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Re: Re: [PATCH 2.6.19-rc3] VFS: per-sb dentry lru list [message #7937 is a reply to message #7887] Tue, 31 October 2006 04:38 Go to previous messageGo to previous message
Neil Brown is currently offline  Neil Brown
Messages: 6
Registered: October 2006
Junior Member
On Monday October 30, dev@sw.ru wrote:
> David,
>
> >>The proposed fix prevents this issue by using per-sb dentry LRU list. It
> >>provides very quickly search for the unused dentries for given super block thus
> >>forcing shrinking always making good progress.
> >
> >
> > We've been down this path before:
> >
> > http://marc.theaimsgroup.com/?l=linux-kernel&m=114861109 717260&w=2
> >
> > A lot of comments on per-sb unused dentry lists were made in
> > the threads associated with the above. other solutions were
> > found to the problem that the above patch addressed, but I don't
> > think any of them have made it to mainline yet. You might want
> > to have a bit of a read of these threads first...
> The major difference between our patch and the one discussed in the link
> it that we keep both global and per-sb dentry LRU lists.
> Thus, when needed normal LRU is used and prune logic is unchanged,
> while umount/remount use per-sb list and do its job faster.

Yes, we have been down this path before - several times I think.
Below is the patch that I like (not tested recently - just rediffed
and reviewed).

NeilBrown

Subject: Reduce contention in dentry_unused when unmounting.

When we unmount a filesystem we need to release all dentries.
We currently
- move a collection of dentries to the end of the dentry_unused list
- call prune_dcache to prune that number of dentries.

If lots of other dentries are added to the end of the list while
the prune_dcache proceeds (e.g. another filesystem is unmounted),
this can involve a lot of wasted time wandering through the
list looking for dentries that we had previously found.

This patch allows the dentry_unused list to temporarily be multiple
lists.
When unmounting, dentries that are found to require pruning are moved
to a temporary list, but accounted as though they were on dentry_unused.
Then this list is passed to prune_dcache for freeing. Any entries
that are not pruned for whatever reason are added to the end of
dentry_unused.

Also change shrink_dcache_sb to simply call shrink_dcache_parent.
This avoids a long walk of the LRU.

Signed-off-by: Neil Brown <neilb@suse.de>

### Diffstat output
./fs/dcache.c | 104 ++++++++++++++++------------------------------------------
1 file changed, 30 insertions(+), 74 deletions(-)

diff .prev/fs/dcache.c ./fs/dcache.c
--- .prev/fs/dcache.c 2006-10-31 15:22:10.000000000 +1100
+++ ./fs/dcache.c 2006-10-31 15:37:22.000000000 +1100
@@ -384,8 +384,8 @@ static void prune_one_dentry(struct dent
/**
* prune_dcache - shrink the dcache
* @count: number of entries to try and free
- * @sb: if given, ignore dentries for other superblocks
- * which are being unmounted.
+ * @list: If given, remove from this list instead of
+ * from dentry_unused.
*
* Shrink the dcache. This is done when we need
* more memory, or simply when we need to unmount
@@ -394,11 +394,21 @@ static void prune_one_dentry(struct dent
*
* This function may fail to free any resources if
* all the dentries are in use.
+ *
+ * Any dentries that were not removed due to the @count
+ * limit will be splice on to the end of dentry_unused,
+ * so they should already be founded in dentry_stat.nr_unused.
*/

-static void prune_dcache(int count, struct super_block *sb)
+static void prune_dcache(int count, struct list_head *list)
{
+ int have_list = list != NULL;
+
spin_lock(&dcache_lock);
+ if (!have_list)
+ /* use the dentry_unused list */
+ list = &dentry_unused;
+
for (; count ; count--) {
struct dentry *dentry;
struct list_head *tmp;
@@ -406,23 +416,11 @@ static void prune_dcache(int count, stru

cond_resched_lock(&dcache_lock);

- tmp = dentry_unused.prev;
- if (sb) {
- /* Try to find a dentry for this sb, but don't try
- * too hard, if they aren't near the tail they will
- * be moved down again soon
- */
- int skip = count;
- while (skip && tmp != &dentry_unused &&
- list_entry(tmp, struct dentry, d_lru)->d_sb != sb) {
- skip--;
- tmp = tmp->prev;
- }
- }
- if (tmp == &dentry_unused)
+ tmp = list->prev;
+ if (tmp == list)
break;
list_del_init(tmp);
- prefetch(dentry_unused.prev);
+ prefetch(list->prev);
dentry_stat.nr_unused--;
dentry = list_entry(tmp, struct dentry, d_lru);

