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Subject[PATCH 16/18] fs: Reduce inode I_FREEING and factor inode disposal
Date
From: Dave Chinner <dchinner@redhat.com>

Inode reclaim can push many inodes into the I_FREEING state before
it actually frees them. During the time it gathers these inodes, it
can call iput(), invalidate_mapping_pages, be preempted, etc. As a
result, holding inodes in I_FREEING can cause pauses.

After the inode scalability work, there is not a big reason to batch
up inodes to reclaim them, so we can dispose them as they are found
from the LRU.

Unmount does a very similar reclaim process via invalidate_list(),
but currently uses the i_lru list to aggregate inodes for a batched
disposal. This requires taking the inode_lru_lock for every inode we
want to dispose. Instead, take the inodes off the superblock inode
list (as we already hold the lock) and use i_sb_list as the
aggregator for inodes to dispose to reduce lock traffic.

Further, iput_final() does the same inode cleanup as reclaim and
unmount, so convert them all to use a single function for destroying
inodes. This is written such that the callers can optimise list
removals to avoid unneccessary lock round trips when removing inodes
from lists.

Based on a patch originally from Nick Piggin.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
fs/inode.c | 116 +++++++++++++++++++++++++++++++++---------------------------
1 files changed, 64 insertions(+), 52 deletions(-)
diff --git a/fs/inode.c b/fs/inode.c
index e58524d..5fed0e5 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -31,6 +31,8 @@
/*
* Locking rules.
*
+ * inode->i_lock is *always* the innermost lock.
+ *
* inode->i_lock protects:
* i_ref i_state
* inode hash lock protects:
@@ -48,8 +50,15 @@
*
* sb inode lock
* inode_lru_lock
- * wb->b_lock
- * inode->i_lock
+ * wb->b_lock
+ * inode->i_lock
+ *
+ * wb->b_lock
+ * sb_lock (pin sb for writeback)
+ * inode->i_lock
+ *
+ * inode_lru
+ * inode->i_lock
*/
/*
* This is needed for the following functions:
@@ -452,6 +461,44 @@ static void evict(struct inode *inode)
}

/*
+ * Free the inode passed in, removing it from the lists it is still connected
+ * to but avoiding unnecessary lock round-trips for the lists it is no longer
+ * on.
+ *
+ * An inode must already be marked I_FREEING so that we avoid the inode being
+ * moved back onto lists if we race with other code that manipulates the lists
+ * (e.g. writeback_single_inode). The caller
+ */
+static void dispose_one_inode(struct inode *inode)
+{
+ BUG_ON(!(inode->i_state & I_FREEING));
+
+ /*
+ * move the inode off the IO lists and LRU once
+ * I_FREEING is set so that it won't get moved back on
+ * there if it is dirty.
+ */
+ if (!list_empty(&inode->i_wb_list))
+ inode_wb_list_del(inode);
+
+ if (!list_empty(&inode->i_lru))
+ inode_lru_list_del(inode);
+
+ if (!list_empty(&inode->i_sb_list)) {
+ spin_lock(&inode->i_sb->s_inodes_lock);
+ list_del_init(&inode->i_sb_list);
+ spin_unlock(&inode->i_sb->s_inodes_lock);
+ }
+
+ evict(inode);
+
+ remove_inode_hash(inode);
+ wake_up_inode(inode);
+ BUG_ON(inode->i_state != (I_FREEING | I_CLEAR));
+ destroy_inode(inode);
+}
+
+/*
* dispose_list - dispose of the contents of a local list
* @head: the head of the list to free
*
@@ -463,18 +510,10 @@ static void dispose_list(struct list_head *head)
while (!list_empty(head)) {
struct inode *inode;

- inode = list_first_entry(head, struct inode, i_lru);
- list_del_init(&inode->i_lru);
-
- evict(inode);
-
- remove_inode_hash(inode);
- spin_lock(&inode->i_sb->s_inodes_lock);
+ inode = list_first_entry(head, struct inode, i_sb_list);
list_del_init(&inode->i_sb_list);
- spin_unlock(&inode->i_sb->s_inodes_lock);

