Re: [PATCH 01/10] mbcache: Don't reclaim used entries

Ritesh Harjani
2022-07-14T11:47:11Z

On 22/07/12 12:54PM, Jan Kara wrote:
> Do not reclaim entries that are currently used by somebody from a
> shrinker. Firstly, these entries are likely useful. Secondly, we will
> need to keep such entries to protect pending increment of xattr block
> refcount.
>
> CC: stable@vger.kernel.org
> Fixes: 82939d7999df ("ext4: convert to mbcache2")
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
> fs/mbcache.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/fs/mbcache.c b/fs/mbcache.c
> index 97c54d3a2227..cfc28129fb6f 100644
> --- a/fs/mbcache.c
> +++ b/fs/mbcache.c
> @@ -288,7 +288,7 @@ static unsigned long mb_cache_shrink(struct mb_cache *cache,
> while (nr_to_scan-- && !list_empty(&cache->c_list)) {
> entry = list_first_entry(&cache->c_list,
> struct mb_cache_entry, e_list);
> - if (entry->e_referenced) {
> + if (entry->e_referenced || atomic_read(&entry->e_refcnt) > 2) {
> entry->e_referenced = 0;
> list_move_tail(&entry->e_list, &cache->c_list);
> continue;
> @@ -302,6 +302,14 @@ static unsigned long mb_cache_shrink(struct mb_cache *cache,
> spin_unlock(&cache->c_list_lock);
> head = mb_cache_entry_head(cache, entry->e_key);
> hlist_bl_lock(head);
> + /* Now a reliable check if the entry didn't get used... */
> + if (atomic_read(&entry->e_refcnt) > 2) {

On taking a look at this patchset again. I think if we move this "if" condition
of checking refcnt to above i.e. before we delete the entry from c_list.
Then we can avoid =>
removing of the entry -> checking it's refcnt under lock -> adding it back
if the refcnt is elevated.

Thoughts?

-ritesh

> + hlist_bl_unlock(head);
> + spin_lock(&cache->c_list_lock);
> + list_add_tail(&entry->e_list, &cache->c_list);
> + cache->c_entry_count++;
> + continue;
> + }
> if (!hlist_bl_unhashed(&entry->e_hash_list)) {
> hlist_bl_del_init(&entry->e_hash_list);
> atomic_dec(&entry->e_refcnt);
> --
> 2.35.3
>