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

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Message-ID: <YPWiRsNaearMNB2g@t490s>
Date:   Mon, 19 Jul 2021 12:03:18 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Tiberiu Georgescu <tiberiu.georgescu@...anix.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>
Subject: Re: [PATCH v5 24/26] mm/pagemap: Recognize uffd-wp bit for
 shmem/hugetlbfs

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).

What we're clear is we know it's uffd wr-protected, so maybe setting PM_UFFD_WP
is still the simplest?

Thanks,

-- 
Peter Xu