Re: [Question] Why PageReadahead is not migrated by migration code?

[Yang Shi]

On 2/13/20 9:33 AM, Matthew Wilcox wrote:
> On Thu, Feb 13, 2020 at 09:06:58AM -0800, Yang Shi wrote:
> > Recently we saw some PageReadahead related bugs, so I did a quick check
> > about the use of PageReadahead. I just found the state is *not* migrated by
> > migrate_page_states().
> >
> > Since migrate_page() won't migrate writeback page, so if PageReadahead is
> > set it should just mean PG_readahead rather than PG_reclaim. So, I didn't
> > think of why it is not migrated.
> >
> > I dig into the history a little bit, but the change in migration code is too
> > overwhelming. But, it looks PG_readahead was added after migration was
> > introduced. Is it just a simple omission?
> It's probably more that it just doesn't matter enough.  If the Readahead
> flag is missing on a page then the application will perform slightly worse
> for a few pages as it ramps its readahead back up again.  On the other
> hand, you just migrated its pages to a different NUMA node, so chances
> are there are bigger perofmrance problems happening at this moment anyway.
>
> I think we probably should migrate it, but I can understand why nobody's
> noticed it before.

Thanks. I tend to agree the slight performance loss might be hidden by 
other things.