[PATCH v2] writeback: Put unused inodes to LRU after writeback completion

[Jan Kara]

Commit 169ebd90 removed iget-iput pair from inode writeback. As a side effect,
inodes that are dirty during iput_final() call won't be ever added to inode LRU
(iput_final() doesn't add dirty inodes to LRU and later when the inode is
cleaned there's noone to add the inode there). Thus inodes are effectively
unreclaimable until someone looks them up again.

Practical effect of this bug is limited by the fact that inodes are
pinned by a dentry for long enough that the inode gets cleaned. But still
the bug can have nasty consequences leading up to OOM conditions under
certain circumstances. Following can easily reproduce the problem:

for (( i = 0; i < 1000; i++ )); do
  mkdir $i
  for (( j = 0; j < 1000; j++ )); do
    touch $i/$j
    echo 2 > /proc/sys/vm/drop_caches
  done
done

then one needs to run 'sync; ls -lR' to make inodes reclaimable again.

We fix the issue by inserting unused clean inodes into the LRU after writeback
finishes in inode_sync_complete().

CC: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
CC: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
CC: stable@xxxxxxxxxxxxxxx # >= 3.5
Reported-by: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
 fs/fs-writeback.c |    2 ++
 fs/inode.c        |   16 ++++++++++++++--
 fs/internal.h     |    1 +
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 51ea267..3e3422f 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -228,6 +228,8 @@ static void requeue_io(struct inode *inode, struct bdi_writeback *wb)
 static void inode_sync_complete(struct inode *inode)
 {
 	inode->i_state &= ~I_SYNC;
+	/* If inode is clean an unused, put it into LRU now... */
+	inode_add_lru(inode);
 	/* Waiters must see I_SYNC cleared before being woken up */
 	smp_mb();
 	wake_up_bit(&inode->i_state, __I_SYNC);
diff --git a/fs/inode.c b/fs/inode.c
index b03c719..8f6396f 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -408,6 +408,19 @@ static void inode_lru_list_add(struct inode *inode)
 	spin_unlock(&inode->i_sb->s_inode_lru_lock);
 }
 
+/*
+ * Add inode to LRU if needed (inode is unused and clean).
+ *
+ * Needs inode->i_lock held.
+ */
+void inode_add_lru(struct inode *inode)
+{
+	if (!(inode->i_state & (I_DIRTY | I_FREEING | I_SYNC)) &&
+	    !atomic_read(&inode->i_count) && inode->i_sb->s_flags & MS_ACTIVE)
+		inode_lru_list_add(inode);
+}
+
+
 static void inode_lru_list_del(struct inode *inode)
 {
 	spin_lock(&inode->i_sb->s_inode_lru_lock);
@@ -1390,8 +1403,7 @@ static void iput_final(struct inode *inode)
 
 	if (!drop && (sb->s_flags & MS_ACTIVE)) {
 		inode->i_state |= I_REFERENCED;
-		if (!(inode->i_state & (I_DIRTY|I_SYNC)))
-			inode_lru_list_add(inode);
+		inode_add_lru(inode);
 		spin_unlock(&inode->i_lock);
 		return;
 	}
diff --git a/fs/internal.h b/fs/internal.h
index 916b7cb..2f6af7f 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -110,6 +110,7 @@ extern int open_check_o_direct(struct file *f);
  * inode.c
  */
 extern spinlock_t inode_sb_list_lock;
+extern void inode_add_lru(struct inode *inode);
 
 /*
  * fs-writeback.c
-- 
1.7.1


--5/uDoXvLw7AC5HRs--
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