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

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Date:   Sat, 18 Dec 2021 21:48:59 +0000
From:   Nadav Amit <namit@...are.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        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>,
        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 Dec 18, 2021, at 10:42 AM, Jason Gunthorpe <jgg@...dia.com> wrote:
> 
> On Fri, Dec 17, 2021 at 07:38:39PM -0800, Linus Torvalds wrote:
>> On Fri, Dec 17, 2021 at 7:30 PM Nadav Amit <namit@...are.com> wrote:
>>> 
>>> In such a case, I do think it makes sense to fail uffd-wp (when
>>> page_count() > 1), and in a prototype I am working on I do something
>>> like that.
>> 
>> Ack. If uddf-wp finds a page that is pinned, just skip it as not
>> write-protectable.
>> 
>> Because some of the pinners might be writing to it, of course - just
>> not through the page tables.
> 
> That doesn't address the qemu use case though. The RDMA pin is the
> 'coherent r/o pin' we discussed before, which requires that the pages
> remain un-write-protected and the HW DMA is read only.
> 
> The VFIO pin will enable dirty page tracking in the system IOMMU so it
> gets the same effect from qemu's perspective as the CPU WP is doing.
> 
> In these operations every single page of the guest will be pinned, so
> skip it just means userfault fd wp doesn't work at all.
> 
> Qemu needs some solution to be able to dirty track the CPU memory for
> migration..

My bad. I misunderstood the scenario.

Yes, I guess that you pin the pages early for RDMA registration, which
is also something you may do for IO-uring buffers. This would render
userfaultfd unusable.

I do not see how it can be solved without custom, potentially
complicated logic, which the page_count() approach wants to avoid.

The only thing I can think of is requiring the pinned regions to be
first madvise’d with MADV_DONTFORK and not COW’ing in such case.
But this would break existing code though.