Ignore zfs_arc_shrinker_limit in direct reclaim mode

zfs_arc_shrinker_limit (default: 10000) avoids ARC collapse
due to excessive memory reclaim. However, when the kernel is
in direct reclaim mode (ie: low on memory), limiting ARC reclaim
increases OOM risk. This is especially true on system without
(or with inadequate) swap.

This patch ignores zfs_arc_shrinker_limit when the kernel is in
direct reclaim mode, avoiding most OOM. It also restores
"echo 3 > /proc/sys/vm/drop_caches" ability to correctly drop
(almost) all ARC.

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

module/os/linux/zfs/arc_os.c

@@ -204,8 +204,11 @@ arc_shrinker_count(struct shrinker *shrink, struct shrink_control *sc)
  *
  * See also the comment above zfs_arc_shrinker_limit.
  */
- int64_t limit = zfs_arc_shrinker_limit != 0 ?
- zfs_arc_shrinker_limit : INT64_MAX;
+ int64_t limit = INT64_MAX;
+ if (current_is_kswapd()) {
+ limit = zfs_arc_shrinker_limit != 0 ?
+ zfs_arc_shrinker_limit : INT64_MAX;
+ }
  return (MIN(limit, btop((int64_t)arc_evictable_memory())));
 }