsum_lower_bound(&arc_sums.arcstat_size) - arc_c -
+ 	    zfs_max_recordsize;
++	int64_t dn_over = aggsum_lower_bound(&arc_sums.arcstat_dnode_size) -
++	    arc_dnode_limit;
+ 
+ 	/* Always allow at least one block of overflow. */
+-	if (over < 0)
++	if (arc_over < 0 && dn_over <= 0)
+ 		return (ARC_OVF_NONE);
+ 
+ 	/* If we are under memory pressure, report severe overflow. */
+@@ -5097,7 +5099,7 @@ arc_is_overflowing(boolean_t lax, boolean_t use_reserve)
+ 	int64_t overflow = (arc_c >> zfs_arc_overflow_shift) / 2;
+ 	if (use_reserve)
+ 		overflow *= 3;
+-	return (over < overflow ? ARC_OVF_SOME : ARC_OVF_SEVERE);
++	return (arc_over < overflow ? ARC_OVF_SOME : ARC_OVF_SEVERE);
+ }
+ 
+ static abd_t *
+@@ -7343,7 +7345,7 @@ arc_kstat_update(kstat_t *ksp, int rw)
+ #if defined(COMPAT_FREEBSD11)
+ 	as->arcstat_other_size.value.ui64 =
+ 	    wmsum_value(&arc_sums.arcstat_bonus_size) +
+-	    wmsum_value(&arc_sums.arcstat_dnode_size) +
++	    aggsum_value(&arc_sums.arcstat_dnode_size) +
+ 	    wmsum_value(&arc_sums.arcstat_dbuf_size);
+ #endif
+ 
+@@ -7385,7 +7387,7 @@ arc_kstat_update(kstat_t *ksp, int rw)
+ 	    &as->arcstat_uncached_evictable_metadata);
+ 
+ 	as->arcstat_dnode_size.value.ui64 =
+-	    wmsum_value(&arc_sums.arcstat_dnode_size);
++	    aggsum_value(&arc_sums.arcstat_dnode_size);
+ 	as->arcstat_bonus_size.value.ui64 =
+ 	    wmsum_value(&arc_sums.arcstat_bonus_size);
+ 	as->arcstat_l2_hits.value.ui64 =
+@@ -7755,7 +7757,7 @@ arc_state_init(void)
+ 	wmsum_init(&arc_sums.arcstat_data_size, 0);
+ 	wmsum_init(&arc_sums.arcstat_metadata_size, 0);
+ 	wmsum_init(&arc_sums.arcstat_dbuf_size, 0);
+-	wmsum_init(&arc_sums.arcstat_dnode_size, 0);
++	aggsum_init(&arc_sums.arcstat_dnode_size, 0);
+ 	wmsum_init(&arc_sums.arcstat_bonus_size, 0);
+ 	wmsum_init(&arc_sums.arcstat_l2_hits, 0);
+ 	wmsum_init(&arc_sums.arcstat_l2_misses, 0);
+@@ -7914,7 +7916,7 @@ arc_state_fini(void)
+ 	wmsum_fini(&arc_sums.arcstat_data_size);
+ 	wmsum_fini(&arc_sums.arcstat_metadata_size);
+ 	wmsum_fini(&arc_sums.arcstat_dbuf_size);
+-	wmsum_fini(&arc_sums.arcstat_dnode_size);
++	aggsum_fini(&arc_sums.arcstat_dnode_size);
+ 	wmsum_fini(&arc_sums.arcstat_bonus_size);
+ 	wmsum_fini(&arc_sums.arcstat_l2_hits);
+ 	wmsum_fini(&arc_sums.arcstat_l2_misses);
diff --git a/debian/patches/series b/debian/patches/series
index f3f297e33..d59978cbb 100644
--- a/debian/patches/series
+++ b/debian/patches/series
@@ -7,3 +7,4 @@
 0007-Add-systemd-unit-for-importing-specific-pools.patch
 0008-Patch-move-manpage-arcstat-1-to-arcstat-8.patch
 0009-zpool-status-tighten-bounds-for-noalloc-stat-availab.patch
+0010-enforce-arc_dnode_limit.patch
-- 
2.39.5



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