linux-kernel - Re: [PATCH 1/1] mm/khugepaged: skip copying lazyfree pages on collapse

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Message-ID: <ZcCuHmCh7a8fU6_3@tiehlicka>
Date: Mon, 5 Feb 2024 10:45:02 +0100
From: Michal Hocko <mhocko@...e.com>
To: Yang Shi <shy828301@...il.com>
Cc: Lance Yang <ioworker0@...il.com>, akpm@...ux-foundation.org,
	zokeefe@...gle.com, david@...hat.com, songmuchun@...edance.com,
	peterx@...hat.com, minchan@...nel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/khugepaged: skip copying lazyfree pages on
 collapse

On Fri 02-02-24 09:42:27, Yang Shi wrote:
> But if the partial range is MADV_FREE, khugepaged won't skip them.
> This is what your second test case does.
> 
> Secondly, I think it depends on the semantics of MADV_FREE,
> particularly how to treat the redirtied pages. TBH I'm always confused
> by the semantics. For example, the page contained "abcd", then it was
> MADV_FREE'ed, then it was written again with "1234" after "abcd". So
> the user should expect to see "abcd1234" or "00001234".

Correct. You cannot assume the content of the first page as it could
have been reclaimed at any time.
 
> I'm supposed it should be "abcd1234" since MADV_FREE pages are still
> valid and available, if I'm wrong please feel free to correct me. If
> so we should always copy MADV_FREE pages in khugepaged regardless of
> whether it is redirtied or not otherwise it may incur data corruption.
> If we don't copy, then the follow up redirty after collapse to the
> hugepage may return "00001234", right?

Right. As pointed above this is a valid outcome if the page has been
dropped. User has means to tell that from /proc/vmstat though. Not in a
great precision but I think it would be really surprising to not see any
pglazyfreed yet the content is gone. I think it would be legit to call
it a bug. One could argue the bug would be in the accounting rather than
the khugepaged implementation because madvised pages could be dropped at
any time. But I think it makes more sense to copy the existing content.
-- 
Michal Hocko
SUSE Labs