Re: [PATCH v3 4/5] mm: introduce MADV_PAGEOUT

[Minchan Kim <minchan@kernel.org>]

On Wed, Jul 10, 2019 at 09:47:19PM +0200, Michal Hocko wrote:
> On Wed 10-07-19 20:53:56, Minchan Kim wrote:
> > On Wed, Jul 10, 2019 at 01:16:22PM +0200, Michal Hocko wrote:
> > > On Wed 10-07-19 19:48:09, Minchan Kim wrote:
> > > > On Tue, Jul 09, 2019 at 11:55:19AM +0200, Michal Hocko wrote:
> > > [...]
> > > > > I am still not convinced about the SWAP_CLUSTER_MAX batching and the
> > > > > udnerlying OOM argument. Is one pmd worth of pages really an OOM risk?
> > > > > Sure you can have many invocations in parallel and that would add on
> > > > > but the same might happen with SWAP_CLUSTER_MAX. So I would just remove
> > > > > the batching for now and think of it only if we really see this being a
> > > > > problem for real. Unless you feel really strong about this, of course.
> > > > 
> > > > I don't have the number to support SWAP_CLUSTER_MAX batching for hinting
> > > > operations. However, I wanted to be consistent with other LRU batching
> > > > logic so that it could affect altogether if someone try to increase
> > > > SWAP_CLUSTER_MAX which is more efficienty for batching operation, later.
> > > > (AFAIK, someone tried it a few years ago but rollback soon, I couldn't
> > > > rebemeber what was the reason at that time, anyway).
> > > 
> > > Then please drop this part. It makes the code more complex while any
> > > benefit is not demonstrated.
> > 
> > The history says the benefit.
> > https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/patch/?id=d37dd5dcb955dd8c2cdd4eaef1f15d1b7ecbc379
> 
> Limiting the number of isolated pages is fine. All I am saying is that
> SWAP_CLUSTER_MAX is an arbitrary number same as 512 pages for one PMD as
> a unit of work. Both can lead to the same effect if there are too many
> parallel tasks doing the same thing.
> 
> I do not want you to change that in the reclaim path. All I am asking
> for is to add a bathing without any actual data to back that because
> that makes the code more complex without any gains.

I understand what you meant and I'm really one to make code simple.
However, my concern was that we have isolated by SWAP_CLUSTER_MAX(32 pages)
for other path(reclaim/compaction) so I want to be consistent with others.
If you think that the consistency(IOW, others are 32 limit but here 256
limit) is no helpful this case, I don't have any strong opinion.
Let's drop the part. I will add it into description, then.

Thanks.

> -- 
> Michal Hocko
> SUSE Labs