quota: improve inode list scanning in add_dquot_ref()

We restarted scan of sb->s_inodes list whenever we had to drop inode_lock
in add_dquot_ref().  This leads to overall quadratic running time and thus
add_dquot_ref() can take several minutes when called on a life filesystem.
We fix the problem by using the fact that inode cannot be removed from
s_inodes list while we hold a reference to it and thus we can safely
restart the scan if we don't drop the reference.  Here we use the fact that
inodes freshly added to s_inodes list are already guaranteed to have quotas
properly initialized and the ordering of inodes on s_inodes list does not
change so we cannot skip any inode.

Thanks goes to Nick <gentuu@gmail.com> for analyzing the problem and
testing the fix.

[akpm@linux-foundation.org: iput(NULL) is legal]
Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Nick <gentuu@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/dquot.c b/fs/dquot.c
index cee7c6f428f0fa518e80bda281eb92421a5ac295..def4e969df77646192eb6ce96d223a1f78ec4544 100644 (file)
--- a/fs/dquot.c
+++ b/fs/dquot.c
@@ -696,9 +696,8 @@ static int dqinit_needed(struct inode *inode, int type)
 /* This routine is guarded by dqonoff_mutex mutex */
 static void add_dquot_ref(struct super_block *sb, int type)
 {
-       struct inode *inode;
+       struct inode *inode, *old_inode = NULL;
 
-restart:
        spin_lock(&inode_lock);
        list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
                if (!atomic_read(&inode->i_writecount))
@@ -711,12 +710,18 @@ restart:
                __iget(inode);
                spin_unlock(&inode_lock);
 
+               iput(old_inode);
                sb->dq_op->initialize(inode, type);
-               iput(inode);
-               /* As we may have blocked we had better restart... */
-               goto restart;
+               /* We hold a reference to 'inode' so it couldn't have been
+                * removed from s_inodes list while we dropped the inode_lock.
+                * We cannot iput the inode now as we can be holding the last
+                * reference and we cannot iput it under inode_lock. So we
+                * keep the reference and iput it later. */
+               old_inode = inode;
+               spin_lock(&inode_lock);
        }
        spin_unlock(&inode_lock);
+       iput(old_inode);
 }
 
 /* Return 0 if dqput() won't block (note that 1 doesn't necessarily mean blocking) */