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Subject: Re: [PATCH next] powerpc/mm: fix _PAGE_SWP_SOFT_DIRTY breaking swapoff
To: Hugh Dickins <hughd@google.com>
References: <alpine.LSU.2.11.1601091651130.9808@eggly.anvils>
Cc: Andrew Morton <akpm@linux-foundation.org>,
 Michael Ellerman <mpe@ellerman.id.au>,
 "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
 Cyrill Gorcunov <gorcunov@gmail.com>,
 Martin Schwidefsky <schwidefsky@de.ibm.com>,
 linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org
From: Laurent Dufour <ldufour@linux.vnet.ibm.com>
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Date: Mon, 11 Jan 2016 17:04:34 +0100
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On 10/01/2016 01:54, Hugh Dickins wrote:
> Swapoff after swapping hangs on the G5, when CONFIG_CHECKPOINT_RESTORE=y
> but CONFIG_MEM_SOFT_DIRTY is not set.  That's because the non-zero
> _PAGE_SWP_SOFT_DIRTY bit, added by CONFIG_HAVE_ARCH_SOFT_DIRTY=y, is not
> discounted when CONFIG_MEM_SOFT_DIRTY is not set: so swap ptes cannot be
> recognized.
> 
> (I suspect that the peculiar dependence of HAVE_ARCH_SOFT_DIRTY on
> CHECKPOINT_RESTORE in arch/powerpc/Kconfig comes from an incomplete
> attempt to solve this problem.)
> 
> It's true that the relationship between CONFIG_HAVE_ARCH_SOFT_DIRTY and
> and CONFIG_MEM_SOFT_DIRTY is too confusing, and it's true that swapoff
> should be made more robust; but nevertheless, fix up the powerpc ifdefs
> as x86_64 and s390 (which met the same problem) have them, defining the
> bits as 0 if CONFIG_MEM_SOFT_DIRTY is not set.
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>

Acked-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>

Thanks, Hugh!