@@ -455,7 +453,7 @@ static void prune_dcache(int count, stru
* If this dentry is for "my" filesystem, then I can prune it
* without taking the s_umount lock (I already hold it).
*/
- if (sb && dentry->d_sb == sb) {
+ if (have_list) {
prune_one_dentry(dentry);
continue;
}
@@ -485,68 +483,25 @@ static void prune_dcache(int count, stru
list_add(&dentry->d_lru, &dentry_unused);
dentry_stat.nr_unused++;
}
+ /* split any remaining entries back onto dentry_unused */
+ if (have_list)
+ list_splice(list, dentry_unused.prev);
spin_unlock(&dcache_lock);
}

-/*
- * Shrink the dcache for the specified super block.
- * This allows us to unmount a device without disturbing
- * the dcache for the other devices.
- *
- * This implementation makes just two traversals of the
- * unused list. On the first pass we move the selected
- * dentries to the most recent end, and on the second
- * pass we free them. The second pass must restart after
- * each dput(), but since the target dentries are all at
- * the end, it's really just a single traversal.
- */
-
/**
* shrink_dcache_sb - shrink dcache for a superblock
* @sb: superblock
*
* Shrink the dcache for the specified super block. This
- * is used to free the dcache before unmounting a file
- * system
+ * is used to reduce the dcache presence of a file system
+ * before re-mounting, and when invalidating the device
+ * holding a file system.
*/

void shrink_dcache_sb(struct super_block * sb)
{
- struct list_head *tmp, *next;
- struct dentry *dentry;
-
- /*
- * Pass one ... move the dentries for the specified
- * superblock to the most recent end of the unused list.
- */
- spin_lock(&dcache_lock);
- list_for_each_safe(tmp, next, &dentry_unused) {
- dentry = list_entry(tmp, struct dentry, d_lru);
- if (dentry->d_sb != sb)
- continue;
- list_move(tmp, &dentry_unused);
- }
-
- /*
- * Pass two ... free the dentries for this superblock.
- */
-repeat:
- list_for_each_safe(tmp, next, &dentry_unused) {
- dentry = list_entry(tmp, struct dentry, d_lru);
- if (dentry->d_sb != sb)
- continue;
- dentry_stat.nr_unused--;
- list_del_init(tmp);
- spin_lock(&dentry->d_lock);
- if (atomic_read(&dentry->d_count)) {
- spin_unlock(&dentry->d_lock);
- continue;
- }
- prune_one_dentry(dentry);
- cond_resched_lock(&dcache_lock);
- goto repeat;
- }
- spin_unlock(&dcache_lock);
+ shrink_dcache_parent(sb->s_root);
}

/*
@@ -739,7 +694,7 @@ positive:

/*
* Search the dentry child list for the specified parent,
- * and move any unused dentries to the end of the unused
+ * and move any unused dentries to the end of a new unused
* list for prune_dcache(). We descend to the next level
* whenever the d_subdirs list is non-empty and continue
* searching.
@@ -751,7 +706,7 @@ positive:
* drop the lock and return early due to latency
* constraints.
*/
-static int select_parent(struct dentry * parent)
+static int select_parent(struct dentry * parent, struct list_head *new)
{
struct dentry *this_parent = parent;
struct list_head *next;
@@ -775,7 +730,7 @@ resume:
* of the unused list for prune_dcache
*/
if (!atomic_read(&dentry->d_count)) {
- list_add_tail(&dentry->d_lru, &dentry_unused);
+ list_add_tail(&dentry->d_lru, new);
dentry_stat.nr_unused++;
found++;
}
@@ -819,9 +774,10 @@ out:
void shrink_dcache_parent(struct dentry * parent)
{
int found;
+ LIST_HEAD(list);

- while ((found = select_parent(parent)) != 0)
- prune_dcache(found, parent->d_sb);
+ while ((found = select_parent(parent, &list)) != 0)
+ prune_dcache(found, &list);
}

/*

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