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From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
To: Hugh Dickins <hughd@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
 Michael Ellerman <mpe@ellerman.id.au>,
 Laurent Dufour <ldufour@linux.vnet.ibm.com>,
 linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org
Subject: Re: [PATCH next] powerpc/mm: fix _PAGE_PTE breaking swapoff
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References: <alpine.LSU.2.11.1601091643060.9808@eggly.anvils>
 <87si24u32t.fsf@linux.vnet.ibm.com>
Date: Mon, 11 Jan 2016 11:25:59 +0530
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"Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> writes:

> Hugh Dickins <hughd@google.com> writes:
>
>> Swapoff after swapping hangs on the G5.  That's because the _PAGE_PTE
>> bit, added by set_pte_at(), is not expected by swapoff: so swap ptes
>> cannot be recognized.
>>
>> I'm not sure whether a swap pte should or should not have _PAGE_PTE set:
>> this patch assumes not, and fixes set_pte_at() to set _PAGE_PTE only on
>> present entries.
>
> One of the reason we added _PAGE_PTE is to enable HUGETLB migration. So
> we want migratio ptes to have _PAGE_PTE set.
>
>>
>> But if that's wrong, a reasonable alternative would be to
>> #define __pte_to_swp_entry(pte)	((swp_entry_t) { pte_val(pte) & ~_PAGE_PTE })
>> #define __swp_entry_to_pte(x)	__pte((x).val | _PAGE_PTE)
>>

You other email w.r.t soft dirty bits explained this. What I missed was
the fact that core kernel expect swp_entry_t to be of an arch neutral
format.  The confusing part was "arch_entry"

static inline pte_t swp_entry_to_pte(swp_entry_t entry)
{
	swp_entry_t arch_entry;
.....
}
	
IMHO we should use the alternative you suggested above. I can write a
patch with additional comments around that if you want me to do that.

-aneesh