linux-kernel - Re: [PATCH v5 24/26] mm/pagemap: Recognize uffd-wp bit for shmem/hugetlbfs

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Date:   Mon, 19 Jul 2021 17:23:14 +0000
From:   Tiberiu Georgescu <tiberiu.georgescu@...anix.com>
To:     Peter Xu <peterx@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Jerome Glisse <jglisse@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Alistair Popple <apopple@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Ivan Teterevkov <ivan.teterevkov@...anix.com>,
        "Carl Waldspurger [C]" <carl.waldspurger@...anix.com>,
        Florian Schmidt <flosch@...anix.com>
Subject: Re: [PATCH v5 24/26] mm/pagemap: Recognize uffd-wp bit for
 shmem/hugetlbfs


> On 19 Jul 2021, at 17:03, Peter Xu <peterx@...hat.com> wrote:
> 
> On Mon, Jul 19, 2021 at 09:53:36AM +0000, Tiberiu Georgescu wrote:
>> 
>> Hello Peter,
> 
> Hi, Tiberiu,
> 
>> 
>>> On 15 Jul 2021, at 21:16, Peter Xu <peterx@...hat.com> wrote:
>>> 
>>> This requires the pagemap code to be able to recognize the newly introduced
>>> swap special pte for uffd-wp, meanwhile the general case for hugetlb that we
>>> recently start to support.  It should make pagemap uffd-wp support complete.
>>> 
>>> Signed-off-by: Peter Xu <peterx@...hat.com>
>>> ---
>>> fs/proc/task_mmu.c | 7 +++++++
>>> 1 file changed, 7 insertions(+)
>>> 
>>> 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(struct pagemapread *pm,
>>> 		flags |= PM_SWAP;
>>> 		if (is_pfn_swap_entry(entry))
>>> 			page = pfn_swap_entry_to_page(entry);
>>> +	} else if (pte_swp_uffd_wp_special(pte)) {
>>> +		flags |= PM_UFFD_WP;
>>> 	}
>> 
>> ^ Would it not be important to also add PM_SWAP to flags?
> 
> Hmm, I'm not sure; it's the same as a none pte in this case, so imho we still
> can't tell if it's swapped out or simply the pte got zapped but page cache 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 swapped
out shmem pages (all bits are cleared out).

> 
> What we're clear is we know it's uffd wr-protected, so maybe setting PM_UFFD_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