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

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Date:   Thu, 16 Jun 2022 19:52:12 +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 22/06/14 06:05PM, 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.

Trying to review the patch series to best of my knowledge, so kindly excuse my
silly queries along the way.

>
> 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) {

Sure, yes, the above "||" conditions looks good.
i.e. if the refcnt is above 2, then we should move the entry to the tail of LRU.
Because that means that there is another user of this entry which might have
incremented the refcnt.

>  			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... */

But not sure why this is more reliable? Anytime we add or remove the entry,
we first always do the list operation and then increment or decrement the
refcnt "atomically".

So could you please help in understanding why will this be more reliable?

-ritesh


> +		if (atomic_read(&entry->e_refcnt) > 2) {
> +			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
>