Date:   Fri, 17 Dec 2021 20:50:41 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Nadav Amit <namit@vmware.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        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>,
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        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
        Michal Hocko <mhocko@kernel.org>,
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        Roman Gushchin <guro@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Peter Xu <peterx@redhat.com>,
        Donald Dutile <ddutile@redhat.com>,
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        Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>,
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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 09:15:45PM +0000, Nadav Amit wrote:

> I think that there is an assumption that once a page is COW-broken,
> it would never have another write-fault that might lead to COW
> breaking later.

Yes, that is what Linus has been explaining, AFAICT

> AFAIK at least after userfaultfd-WP followed by
> userfaultfd-write-unprotect a page might be write-protected and
> go through do_wp_page() a second time to be COW-broken again. In
> such case, I think the FOLL_FORCE|FOLL_WRITE would not help.

Right, and this is a good reason why refcount is running into trouble,
it COW's too much in cases like that because userfaultfd-WP doesn't
align to the first assumption.

Jason