Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb)

[Nadav Amit]

> On Dec 17, 2021, at 12:47 PM, Jason Gunthorpe <jgg@xxxxxxxxxx> 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.