xfs
[Top] [All Lists]

Re: [PATCH 3/5] superblock: introduce per-sb cache shrinker infrastructu

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 3/5] superblock: introduce per-sb cache shrinker infrastructure
From: Nick Piggin <npiggin@xxxxxxx>
Date: Thu, 27 May 2010 12:19:05 +1000
Cc: linux-kernel@xxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-mm@xxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <20100526231214.GB1395@dastard>
References: <1274777588-21494-1-git-send-email-david@xxxxxxxxxxxxx> <1274777588-21494-4-git-send-email-david@xxxxxxxxxxxxx> <20100526164116.GD22536@laptop> <20100526231214.GB1395@dastard>
User-agent: Mutt/1.5.20 (2009-06-14)
On Thu, May 27, 2010 at 09:12:14AM +1000, Dave Chinner wrote:
> On Thu, May 27, 2010 at 02:41:16AM +1000, Nick Piggin wrote:
> > On Tue, May 25, 2010 at 06:53:06PM +1000, Dave Chinner wrote:
> > > @@ -456,21 +456,16 @@ static void prune_one_dentry(struct dentry * dentry)
> > > + /*
> > > +  * if we can't get the umount lock, then there's no point having the
> > > +  * shrinker try again because the sb is being torn down.
> > > +  */
> > > + if (!down_read_trylock(&sb->s_umount))
> > > +         return -1;
> > 
> > Would you just elaborate on the lock order problem somewhere? (the
> > comment makes it look like we *could* take the mutex if we wanted
> > to).
> 
> The shrinker is unregistered in deactivate_locked_super() which is
> just before ->kill_sb is called. The sb->s_umount lock is held at
> this point. hence is the shrinker is operating, we will deadlock if
> we try to lock it like this:
> 
>       unmount:                        shrinker:
>                                       down_read(&shrinker_lock);
>       down_write(&sb->s_umount)
>       unregister_shrinker()
>       down_write(&shrinker_lock)
>                                       prune_super()
>                                         down_read(&sb->s_umount);
>                                         (deadlock)
> 
> hence if we can't get the sb->s_umount lock in prune_super(), then
> the superblock must be being unmounted and the shrinker should abort
> as the ->kill_sb method will clean up everything after the shrinker
> is unregistered. Hence the down_read_trylock().

You added it to the comment in your updated patch, that was the main
thing I wanted. Thanks.


> > > + if (!sb->s_root) {
> > > +         up_read(&sb->s_umount);
> > > +         return -1;
> > > + }
> > > +
> > > + if (nr_to_scan) {
> > > +         /* proportion the scan between the two cacheѕ */
> > > +         int total;
> > > +
> > > +         total = sb->s_nr_dentry_unused + sb->s_nr_inodes_unused + 1;
> > > +         count = (nr_to_scan * sb->s_nr_dentry_unused) / total;
> > > +
> > > +         /* prune dcache first as icache is pinned by it */
> > > +         prune_dcache_sb(sb, count);
> > > +         prune_icache_sb(sb, nr_to_scan - count);
> > > + }
> > > +
> > > + count = ((sb->s_nr_dentry_unused + sb->s_nr_inodes_unused) / 100)
> > > +                                         * sysctl_vfs_cache_pressure;
> > 
> > Do you think truncating in the divisions is at all a problem? It
> > probably doesn't matter much I suppose.
> 
> Same code as currently exists. IIRC, the reasoning is that if we've
> got less that 100 objects to reclaim, then we're unlikely to be able
> to free up any memory from the caches, anyway.

Yeah, which is why I stop short of saying you should change it in
this patch.

But I think we should ensure things can get reclaimed eventually.
100 objects could be 100 slabs, which could be anything from
half a meg to half a dozen. Multiplied by each of the caches.
Could be significant in small systems.

<Prev in Thread] Current Thread [Next in Thread>