linux-kernel - Re: [PATCH] mm: softdirty: respect VM

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Message-ID: <20140801070156.GE17343@moon>
Date:	Fri, 1 Aug 2014 11:01:56 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Peter Feiner <pfeiner@...gle.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Pavel Emelyanov <xemul@...allels.com>,
	Hugh Dickins <hughd@...gle.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: softdirty: respect VM_SOFTDIRTY in PTE holes

On Thu, Jul 31, 2014 at 06:43:25PM -0400, Peter Feiner wrote:
> After a VMA is created with the VM_SOFTDIRTY flag set,
> /proc/pid/pagemap should report that the VMA's virtual pages are
> soft-dirty until VM_SOFTDIRTY is cleared (i.e., by the next write of
> "4" to /proc/pid/clear_refs). However, pagemap ignores the
> VM_SOFTDIRTY flag for virtual addresses that fall in PTE holes (i.e.,
> virtual addresses that don't have a PMD, PUD, or PGD allocated yet).
> 
> To observe this bug, use mmap to create a VMA large enough such that
> there's a good chance that the VMA will occupy an unused PMD, then
> test the soft-dirty bit on its pages. In practice, I found that a VMA
> that covered a PMD's worth of address space was big enough.
> 
> This patch adds the necessary VMA lookup to the PTE hole callback in
> /proc/pid/pagemap's page walk and sets soft-dirty according to the
> VMAs' VM_SOFTDIRTY flag.
> 
> Signed-off-by: Peter Feiner <pfeiner@...gle.com>
Acked-by: Cyrill Gorcunov <gorcunov@...nvz.org>
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