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charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Sep 22, 2020 at 12:30 AM Peter Xu wrote: > On Mon, Sep 21, 2020 at 11:43:38PM +0200, Jann Horn wrote: > > On Mon, Sep 21, 2020 at 11:17 PM Peter Xu wrote: > > > (Commit message collected from Jason Gunthorpe) > > > > > > Reduce the chance of false positive from page_maybe_dma_pinned() by keeping > > > track if the mm_struct has ever been used with pin_user_pages(). mm_structs > > > that have never been passed to pin_user_pages() cannot have a positive > > > page_maybe_dma_pinned() by definition. > > > > There are some caveats here, right? E.g. this isn't necessarily true > > for pagecache pages, I think? > > Sorry I didn't follow here. Could you help explain with some details? The commit message says "mm_structs that have never been passed to pin_user_pages() cannot have a positive page_maybe_dma_pinned() by definition"; but that is not true for pages which may also be mapped in a second mm and may have been passed to pin_user_pages() through that second mm (meaning they must be writable over there and not shared with us via CoW). For example: Process A: fd_a = open("/foo/bar", O_RDWR); mapping_a = mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_SHARED, fd_a, 0); pin_user_pages(mapping_a, 1, ...); Process B: fd_b = open("/foo/bar", O_RDONLY); mapping_b = mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd_b, 0); *(volatile char *)mapping_b; At this point, process B has never called pin_user_pages(), but page_maybe_dma_pinned() on the page at mapping_b would return true. I don't think this is a problem for the use of page_maybe_dma_pinned() in fork(), but I do think that the commit message is not entirely correct.