- wake_up_inode(inode);
- destroy_inode(inode);
+ dispose_one_inode(inode);
}
}

@@ -515,17 +554,8 @@ static int invalidate_list(struct super_block *sb, struct list_head *head,
inode->i_state |= I_FREEING;
spin_unlock(&inode->i_lock);

- /*
- * move the inode off the IO lists and LRU once
- * I_FREEING is set so that it won't get moved back on
- * there if it is dirty.
- */
- inode_wb_list_del(inode);
-
- spin_lock(&inode_lru_lock);
- list_move(&inode->i_lru, dispose);
- percpu_counter_dec(&nr_inodes_unused);
- spin_unlock(&inode_lru_lock);
+ /* save a lock round trip by removing the inode here. */
+ list_move(&inode->i_sb_list, dispose);
continue;
}
spin_unlock(&inode->i_lock);
@@ -544,17 +574,17 @@ static int invalidate_list(struct super_block *sb, struct list_head *head,
*/
int invalidate_inodes(struct super_block *sb)
{
- int busy;
LIST_HEAD(throw_away);
+ int busy;

down_write(&iprune_sem);
spin_lock(&sb->s_inodes_lock);
fsnotify_unmount_inodes(&sb->s_inodes);
busy = invalidate_list(sb, &sb->s_inodes, &throw_away);
spin_unlock(&sb->s_inodes_lock);
+ up_write(&iprune_sem);

dispose_list(&throw_away);
- up_write(&iprune_sem);

return busy;
}
@@ -592,7 +622,6 @@ static int can_unuse(struct inode *inode)
*/
static void prune_icache(int nr_to_scan)
{
- LIST_HEAD(freeable);
int nr_scanned;
unsigned long reap = 0;

@@ -652,15 +681,15 @@ static void prune_icache(int nr_to_scan)
inode->i_state |= I_FREEING;
spin_unlock(&inode->i_lock);

- /*
- * move the inode off the io lists and lru once
- * i_freeing is set so that it won't get moved back on
- * there if it is dirty.
- */
- inode_wb_list_del(inode);
-
- list_move(&inode->i_lru, &freeable);
+ /* save a lock round trip by removing the inode here. */
+ list_del_init(&inode->i_lru);
percpu_counter_dec(&nr_inodes_unused);
+ spin_unlock(&inode_lru_lock);
+
+ dispose_one_inode(inode);
+ cond_resched();
+
+ spin_lock(&inode_lru_lock);
}
if (current_is_kswapd())
__count_vm_events(KSWAPD_INODESTEAL, reap);
@@ -668,7 +697,6 @@ static void prune_icache(int nr_to_scan)
__count_vm_events(PGINODESTEAL, reap);
spin_unlock(&inode_lru_lock);

- dispose_list(&freeable);
up_read(&iprune_sem);
}

@@ -1453,23 +1481,7 @@ static void iput_final(struct inode *inode)
inode->i_state |= I_FREEING;
spin_unlock(&inode->i_lock);

- /*
- * After we delete the inode from the LRU and IO lists here, we avoid
- * moving dirty inodes back onto the LRU now because I_FREEING is set
- * and hence writeback_single_inode() won't move the inode around.
- */
- inode_wb_list_del(inode);
- inode_lru_list_del(inode);
-
- spin_lock(&sb->s_inodes_lock);
- list_del_init(&inode->i_sb_list);
- spin_unlock(&sb->s_inodes_lock);
-
- evict(inode);
- remove_inode_hash(inode);
- wake_up_inode(inode);
- BUG_ON(inode->i_state != (I_FREEING | I_CLEAR));
- destroy_inode(inode);
+ dispose_one_inode(inode);
}

/**
--
1.7.1


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