Re: [PATCH] mm/swap: piggyback lru_add_drain_all() calls

[Konstantin Khlebnikov]

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.