Re: [PATCH -V3] swap: Reduce lock contention on swap cache from swap slots allocation

[Daniel Jordan <daniel.m.jordan@oracle.com>]

On Thu, May 28, 2020 at 01:32:40PM +0800, Huang, Ying wrote:
> Daniel Jordan <daniel.m.jordan@oracle.com> writes:
> 
> > On Mon, May 25, 2020 at 08:26:48AM +0800, Huang Ying wrote:
> >> diff --git a/mm/swapfile.c b/mm/swapfile.c
> >> index 423c234aca15..0abd93d2a4fc 100644
> >> --- a/mm/swapfile.c
> >> +++ b/mm/swapfile.c
> >> @@ -615,7 +615,8 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
> >>  			 * discarding, do discard now and reclaim them
> >>  			 */
> >>  			swap_do_scheduled_discard(si);
> >> -			*scan_base = *offset = si->cluster_next;
> >> +			*scan_base = this_cpu_read(*si->cluster_next_cpu);
> >> +			*offset = *scan_base;
> >>  			goto new_cluster;
> >
> > Why is this done?  As far as I can tell, the values always get overwritten at
> > the end of the function with tmp and tmp isn't derived from them.  Seems
> > ebc2a1a69111 moved some logic that used to make sense but doesn't have any
> > effect now.
> 
> If we fail to allocate from cluster, "scan_base" and "offset" will not
> be overridden.

Ok, if another task races to allocate the clusters the first just discarded.

> And "cluster_next" or "cluster_next_cpu" may be changed
> in swap_do_scheduled_discard(), because the lock is released and
> re-acquired there.

I see, by another task on the same cpu for cluster_next_cpu.

Both probably unlikely, but at least it tries to pick up where the racing task
left off.  You might tack this onto the comment:

		 * discarding, do discard now and reclaim them, then reread
                 * cluster_next_cpu since we dropped si->lock
                /*

> The code may not have much value.

No, it makes sense.

> > These aside, patch looks good to me.
> 
> Thanks for your review!  It really help me to improve the quality of the
> patch.  Can I add your "Reviewed-by" in the next version?

Sure,
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>