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Message-ID: <20220714144902.vptbdgmbkd5nxara@riteshh-domain> Date: Thu, 14 Jul 2022 20:19:02 +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 22/07/14 04:36PM, Jan Kara wrote: > 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 Ok, yes. I tried implementing it and it becomes lock()/unlock() mess. > 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... I agree. Thanks for re-checking it. -ritesh
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