On Tue, May 17, 2016 at 04:56:02PM +0530, Ashish Srivastava wrote:
> Yes, the original repro was using a custom allocator but I was seeing the
> issue with malloc'd memory as well on my (ARMv7) platform.
Test-case for that would be helpful, as normal malloc()'ed anon memory
cannot be subject for the bug. Unless I miss something obvious.
> I agree that the repro code won't reliably work so have modified the repro
> code attached to the bug to use file backed memory.
>
> That really is the root cause of the problem. I can make the following
> change in the kernel that can make the slow writes problem go away.
> This makes vma_set_page_prot return the value of vma_wants_writenotify to
> the caller after setting vma->vmpage_prot.
>
> In vma_set_page_prot:
> -void vma_set_page_prot(struct vm_area_struct *vma)
> +bool vma_set_page_prot(struct vm_area_struct *vma)
> {
> unsigned long vm_flags = vma->vm_flags;
>
> vma->vm_page_prot = vm_pgprot_modify(vma->vm_page_prot, vm_flags);
> if (vma_wants_writenotify(vma)) {
> vm_flags &= ~VM_SHARED;
> vma->vm_page_prot = vm_pgprot_modify(vma->vm_page_prot,
> vm_flags);
> + return 1;
> }
> + return 0;
> }
>
> In mprotect_fixup:
>
> * held in write mode.
> */
> vma->vm_flags = newflags;
> - dirty_accountable = vma_wants_writenotify(vma);
> - vma_set_page_prot(vma);
> + dirty_accountable = vma_set_page_prot(vma);
>
> change_protection(vma, start, end, vma->vm_page_prot,
> dirty_accountable, 0)
>
That looks good to me. Please prepare proper patch.
--
Kirill A. Shutemov
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