Re: [Bug 117731] New: Doing mprotect for PROT_NONE and then for PROT_READ|PROT_WRITE reduces CPU write B/W on buffer

["Kirill A. Shutemov" <kirill@shutemov.name>]

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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