linux-kernel - Re: oops in copy_page

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Message-ID: <CA+55aFzaTvF7nYxWBT-G_b=xGz+_akRAeJ=U9iHy+Y=ZPo=pbA@mail.gmail.com>
Date:	Tue, 8 Jan 2013 08:52:14 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
Cc:	Hillf Danton <dhillf@...il.com>, Hugh Dickins <hughd@...gle.com>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Linux-MM <linux-mm@...ck.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: oops in copy_page_rep()

On Tue, Jan 8, 2013 at 8:31 AM, Kirill A. Shutemov <kirill@...temov.name> wrote:
>>
>> Heh. I was more thinking about why do_huge_pmd_wp_page() needs it, but
>> do_huge_pmd_numa_page() does not.
>
> It does. The check should be moved up.
>
>> Also, do we actually need it for huge_pmd_set_accessed()? The
>> *placement* of that thing confuses me. And because it confuses me, I'd
>> like to understand it.
>
> We need it for huge_pmd_set_accessed() too.
>
> Looks like a mis-merge. The original patch for huge_pmd_set_accessed() was
> correct: http://lkml.org/lkml/2012/10/25/402

Not a merge error: the pmd_trans_splitting() check was removed by
commit d10e63f29488 ("mm: numa: Create basic numa page hinting
infrastructure").

Now, *why* it was removed, I can't tell. And it's not clear why the
original code just had it in a conditional, while the suggested patch
has that "goto repeat" thing. I suspect re-trying the fault (which I
assume the original code did) is actually better, because that way you
go through all the "should I reschedule as I return through the
exception" stuff. I dunno.

Mel, that original patch came from you , although it was based on
previous work by Peter/Ingo/Andrea. Can you walk us through the
history and thinking about the loss of pmd_trans_splitting(). Was it
purely a mistake? It looks intentional.

               Linus
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