Restructure per-filesystem reclaim

Originally when the ARC prune callback was introduced the idea was
to register a single callback for the ZPL.  The ARC could invoke this
call back if it needed the ZPL to drop dentries, inodes, or other
cache objects which might be pinning buffers in the ARC.  The ZPL
would iterate over all ZFS super blocks and perform the reclaim.

For the most part this design has worked well but due to limitations
in 2.6.35 and earlier kernels there were some problems.  This patch
is designed to address those issues.

1) iterate_supers_type() is not provided by all kernels which makes
it impossible to safely iterate over all zpl_fs_type filesystems in
a single callback.  The most straight forward and portable way to
resolve this is to register a callback per-filesystem during mount.
The arc_*_prune_callback() functions have always supported multiple
callbacks so this is functionally a very small change.

2) Commit 050d22b removed the non-portable shrink_dcache_memory()
and shrink_icache_memory() functions and didn't replace them with
equivalent functionality.  This meant that for Linux 3.1 and older
kernels the ARC had no mechanism to drop dentries and inodes from
the caches if needed.  This patch adds that missing functionality
by calling shrink_dcache_parent() to release dentries which may be
pinning inodes.  This will result in all unused cache entries being
dropped which is a bit heavy handed but it's the only interface
available for old kernels.

3) A zpl_drop_inode() callback is registered for kernels older than
2.6.35 which do not support the .evict_inode callback.  This ensures
that when the last reference on an inode is dropped it is immediately
removed from the cache.  If this isn't done than inode can end up on
the global unused LRU with no mechanism available to ZFS to drop them.
Since the ARC buffers are not dropped the hottest inodes can still
be recreated without performing disk IO.

Signed-off-by: Brian Behlendorf <[email protected]>
Signed-off-by: Pavel Snajdr <[email protected]>
Issue #3160

1 files changed, 3 insertions, 3 deletions

diff --git a/module/zfs/arc.c b/module/zfs/arc.c
index 188086767..f9f0008c0 100644
--- a/module/zfs/arc.c
+++ b/module/zfs/arc.c
@@ -158,8 +158,8 @@ static kmutex_t		arc_reclaim_thr_lock;
 static kcondvar_t	arc_reclaim_thr_cv;	/* used to signal reclaim thr */
 static uint8_t		arc_thread_exit;
 
-/* number of bytes to prune from caches when at arc_meta_limit is reached */
-int zfs_arc_meta_prune = 1048576;
+/* number of objects to prune from caches when arc_meta_limit is reached */
+int zfs_arc_meta_prune = 10000;
 
 typedef enum arc_reclaim_strategy {
 	ARC_RECLAIM_AGGR,		/* Aggressive reclaim strategy */
@@ -5607,7 +5607,7 @@ module_param(zfs_arc_meta_limit, ulong, 0644);
 MODULE_PARM_DESC(zfs_arc_meta_limit, "Meta limit for arc size");
 
 module_param(zfs_arc_meta_prune, int, 0644);
-MODULE_PARM_DESC(zfs_arc_meta_prune, "Bytes of meta data to prune");
+MODULE_PARM_DESC(zfs_arc_meta_prune, "Meta objects to scan for prune");
 
 module_param(zfs_arc_grow_retry, int, 0644);
 MODULE_PARM_DESC(zfs_arc_grow_retry, "Seconds before growing arc size");