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

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Date:   Fri, 17 Dec 2021 21:15:45 +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 17, 2021, at 12:47 PM, Jason Gunthorpe <jgg@...dia.com> wrote:
> 
> On Fri, Dec 17, 2021 at 12:36:43PM -0800, Linus Torvalds wrote:
> 
>>> 5. Take a R/O pin (RDMA, VFIO, ...)
>>> -> refcount > 1
>>> 
>>> 6. memset(mem, 0xff, pagesize);
>>> -> Write fault -> COW
>> 
>> I do not believe this is actually a bug.
>> 
>> You asked for a R/O pin, and you got one.
>> 
>> 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.
> 
> 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.
> 
> The above #5 can occur for O_DIRECT read and in that case the
> 'snapshot the data' is perfectly fine as racing the COW with the
> O_DIRECT read just resolves the race toward the read() direction.
> 
> IIRC there is some other scenario that motivated this patch?

I think that there is an assumption that once a page is COW-broken,
it would never have another write-fault that might lead to COW
breaking later.

AFAIK at least after userfaultfd-WP followed by
userfaultfd-write-unprotect a page might be write-protected and
go through do_wp_page() a second time to be COW-broken again. In
such case, I think the FOLL_FORCE|FOLL_WRITE would not help.

I suspect (not sure) that this might even happen with mprotect()
since I do not see all code-paths preserving whether the page
was writable.