linux-kernel - Re: oops in copy_page

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Message-ID: <CA+55aFwcDs_R0Cv=RS2LD8ggP3EdvODjENAsXNe126xNwYatOQ@mail.gmail.com>
Date:	Tue, 8 Jan 2013 10:21:00 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Hillf Danton <dhillf@...il.com>,
	Hugh Dickins <hughd@...gle.com>, Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	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 10:03 AM, Andrea Arcangeli <aarcange@...hat.com> wrote:
>
> It looks very fine to me, but I suggest to move it above the
> pmd_numa() check because of the newly introduced
> migrate_misplaced_transhuge_page method relying on pmd_same too.

Hmm. If we need it there, then we need to fix the *later* case of
pmd_numa() too:

        if (pmd_numa(*pmd))
                return do_pmd_numa_page(mm, vma, address, pmd);

Also, and more fundamentally, since do_pmd_numa_page() doesn't take
the orig_pmd thing as an argument (and re-check it under the
page-table lock), testing pmd_trans_splitting() on it is pointless,
since it can change later.

So no, moving the check up does *not* make sense, at least not without
other changes. Because if I read things right, pmd_trans_splitting()
really has to be done with the page-table lock protection (where "with
page-table lock protection" does *not* mean that it has to be done
under the page table lock, but if it is done outside, then the pmd
entry has to be re-verified after getting the lock - which both
do_huge_pmd_wp_page() and huge_pmd_set_accessed() correctly do).

Comments?

                Linus
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