Re: [PATCH] mm, swap: Make VMA based swap readahead configurable

[Michal Hocko]

On Thu 28-09-17 09:02:20, Huang, Ying wrote:
> Hi, Michal,
> 
> Michal Hocko <mhocko@xxxxxxxxxx> writes:
> 
> > On Wed 27-09-17 23:10:08, Minchan Kim wrote:
> >> On Wed, Sep 27, 2017 at 03:50:34PM +0200, Michal Hocko wrote:
> >> > On Wed 27-09-17 22:41:17, Minchan Kim wrote:
> >> > > On Wed, Sep 27, 2017 at 03:22:41PM +0200, Michal Hocko wrote:
> >> > [...]
> >> > > > simply cannot disable swap readahead when page-cluster is 0?
> >> > > 
> >> > > That's was what I want really but Huang want to use two readahead
> >> > > algorithms in parallel so he wanted to keep two separated disable
> >> > > knobs.
> >> > 
> >> > If it breaks existing and documented behavior then it is a clear
> >> > regression and it should be fixed. I do not see why this should be
> >> > disputable at all.
> >> 
> >> Indeed but Huang doesn't think so. He has thought it's not a regression.
> >> Frankly speaking, I'm really bored of discussing with it.
> >> https://marc.info/?l=linux-mm&m=150526413319763&w=2
> >
> > Then send a patch explaining why you consider this a regression with
> > some numbers backing it and I will happily ack it.
> 
> I still think there may be a performance regression for some users
> because of the change of the algorithm and the knobs, and the
> performance regression can be resolved via setting the new knob.  But I
> don't think there will be a functionality regression.  Do you agree?

I am not sure I understand. One thing is clear though. Your change has
introduced a regression as described by Minchan. And that has to be
resolved no matter what. You cannot expect users will tweak the system
to resolve it or configure their systems in a specific way.

-- 
Michal Hocko
SUSE Labs