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

[Jason Gunthorpe]

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?

Jason