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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 17 Dec 2021 11:04:47 -0800
Message-ID: <CAHk-=wgL5u3XMgfUN6BOqVO0OvPx3-LEri1ju-1TW4dFhHQO4g@mail.gmail.com>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)
To:     David Hildenbrand <david@redhat.com>
Cc:     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>
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On Fri, Dec 17, 2021 at 3:34 AM David Hildenbrand <david@redhat.com> wrote:
>
> + * If the child takes a read-only pin on such a page (i.e., FOLL_WRITE is not
> + * set) and then unmaps the target page, we have:
> + *
> + * * page has mapcount == 1 and refcount > 1

All these games with mapcount makes me think this is still broken.

mapcount has been a horribly broken thing in the past, and I'm not
convinced it's not a broken thing now.

> +       vmf->page = vm_normal_page(vmf->vma, vmf->address, vmf->orig_pte);
> +       if (vmf->page && PageAnon(vmf->page) && !PageKsm(vmf->page) &&
> +           page_mapcount(vmf->page) > 1) {

What keeps the mapcount stable in here?

And I still believe that the whole notion that "COW should use
mapcount" is pure and utter garbage.

If we are doing a COW, we need an *exclusive* access to the page. That
is not mapcount, that is the page ref.

mapcount is insane, and I think this is making this worse again.

                Linus