linux-ext4 - Re: [PATCH v3 4/6] ext4: change lru to round-robin in extent status tree shrinker

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Message-ID: <20140904075010.GJ20473@dastard>
Date:	Thu, 4 Sep 2014 17:50:10 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Jan Kara <jack@...e.cz>, Zheng Liu <gnehzuil.liu@...il.com>,
	linux-ext4@...r.kernel.org,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Zheng Liu <wenqing.lz@...bao.com>
Subject: Re: [PATCH v3 4/6] ext4: change lru to round-robin in extent status
 tree shrinker

On Wed, Sep 03, 2014 at 04:00:39PM -0400, Theodore Ts'o wrote:
> On Wed, Sep 03, 2014 at 05:31:22PM +0200, Jan Kara wrote:
> > Also one question:
> > 
> > > -		if (ei == locked_ei || !write_trylock(&ei->i_es_lock)) {
> > > -			nr_skipped++;
> > > -			spin_lock(&sbi->s_es_lock);
> > >  			__ext4_es_list_add(sbi, ei);
> > > +			if (spin_is_contended(&sbi->s_es_lock)) {
> > > +				spin_unlock(&sbi->s_es_lock);
> > > +				spin_lock(&sbi->s_es_lock);
> > > +			}
> >   Why not cond_resched_lock(&sbi->s_es_lock)?
> 
> I didn't think we were allowed to reschedule or sleep while in
> shrinker context?

You are allowed to block shrinkers if there is no possibility of
deadlock. i.e. that's what the __GFP_FS flag check in filesystem
shrinkers is for - so that they only run in GFP_KERNEL context
and not GFP_NOFS/GFP_NOIO/GFP_ATOMIC context where blocking reclaim
can cause deadlocks...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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