Re: [PATCH -mm -v4 3/5] mm, swap: VMA based swap readahead

[Minchan Kim <minchan@kernel.org>]

On Wed, Sep 13, 2017 at 02:02:29PM -0700, Andrew Morton wrote:
> On Wed, 13 Sep 2017 10:40:19 +0900 Minchan Kim <minchan@kernel.org> wrote:
> 
> > Every zram users like low-end android device has used 0 page-cluster
> > to disable swap readahead because it has no seek cost and works as
> > synchronous IO operation so if we do readahead multiple pages,
> > swap falut latency would be (4K * readahead window size). IOW,
> > readahead is meaningful only if it doesn't bother faulted page's
> > latency.
> > 
> > However, this patch introduces additional knob /sys/kernel/mm/swap/
> > vma_ra_max_order as well as page-cluster. It means existing users
> > has used disabled swap readahead doesn't work until they should be
> > aware of new knob and modification of their script/code to disable
> > vma_ra_max_order as well as page-cluster.
> > 
> > I say it's a *regression* and wanted to fix it but Huang's opinion
> > is that it's not a functional regression so userspace should be fixed
> > by themselves.
> > Please look into detail of discussion in
> > http://lkml.kernel.org/r/%3C1505183833-4739-4-git-send-email-minchan@kernel.org%3E
> 
> hm, tricky problem.  I do agree that linking the physical and virtual
> readahead schemes in the proposed fashion is unfortunate.  I also agree
> that breaking existing setups (a bit) is also unfortunate.
> 
> Would it help if, when page-cluster is written to zero, we do
> 
> printk_once("physical readahead disabled, virtual readahead still
> enabled.  Disable virtual readhead via
> /sys/kernel/mm/swap/vma_ra_max_order").
> 
> Or something like that.  It's pretty lame, but it should help alert the
> zram-readahead-disabling people to the issue?

It was my last resort. If we cannot find other ways after all, yes, it would
be a minimum we should do. But it still breaks users don't/can't read/modify
alert and program.

How about this?

Can't we make vma-based readahead config option?
With that, users who no interest on readahead don't enable vma-based
readahead. In this case, page-cluster works as expected "disable readahead
completely" so it doesn't break anything.

People who want to use upcoming vma-based readahead can enable the feature
and we can say such unfortunate things in config/document description
somewhere so upcoming users will be aware of that unforunate two knobs.

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