On Thu 30-11-17 13:17:06, Andrew Morton wrote: > On Thu, 30 Nov 2017 07:53:35 +0100 Michal Hocko <mho...@kernel.org> wrote: > > > > mm... So we have a caller which hopes to be getting highmem pages but > > > isn't. Caller then proceeds to pointlessly kmap the page and wonders > > > why it isn't getting as much memory as it would like on 32-bit systems, > > > etc. > > > > How he can kmap the page when he gets a _virtual_ address? > > doh. > > > > I do think we should help ferret out such bogosity. A WARN_ON_ONCE > > > would suffice. > > > > This function has always been about lowmem pages. I seriously doubt we > > have anybody confused and asking for a highmem page in the kernel. I > > haven't checked that but it would already blow up as VM_BUG_ON tends to > > be enabled on many setups. > > OK. But silently accepting __GFP_HIGHMEM is a bit weird - callers > shouldn't be doing that in the first place.
Yes, they shouldn't be. > I wonder what happens if we just remove the WARN_ON and pass any > __GFP_HIGHMEM straight through. The caller gets a weird address from > page_to_virt(highmem page) and usually goes splat? Good enough > treatment for something which never happens anyway? page_address will return NULL so they will blow up and leak the freshly allocated memory. I do not think this is really desirable. We _could_ handle this case but I am not really sure this is a win. A silent fixup sounds like the most simply protection. -- Michal Hocko SUSE Labs
