On Tue, May 27, 2014 at 4:10 PM, vinayak menon <vinayakm.list@xxxxxxxxx> wrote: > On Tue, May 27, 2014 at 5:18 PM, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote: >> On Tue, May 27, 2014 at 3:43 PM, vinayak menon <vinayakm.list@xxxxxxxxx> wrote: >>> On Tue, May 27, 2014 at 5:01 PM, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote: >>>> On Tue, May 27, 2014 at 2:31 PM, Kirill A. Shutemov >>>> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote: >>>>> Vinayak Menon wrote: >>>>>> An issue was observed when a userspace task exits. >>>>>> The page which hits error here is the zero page. >>>>>> In zap_pte_range, vm_normal_page gets called, and it >>>>>> returns a page address and not NULL, even though the >>>>>> pte corresponds to zero pfn. In this case, >>>>>> HAVE_PTE_SPECIAL is not set, and VM_MIXEDMAP is set >>>>>> in vm_flags. In the case of VM_MIXEDMAP , only pfn_valid >>>>>> is checked, and not is_zero_pfn. This results in >>>>>> zero page being returned instead of NULL. >>>>>> >>>>>> BUG: Bad page map in process mediaserver pte:9dff379f pmd:9bfbd831 >>>>>> page:c0ed8e60 count:1 mapcount:-1 mapping: (null) index:0x0 >>>>>> page flags: 0x404(referenced|reserved) >>>>>> addr:40c3f000 vm_flags:10220051 anon_vma: (null) mapping:d9fe0764 index:fd >>>>>> vma->vm_ops->fault: (null) >>>>>> vma->vm_file->f_op->mmap: binder_mmap+0x0/0x274 >>>>> >>>>> How do we get zero_pfn there. We shouldn't use zero page for file mappings. >>>>> binder does some tricks? >>>> >>>> Its vm_ops doesn't provide ->fault method at all. >>>> Seems like all ptes must be populated at the mmap time. >>>> For some reason read page fault had happened and handle_pte_fault() >>>> handled it in do_anonymous_page() which maps zero_page. >>>> >>> When the task crashed, it was ptraced by debuggered and the areas were >>> dumped. And this resulted in the read page fault. >> >> Anyway, this bug in the binder. It must either populate all PTEs in ->mmap() >> or provide ->fault() method. Falling into do_anonymous_page() isn't funny. > > Ok. But in vm_normal_page shouldn't we check for zero_pfn in the case > of VM_MIXEDMAP ? I don't think so. VM_MIXEDMAP shouldn't have zero_pfn. It isn't expected to be here, for example in your case next fault for write will remap that zero page for write becaue do_wp_page() doesn't expect to see it here. In case of HAVE_PTE_SPECIAL it's checked because zero_page _also_ installed as special pte and this must be the only one possible special-pte in non-mixedmap/pfnmap vmas. -- 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>