Date:   Sun, 19 Dec 2021 01:52:11 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
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>,
        Jason Gunthorpe <jgg@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>,
        Peter Xu <peterx@redhat.com>,
        Donald Dutile <ddutile@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>,
        Linux-MM <linux-mm@kvack.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)
Message-ID: <20211218225211.epa4u6mtjnvgkw4x@box.shutemov.name>
References: <20211217113049.23850-1-david@redhat.com>
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 <CAHk-=wgKft6E_EeLA1GnEXcQBA9vu8m2B-M-U7PuiNa0+9gpHA@mail.gmail.com>
 <54c492d7-ddcd-dcd0-7209-efb2847adf7c@redhat.com>
 <CAHk-=wgjOsHAXttQa=csLG10Cp2hh8Dk8CnNC3_WDpBpTzBESQ@mail.gmail.com>
 <17bfb2fd-da51-1264-513f-f9e928ec36c6@redhat.com>
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On Fri, Dec 17, 2021 at 12:45:45PM -0800, Linus Torvalds wrote:
> On Fri, Dec 17, 2021 at 12:42 PM David Hildenbrand <david@redhat.com> wrote:
> >
> > > 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.
> >
> > Where in the code did I ask for a COW event? I asked for a R/O pin, not
> > any kind of memory protection.
> 
> Why didn't you ask for a shared pin, if that is what you want?
> 
> We already support that.
> 
> If you don't like the read-only pins, don't use them. It's that simple.

So you are saying that if a GUP user wants to see changes made by
userspace to the page after the GUP it must ask for FOLL_WRITE, even if it
doesn't have intend to write to the page?

That's news to me.

Or did I misunderstand you?

-- 
 Kirill A. Shutemov