On Sun, Aug 16, 2020 at 10:45 PM Christoph Hellwig <hch@xxxxxx> wrote: > > On Sun, Aug 16, 2020 at 10:55:09AM -0700, Cong Wang wrote: > > On Sun, Aug 16, 2020 at 1:36 AM Coly Li <colyli@xxxxxxx> wrote: > > > > > > The original problem was from nvme-over-tcp code, who mistakenly uses > > > kernel_sendpage() to send pages allocated by __get_free_pages() without > > > __GFP_COMP flag. Such pages don't have refcount (page_count is 0) on > > > tail pages, sending them by kernel_sendpage() may trigger a kernel panic > > > from a corrupted kernel heap, because these pages are incorrectly freed > > > in network stack as page_count 0 pages. > > > > > > This patch introduces a helper sendpage_ok(), it returns true if the > > > checking page, > > > - is not slab page: PageSlab(page) is false. > > > - has page refcount: page_count(page) is not zero > > > > > > All drivers who want to send page to remote end by kernel_sendpage() > > > may use this helper to check whether the page is OK. If the helper does > > > not return true, the driver should try other non sendpage method (e.g. > > > sock_no_sendpage()) to handle the page. > > > > Can we leave this helper to mm subsystem? > > > > I know it is for sendpage, but its implementation is all about some > > mm details and its two callers do not belong to net subsystem either. > > > > Think this in another way: who would fix it if it is buggy? I bet mm people > > should. ;) > > No. This is all about a really unusual imitation in sendpage, which So netdev people will have to understand and support PageSlab() or page_count()? If it is unusual even for mm people, how could netdev people suppose to understand this unusual mm bug? At least not any better. > is pretty much unexpected. In fact the best thing would be to make > sock_sendpage do the right thing and call sock_no_sendpage based > on this condition, so that driver writers don't have to worry at all. Agreed, but kernel_sendpage() still relies on mm to provide a helper to make the decision and ensure this helper is always up-to-date. In short, it is all about ownership. Thanks.
