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>

1 files changed, 10 insertions, 5 deletions

diff --git a/fs/dquot.c b/fs/dquot.c
index cee7c6f428f..def4e969df7 100644
--- 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) */