On Mon 20-08-18 10:07:44, Matthew Wilcox wrote: > On Mon, Aug 20, 2018 at 06:24:06PM +0200, Michal Hocko wrote: > > On Mon 20-08-18 07:49:23, Matthew Wilcox wrote: > > > On Mon, Aug 20, 2018 at 04:41:16PM +0200, Michal Hocko wrote: > > > > On Sat 18-08-18 20:49:01, Li RongQing wrote: > > > > > The new helper returns address mapping page, which has several users > > > > > in individual subsystem, like mem_to_page in xfs_buf.c and pgv_to_page > > > > > in af_packet.c, unify them > > > > > > > > kvvirt_to_page is a weird name. I guess you wanted it to fit into > > > > kv*alloc, kvfree naming, right? If yes then I guess kvmem_to_page > > > > would be slightly better. > > > > > > > > Other than that the patch makes sense to me. It would be great to add > > > > some documentation and be explicit that the call is only safe on > > > > directly mapped kernel address and vmalloc areas. > > > > > > ... and not safe if the length crosses a page boundary. I don't want to > > > see new code emerge that does kvmalloc(PAGE_SIZE * 2, ...); kvmem_to_page() > > > and have it randomly crash when kvmalloc happens to fall back to vmalloc() > > > under heavy memory pressure. > > > > > > Also, people are going to start using this for stack addresses. Perhaps > > > we should have a debug option to guard against them doing that. > > > > I do agree that such an interface is quite dangerous. That's why I was > > stressing out the proper documentation. I would be much happier if we > > could do without it altogether. Maybe the existing users can be rewoked > > to not rely on the addr2page functionality. If that is not the case then > > we should probably offer a helper. With some WARN_ONs to catch misuse > > would be really nice. I am not really sure how many abuses can we catch > > actually though. > > I certainly understand the enthusiasm for sharing this code rather than > having dozens of places outside the VM implement their own version of it. > But I think most of these users are using code that's working at the wrong > level. Most of them seem to have an address range which may-or-may-not-be > virtually mapped and they want to get an array-of-pages for that. > > Perhaps we should offer -that- API instead. vmalloc/vmap already has > an array-of-pages, and the various users could be given a pointer to > that array. If the memory isn't vmapped, maybe the caller could pass > an array pointer like XFS does, or we could require them to pass GFP flags > to allocate a new array. Sure, I wouldn't be opposed if there was a model which doesn't force them to do hacks like this. -- Michal Hocko SUSE Labs
