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SubjectRe: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb)
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On 17.12.21 22:15, Nadav Amit wrote:
>
>
>> On Dec 17, 2021, at 12:47 PM, Jason Gunthorpe <jgg@nvidia.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.
>

uffd-wp and mprotect() are broken as well, yes. But the easiest example
is just swap + read fault.

Section 2 and 3 in [1], along with reproducers.

Note that I didn't mention uffd-wp and mprotect(), because these require
"manual intervention". With swap, it's not your application doing
something "special".

[1]
https://lore.kernel.org/r/3ae33b08-d9ef-f846-56fb-645e3b9b4c66@redhat.com

--
Thanks,

David / dhildenb

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