linux-ext4 - Re: [PATCH 01/10] mbcache: Don't reclaim used entries

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Date:   Thu, 14 Jul 2022 17:17:02 +0530
From:   Ritesh Harjani <ritesh.list@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Ted Tso <tytso@....edu>, linux-ext4@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 01/10] mbcache: Don't reclaim used entries

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@...r.kernel.org
> Fixes: 82939d7999df ("ext4: convert to mbcache2")
> Signed-off-by: Jan Kara <jack@...e.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
>