trylock_super(): replacement for grab_super_passive()

I've noticed significant locking contention in memory reclaimer around
sb_lock inside grab_super_passive(). Grab_super_passive() is called from
two places: in icache/dcache shrinkers (function super_cache_scan) and
from writeback (function __writeback_inodes_wb). Both are required for
progress in memory allocator.

Grab_super_passive() acquires sb_lock to increment sb->s_count and check
sb->s_instances. It seems sb->s_umount locked for read is enough here:
super-block deactivation always runs under sb->s_umount locked for write.
Protecting super-block itself isn't a problem: in super_cache_scan() sb
is protected by shrinker_rwsem: it cannot be freed if its slab shrinkers
are still active. Inside writeback super-block comes from inode from bdi
writeback list under wb->list_lock.

This patch removes locking sb_lock and checks s_instances under s_umount:
generic_shutdown_super() unlinks it under sb->s_umount locked for write.
New variant is called trylock_super() and since it only locks semaphore,
callers must call up_read(&sb->s_umount) instead of drop_super(sb) when
they're done.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 3 insertions, 3 deletions

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 073657f755d4..e907052eeadb 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -769,9 +769,9 @@ static long __writeback_inodes_wb(struct bdi_writeback *wb,
                 struct inode *inode = wb_inode(wb->b_io.prev);
                 struct super_block *sb = inode->i_sb;
 
-                if (!grab_super_passive(sb)) {
+                if (!trylock_super(sb)) {
                         /*
-                         * grab_super_passive() may fail consistently due to
+                         * trylock_super() may fail consistently due to
                          * s_umount being grabbed by someone else. Don't use
                          * requeue_io() to avoid busy retrying the inode/sb.
                          */
@@ -779,7 +779,7 @@ static long __writeback_inodes_wb(struct bdi_writeback *wb,
                         continue;
                 }
                 wrote += writeback_sb_inodes(sb, wb, work);
-                drop_super(sb);
+                up_read(&sb->s_umount);
 
                 /* refer to the same tests at the end of writeback_sb_inodes */
                 if (wrote) {