> 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.
