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

[Huang\, Ying]

Hi, Andrew,

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:

> On Wed, 13 Sep 2017 10:40:19 +0900 Minchan Kim <minchan@xxxxxxxxxx> 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@xxxxxxxxxx%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?

This sounds good for me.

Hi, Minchan, what do you think about this?  I think for low-end android
device, the end-user may have no opportunity to upgrade to the latest
kernel, the device vendor should care about this.  For desktop users,
the warning proposed by Andrew may help to remind them for the new knob.

Best Regards,
Huang, Ying