Messages in this thread | | | Date | Mon, 7 Jul 2008 18:48:51 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: Lockless/Get_User_Pages_Fast causes Xorg 1.4.99.* to lock |
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On Mon, 7 Jul 2008, Carsten Otte wrote: > Nick Piggin wrote: > > I think we need a similar fix for s390 too. If so, then it really should > > get into 2.6.26, but this late in the release, I hope an s390 maintainer > > might be able to test and verify the fix? > > I've done my best to combine mprotect, mmap and munmap a MAP_PRIVATE mapping > on a xip file system. The system runs stable with and without this patch. > Could someone please enlighten me on how to reproduce the problem so that I > can verify the fix?
Though it would be more obvious to use a MAP_SHARED mapping, we had that earlier thread in which it emerged that you're not using shared writable xip mappings, IIRC. So, sticking to MAP_PRIVATE, I'd expect the following sequence
ptr = mmap(NULL, PAGE_SIZE, PROT_READ, MAP_PRIVATE, xip_fd, 0); var = *ptr; mprotect(ptr, PAGE_SIZE, PROT_NONE); munmap(ptr, PAGE_SIZE);
to do a put_page on a non-existent struct page derived from the pfn: perhaps corrupting other memory without being noticed? Or if you have CONFIG_DEBUG_VM=y (that would be a good move), to hit vm_normal_page's VM_BUG_ON(!pfn_valid(pte_pfn(pte))) before that.
(I think you can just as well use PROT_READ|PROT_WRITE rather than PROT_NONE there, but in principle mprotect could optimize away that pte modification - though I think it goes ahead and does it anyway.)
Hugh
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