Re: [PATCH RFC 13/24] mm: merge parameters for change_protection()

[Jerome Glisse]

On Mon, Jan 21, 2019 at 03:57:11PM +0800, Peter Xu wrote:
> change_protection() was used by either the NUMA or mprotect() code,
> there's one parameter for each of the callers (dirty_accountable and
> prot_numa).  Further, these parameters are passed along the calls:
> 
>   - change_protection_range()
>   - change_p4d_range()
>   - change_pud_range()
>   - change_pmd_range()
>   - ...
> 
> Now we introduce a flag for change_protect() and all these helpers to
> replace these parameters.  Then we can avoid passing multiple parameters
> multiple times along the way.
> 
> More importantly, it'll greatly simplify the work if we want to
> introduce any new parameters to change_protection().  In the follow up
> patches, a new parameter for userfaultfd write protection will be
> introduced.
> 
> No functional change at all.

There is one change i could spot and also something that looks wrong.

> 
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---

[...]

> @@ -428,8 +431,7 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev,
>  	dirty_accountable = vma_wants_writenotify(vma, vma->vm_page_prot);
>  	vma_set_page_prot(vma);
>  
> -	change_protection(vma, start, end, vma->vm_page_prot,
> -			  dirty_accountable, 0);
> +	change_protection(vma, start, end, vma->vm_page_prot, MM_CP_DIRTY_ACCT);

Here you unconditionaly see the DIRTY_ACCT flag instead it should be
something like:

    s/dirty_accountable/cp_flags
    if (vma_wants_writenotify(vma, vma->vm_page_prot))
        cp_flags = MM_CP_DIRTY_ACCT;
    else
        cp_flags = 0;

    change_protection(vma, start, end, vma->vm_page_prot, cp_flags);

Or any equivalent construct.

>  	/*
>  	 * Private VM_LOCKED VMA becoming writable: trigger COW to avoid major
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 005291b9b62f..23d4bbd117ee 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -674,7 +674,7 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
>  		newprot = vm_get_page_prot(dst_vma->vm_flags);
>  
>  	change_protection(dst_vma, start, start + len, newprot,
> -				!enable_wp, 0);
> +			  enable_wp ? 0 : MM_CP_DIRTY_ACCT);

We had a discussion in the past on that, i have not look at other
patches but this seems wrong to me. MM_CP_DIRTY_ACCT is an
optimization to keep a pte with write permission if it is dirty
while my understanding is that you want to set write flag for pte
unconditionaly.

So maybe this patch that adds flag should be earlier in the serie
so that you can add a flag to do that before introducing the UFD
mwriteprotect_range() function.

Cheers,
Jérôme