linux-kernel - Re: mm: fix BUG in __split_huge_page

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Message-ID: <1381865330-8nb86ucy-mutt-n-horiguchi@ah.jp.nec.com>
Date:	Tue, 15 Oct 2013 15:28:50 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Hugh Dickins <hughd@...gle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: mm: fix BUG in __split_huge_page_pmd

On Tue, Oct 15, 2013 at 08:55:10PM +0200, Andrea Arcangeli wrote:
> On Tue, Oct 15, 2013 at 10:53:10AM -0700, Hugh Dickins wrote:
> > I'm afraid Andrea's mail about concurrent madvises gives me far more
> > to think about than I have time for: seems to get into problems he
> > knows a lot about but I'm unfamiliar with.  If this patch looks good
> > for now on its own, let's put it in; but no problem if you guys prefer
> > to wait for a fuller solution of more problems, we can ride with this
> > one internally for the moment.
> 
> I'm very happy with the patch and I think it's a correct fix for the
> COW scenario which is deterministic so the looping makes a meaningful
> difference for it. If we wouldn't loop, part of the copied page
> wouldn't be zapped after the COW.

I like this patch, too.

If we have the loop in __split_huge_page_pmd as suggested in this patch,
can we assume that the pmd is stable after __split_huge_page_pmd returns?
If it's true, we can remove pmd_none_or_trans_huge_or_clear_bad check
in the callers side (zap_pmd_range and some other page table walking code.)

Naoya Horiguchi
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