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

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Date:   Thu, 14 Jul 2022 16:36:12 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ritesh Harjani <ritesh.list@...il.com>
Cc:     Jan Kara <jack@...e.cz>, 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 Thu 14-07-22 17:17:02, Ritesh Harjani wrote:
> 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?

Well, but synchronization would get more complicated because we don't want
to acquire hlist_bl_lock() under c_list_lock (technically we could at this
point in the series but it would make life harder for the last patch in the
series). And we need c_list_lock to remove entry from the LRU list. It
could be all done but I don't think what you suggest is really that simpler
and this code will go away later in the patchset anyway...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR