linux-kernel - Re: [PATCH] mm/swap: piggyback lru_add_drain

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Message-ID: <20191004123718.GI9578@dhcp22.suse.cz>
Date:   Fri, 4 Oct 2019 14:37:18 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/swap: piggyback lru_add_drain_all() calls

On Fri 04-10-19 15:32:01, Konstantin Khlebnikov wrote:
> 
> 
> On 04/10/2019 15.27, Michal Hocko wrote:
> > On Fri 04-10-19 05:10:17, Matthew Wilcox wrote:
> > > On Fri, Oct 04, 2019 at 01:11:06PM +0300, Konstantin Khlebnikov wrote:
> > > > This is very slow operation. There is no reason to do it again if somebody
> > > > else already drained all per-cpu vectors after we waited for lock.
> > > > +	seq = raw_read_seqcount_latch(&seqcount);
> > > > +
> > > >   	mutex_lock(&lock);
> > > > +
> > > > +	/* Piggyback on drain done by somebody else. */
> > > > +	if (__read_seqcount_retry(&seqcount, seq))
> > > > +		goto done;
> > > > +
> > > > +	raw_write_seqcount_latch(&seqcount);
> > > > +
> > > 
> > > Do we really need the seqcount to do this?  Wouldn't a mutex_trylock()
> > > have the same effect?
> > 
> > Yeah, this makes sense. From correctness point of view it should be ok
> > because no caller can expect that per-cpu pvecs are empty on return.
> > This might have some runtime effects that some paths might retry more -
> > e.g. offlining path drains pcp pvces before migrating the range away, if
> > there are pages still waiting for a worker to drain them then the
> > migration would fail and we would retry. But this not a correctness
> > issue.
> > 
> 
> Caller might expect that pages added by him before are drained.
> Exiting after mutex_trylock() will not guarantee that.
> 
> For example POSIX_FADV_DONTNEED uses that.

OK, I was not aware of this case. Please make sure to document that in
the changelog and a comment in the code wouldn't hurt either. It would
certainly explain more thatn "Piggyback on drain done by somebody
else.".

Thanks!
-- 
Michal Hocko
SUSE Labs