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 ? -- 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>