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From:   Tiberiu Georgescu <tiberiu.georgescu@nutanix.com>
To:     Peter Xu <peterx@redhat.com>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Axel Rasmussen <axelrasmussen@google.com>,
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        Jerome Glisse <jglisse@redhat.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Alistair Popple <apopple@nvidia.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>,
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        Matthew Wilcox <willy@infradead.org>,
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        Miaohe Lin <linmiaohe@huawei.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Ivan Teterevkov <ivan.teterevkov@nutanix.com>,
        "Carl Waldspurger [C]" <carl.waldspurger@nutanix.com>,
        Florian Schmidt <flosch@nutanix.com>
Subject: Re: [PATCH v5 24/26] mm/pagemap: Recognize uffd-wp bit for
 shmem/hugetlbfs
Thread-Topic: [PATCH v5 24/26] mm/pagemap: Recognize uffd-wp bit for
 shmem/hugetlbfs
Thread-Index: AQHXebZmBTPHtO67e0+5F2gnOfKvWatKFHUAgABnSwCAABZVAA==
Date:   Mon, 19 Jul 2021 17:23:14 +0000
Message-ID: <D2FD5D85-BA6D-492E-801F-E5003452DA70@nutanix.com>
References: <20210715201422.211004-1-peterx@redhat.com>
 <20210715201651.212134-1-peterx@redhat.com>
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> On 19 Jul 2021, at 17:03, Peter Xu <peterx@redhat.com> wrote:
>=20
> On Mon, Jul 19, 2021 at 09:53:36AM +0000, Tiberiu Georgescu wrote:
>>=20
>> Hello Peter,
>=20
> Hi, Tiberiu,
>=20
>>=20
>>> On 15 Jul 2021, at 21:16, Peter Xu <peterx@redhat.com> wrote:
>>>=20
>>> This requires the pagemap code to be able to recognize the newly introd=
uced
>>> swap special pte for uffd-wp, meanwhile the general case for hugetlb th=
at we
>>> recently start to support.  It should make pagemap uffd-wp support comp=
lete.
>>>=20
>>> Signed-off-by: Peter Xu <peterx@redhat.com>
>>> ---
>>> fs/proc/task_mmu.c | 7 +++++++
>>> 1 file changed, 7 insertions(+)
>>>=20
>>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>>> index 9c5af77b5290..988e29fa1f00 100644
>>> --- a/fs/proc/task_mmu.c
>>> +++ b/fs/proc/task_mmu.c
>>> @@ -1389,6 +1389,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struc=
t pagemapread *pm,
>>> 		flags |=3D PM_SWAP;
>>> 		if (is_pfn_swap_entry(entry))
>>> 			page =3D pfn_swap_entry_to_page(entry);
>>> +	} else if (pte_swp_uffd_wp_special(pte)) {
>>> +		flags |=3D PM_UFFD_WP;
>>> 	}
>>=20
>> ^ Would it not be important to also add PM_SWAP to flags?
>=20
> Hmm, I'm not sure; it's the same as a none pte in this case, so imho we s=
till
> can't tell if it's swapped out or simply the pte got zapped but page cach=
e will
> still hit (even if being swapped out may be the most possible case).

Yeah, that's true. Come to think of it, we also can't tell none pte from sw=
apped
out shmem pages (all bits are cleared out).

>=20
> What we're clear is we know it's uffd wr-protected, so maybe setting PM_U=
FFD_WP
> is still the simplest?

That's right, but if we were to require any of the differentiations above, =
how
does keeping another bit on the special pte sound to you? One to signal the=
 location on swap or otherwise (none or zapped).

Is there any other clearer way to do it? We wouldn't want to overload the
special pte unnecessarily.

Thanks,

--
Tibi