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

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Message-Id: <20131015113254.14E88E0090@blue.fi.intel.com>
Date:	Tue, 15 Oct 2013 14:32:54 +0300 (EEST)
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: RE: mm: fix BUG in __split_huge_page_pmd

Hugh Dickins wrote:
> Occasionally we hit the BUG_ON(pmd_trans_huge(*pmd)) at the end of
> __split_huge_page_pmd(): seen when doing madvise(,,MADV_DONTNEED).
> 
> It's invalid: we don't always have down_write of mmap_sem there:
> a racing do_huge_pmd_wp_page() might have copied-on-write to another
> huge page before our split_huge_page() got the anon_vma lock.
> 
> Forget the BUG_ON, just go back and try again if this happens.
>     
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> Cc: stable@...r.kernel.org

Looks reasonable to me.

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

madvise(MADV_DONTNEED) was aproblematic with THP before. Is a big win having
mmap_sem taken on read rather than on write for it?

-- 
 Kirill A. Shutemov
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