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

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Message-ID: <20200213173348.GS7778@bombadil.infradead.org>
Date:   Thu, 13 Feb 2020 09:33:48 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [Question] Why PageReadahead is not migrated by migration code?

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.