On Tue, Jan 26, 2016 at 09:44:24PM -0800, Andy Lutomirski wrote: > On Tue, Jan 26, 2016 at 8:40 PM, Matthew Wilcox <willy@xxxxxxxxxxxxxxx> wrote: > > On Mon, Jan 25, 2016 at 09:33:35AM -0800, Andy Lutomirski wrote: > >> On Mon, Jan 25, 2016 at 9:25 AM, Matthew Wilcox > >> <matthew.r.wilcox@xxxxxxxxx> wrote: > >> > From: Matthew Wilcox <willy@xxxxxxxxxxxxxxx> > >> > > >> > track_pfn_insert() overwrites the pgprot that is passed in with a value > >> > based on the VMA's page_prot. This is a problem for people trying to > >> > do clever things with the new vm_insert_pfn_prot() as it will simply > >> > overwrite the passed protection flags. If we use the current value of > >> > the pgprot as the base, then it will behave as people are expecting. > >> > > >> > Also fix track_pfn_remap() in the same way. > >> > >> Well that's embarrassing. Presumably it worked for me because I only > >> overrode the cacheability bits and lookup_memtype did the right thing. > >> > >> But shouldn't the PAT code change the memtype if vm_insert_pfn_prot > >> requests it? Or are there no callers that actually need that? (HPET > >> doesn't, because there's a plain old ioremapped mapping.) > > > > I'm confused. Here's what I understand: > > > > - on x86, the bits in pgprot can be considered as two sets of bits; > > the 'cacheability bits' -- those in _PAGE_CACHE_MASK and the > > 'protection bits' -- PRESENT, RW, USER, ACCESSED, NX > > - The purpose of track_pfn_insert() is to ensure that the cacheability bits > > are the same on all mappings of a given page, as strongly advised by the > > Intel manuals [1]. So track_pfn_insert() is really only supposed to > > modify _PAGE_CACHE_MASK of the passed pgprot, but in fact it ends up > > modifying the protection bits as well, due to the bug. > > > > I don't think you overrode the cacheability bits at all. It looks to > > me like your patch ends up mapping the HPET into userspace writable. > > I sure hope not. If vm_page_prot was writable, something was already > broken, because this is the vvar mapping, and the vvar mapping is > VM_READ (and not even VM_MAYREAD). I do beg yor pardon. I thought you were inserting a readonly page into the middle of a writable mapping. Instead you're inserting a non-executable page into the middle of a VM_READ | VM_EXEC mapping. Sorry for the confusion. I should have written: "like your patch ends up mapping the HPET into userspace executable" which is far less exciting. > > I don't think the vm_insert_pfn_prot() call gets to change the memtype. > > For one, that page may already be mapped into a differet userspace using > > the pre-existing memtype, and [1] continues to bite you. Then there > > may be outstanding kernel users of the page that's being mapped in. > > So why was remap_pfn_range different? I'm sure there was a reason. Yeah, doesn't make sense to me either. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>