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

[Michal Hocko]

On Wed 27-09-17 17:04:32, Minchan Kim wrote:
> On Wed, Sep 27, 2017 at 09:48:35AM +0200, Michal Hocko wrote:
> > On Wed 27-09-17 14:04:01, Minchan Kim wrote:
[...]
> > > The problem is users have disabled swap readahead by echo 0 > /proc/sys/
> > > vm/page-cluster are regressed by this new interface /sys/kernel/mm/swap/
> > > vma_ra_max_order. Because for disabling readahead completely, they should
> > > disable vma_ra_max_order as well as page-cluster from now on.
> > > 
> > > So, goal of new config to notice new feature to admins so they can be aware
> > > of new konb vma_ra_max_order as well as page-cluster.
> > > I canont think other better idea to preventing such regression.
> > > 
> > > http://lkml.kernel.org/r/%3C20170913014019.GB29422@bbox%3E
> > 
> > So, how are you going to configure this when you do not know whether
> > zram will be used? In other words what should e.g. distribution set this
> > to?
> 
> I have no idea. Unfortunately, it depends on them. If they want to use
> zram as swap, they should fix the script. Surely, I don't like it.
> Instead, I wanted that page-cluster zeroing disables both virtual/pysical
> swap readahead not to break current userspace. However, Huang doesn't
> liek it.
> If you have better idea, please suggest.

I understand your frustration but config options are not there to bypass
proper design decisions. Why cannot we unconditionally disable all the
read ahead when zram is enabled?
-- 
Michal Hocko
SUSE Labs