On Tue, 26 Mar 2013, Toralf Foerster wrote: > On 03/25/2013 11:53 PM, Andrew Morton wrote: > > On Fri, 22 Mar 2013 18:28:36 +0100 Toralf Foerster <toralf.foerster@xxxxxx> wrote: > > > >> > Using trinity I often trigger under a user mode linux image with host kernel 3.8.4 > >> > and guest kernel linux-v3.9-rc3-244-g9217cbb the following : > >> > (The UML guest is a 32bit stable Gentoo Linux) > > I assume 3.8 is OK? > > > With UML kernel 3.7.10 (host kernel still 3.8.4) I can trigger this > issue too. > Just to clarify it - here the bug appears in the UML kernel - the host > kernel is ok (I can of course crash a host kernel too by trinity'ing an > UML guest, but that's another thread - see [1]) > > > FWIW he trinity command is just a test of 1 syscall: > > $> trinity --children 1 --victims /mnt/nfs/n22/victims -c mremap > > > > [1] https://lkml.org/lkml/2013/3/24/174 I should think it's been like this for five years, or even more: maybe you are the first person to try unmapping user address 0x100000 on UML; though it's odd that you find it using mremap than the more common munmap. uml_setup_stubs() sets up the special vma with install_special_mapping(), but instead of then faulting in the two pages concerned, it has preset the ptes with init_stub_pte(), which did not increment page mapcount. munmap() that area (or set up another mapping in that place), and zap_pte_range() will decrement page mapcount negative, hence the "Bad page" errors. Whereas UML uses an arch_exit_mmap() hook to clear the ptes at exit time, to avoid encountering such errors. I think that adding VM_PFNMAP to those install_special_mapping() flags would be enough to fix it (and avoid the need for the arch_exit_mmap(), and let vm_insert_pfn() do the work of init_stub_pte()); but I'm not certain that would be the approved way, and I may have missed problems doing it like this (which would disallow get_user_pages(), e.g. ptrace, on that area: which might or might not be a good thing, I don't know). I'm saying this just by examination, I've not tried any of it at all. Over to Richard. Hugh -- 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>