avoid meta/data shrink when dnode is over quota

Fix: #17487

When iterating over millions of files, dnode cache grows almost unbounded
consuming nearly all available ARC memory and pushing out valuable
metadata and data. This is probably due to the kernel not releasing its
dentries and inodes caches, keeping dnodes pinned and unable to be
pruned.

This patch avoid shrinking metadata and data when dnode is over quota,
forcing the kernel to drop its caches and, in turn, enabling the zfs
shrinker thread to prune the now-unpinned dnodes.

Signed-off-by: Gionatan Danti <[email protected]>

Author: shodanshok

module/zfs/arc.c

@@ -4426,18 +4426,6 @@ arc_evict_adj(uint64_t frac, uint64_t total, uint64_t up, uint64_t down,
 	return (frac + up - down);
 }
 
-/*
- * Calculate (x * multiplier / divisor) without unnecesary overflows.
- */
-static uint64_t
-arc_mf(uint64_t x, uint64_t multiplier, uint64_t divisor)
-{
-	uint64_t q = (x / divisor);
-	uint64_t r = (x % divisor);
-
-	return ((q * multiplier) + ((r * multiplier) / divisor));
-}
-
 /*
  * Evict buffers from the cache, such that arcstat_size is capped by arc_c.
  */
@@ -4485,29 +4473,17 @@ arc_evict(void)
 	uint64_t ac = arc_c;
 	int64_t wt = t - (asize - ac);
 
-	/*
-	 * Try to reduce pinned dnodes if more than 3/4 of wanted metadata
-	 * target is not evictable or if they go over arc_dnode_limit.
-	 */
+	/* Avoid meta/data shrink to force dentries and inodes reclaim. */
 	int64_t prune = 0;
 	int64_t dn = wmsum_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])
-	    - zfs_refcount_count(&arc_mfu->arcs_esize[ARC_BUFC_METADATA]);
-	w = wt * (int64_t)(arc_meta >> 16) >> 16;
-	if (nem > w * 3 / 4) {
-		prune = dn / sizeof (dnode_t) *
-		    zfs_arc_dnode_reduce_percent / 100;
-		if (nem < w && w > 4)
-			prune = arc_mf(prune, nem - w * 3 / 4, w / 4);
-	}
 	if (dn > arc_dnode_limit) {
 		prune = MAX(prune, (dn - arc_dnode_limit) / sizeof (dnode_t) *
-		    zfs_arc_dnode_reduce_percent / 100);
+				zfs_arc_dnode_reduce_percent / 100);
 	}
-	if (prune > 0)
+	if (prune > 0) {
 		arc_prune_async(prune);
+		return 0;
+	}
 
 	/* Evict MRU metadata. */
 	w = wt * (int64_t)(arc_meta * arc_pm >> 48) >> 16;
@@ -4698,6 +4674,11 @@ arc_async_flush_guid_inuse(uint64_t spa_guid)
 uint64_t
 arc_reduce_target_size(uint64_t to_free)
 {
+	/* Avoid meta/data shrink to force dentries and inodes reclaim. */
+	int64_t dn = wmsum_value(&arc_sums.arcstat_dnode_size);
+	if (dn > arc_dnode_limit)
+		return 0;
+
 	/*
 	 * Get the actual arc size.  Even if we don't need it, this updates
 	 * the aggsum lower bound estimate for arc_is_overflowing().