From: Matthew Wilcox <willy@infradead.org>
To: Yang Shi <yang.shi@linux.alibaba.com>
Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [Question] Why PageReadahead is not migrated by migration code?
Date: Thu, 13 Feb 2020 09:33:48 -0800 [thread overview]
Message-ID: <20200213173348.GS7778@bombadil.infradead.org> (raw)
In-Reply-To: <7691ab12-2e84-2531-f27d-2fae9045576d@linux.alibaba.com>
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.
next prev parent reply other threads:[~2020-02-13 17:33 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-13 17:06 [Question] Why PageReadahead is not migrated by migration code? Yang Shi
2020-02-13 17:33 ` Matthew Wilcox [this message]
2020-02-13 18:04 ` Yang Shi
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