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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 17 Dec 2021 12:56:21 -0800
Message-ID: <CAHk-=wgO9Zmc3S+cG=xFuUAgkZ3Ti9fOdUr4Q72EPwrxX67YAA@mail.gmail.com>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)
To:     Jason Gunthorpe <jgg@nvidia.com>
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>,
        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>,
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On Fri, Dec 17, 2021 at 12:47 PM Jason Gunthorpe <jgg@nvidia.com> wrote:
>
> 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.

I agree.

And my argument is that those kinds of things that ask for a R/O pin
are broken, and should just make sure to use the shared pins.

If the page was already writable, you can just re-use the page
directly (marking it pinned, so that any subsequent fork() does the
right pre-cow thing)

And if the page was *NOT* already writable, you do a COW - which might
be sharing the page directly too, if it has no other references.

What's the downside of just doing this properly?

Again: if a DMA user wants coherent memory, then it should use the
coherent pinning. Not some kind of "read-only sharing that looks at
crazy mapcounts that have absolutely zero relevance to whether the
page is coherent or not".

                   Linus