Date:   Fri, 17 Dec 2021 16:47:05 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     David Hildenbrand <david@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        David Rientjes <rientjes@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Nadav Amit <namit@vmware.com>, Rik van Riel <riel@surriel.com>,
        Roman Gushchin <guro@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
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        "open list:KERNEL SELFTEST FRAMEWORK" 
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Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)
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On Fri, Dec 17, 2021 at 12:36:43PM -0800, Linus Torvalds wrote:

> > 5. Take a R/O pin (RDMA, VFIO, ...)
> > -> refcount > 1
> >
> > 6. memset(mem, 0xff, pagesize);
> > -> Write fault -> COW
> 
> I do not believe this is actually a bug.
> 
> You asked for a R/O pin, and you got one.
> 
> Then somebody else modified that page, and you got exactly what you
> asked for - a COW event. The original R/O pin has the original page
> that it asked for, and can read it just fine.

To remind all, the GUP users, like RDMA, VFIO use
FOLL_FORCE|FOLL_WRITE to get a 'r/o pin' specifically because of the
COW breaking the coherence. In these case 'r/o pin' does not mean
"snapshot the data", but its only a promise not to write to the pages
and still desires coherence with the memory map.

Eg in RDMA we know of apps asking for a R/O pin of something in .bss
then filling that something with data finally doing the actual
DMA. Breaking COW after pin breaks those apps.

The above #5 can occur for O_DIRECT read and in that case the
'snapshot the data' is perfectly fine as racing the COW with the
O_DIRECT read just resolves the race toward the read() direction.

IIRC there is some other scenario that motivated this patch?

Jason