linux-kernel - Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG

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Date:   Sun, 19 Dec 2021 01:52:11 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Hildenbrand <david@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Linux-MM <linux-mm@...ck.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)

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@...hat.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