set zfs_arc_shrinker_limit to 0 by default

zfs_arc_shrinker_limit was introduced to avoid ARC collapse due to
aggressive kernel reclaim. While useful, the current default (10000) is
too prone to OOM especially when MGLRU-enabled kernels with default
min_ttl_ms are used. Even when no OOM happens, it often causes too much
swap usage.

This patch sets zfs_arc_shrinker_limit=0 to not ignore kernel reclaim
requests. ARC now plays better with both kernel shrinker and pagecache
but, should ARC collapse happen again, MGLRU behavior can be tuned or
even disabled.

Anyway, zfs should not cause OOM when ARC can be released.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Signed-off-by: Gionatan Danti <[email protected]>
Closes #16909

man/man4/zfs.4

@@ -867,14 +867,14 @@ where that percent may exceed
 This
 only operates during memory pressure/reclaim.
 .
-.It Sy zfs_arc_shrinker_limit Ns = Ns Sy 10000 Pq int
+.It Sy zfs_arc_shrinker_limit Ns = Ns Sy 0 Pq int
 This is a limit on how many pages the ARC shrinker makes available for
 eviction in response to one page allocation attempt.
 Note that in practice, the kernel's shrinker can ask us to evict
 up to about four times this for one allocation attempt.
 To reduce OOM risk, this limit is applied for kswapd reclaims only.
 .Pp
-The default limit of
+For example a value of
 .Sy 10000 Pq in practice, Em 160 MiB No per allocation attempt with 4 KiB pages
 limits the amount of time spent attempting to reclaim ARC memory to
 less than 100 ms per allocation attempt,

module/os/linux/zfs/arc_os.c

@@ -63,15 +63,15 @@
  * practice, the kernel's shrinker can ask us to evict up to about 4x this
  * for one allocation attempt.
  *
- * The default limit of 10,000 (in practice, 160MB per allocation attempt
+ * For example a value of 10,000 (in practice, 160MB per allocation attempt
  * with 4K pages) limits the amount of time spent attempting to reclaim ARC
  * memory to less than 100ms per allocation attempt, even with a small
  * average compressed block size of ~8KB.
  *
  * See also the comment in arc_shrinker_count().
  * Set to 0 to disable limit.
  */
-static int zfs_arc_shrinker_limit = 10000;
+static int zfs_arc_shrinker_limit = 0;
 
 /*
  * Relative cost of ARC eviction, AKA number of seeks needed to restore evicted