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. -- 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>