Re: [PATCH 09/13] mm/gup: Cache *pudp in follow_pud_mask()

[James Houghton]

On Tue, Dec 19, 2023 at 2:57 AM <peterx@xxxxxxxxxx> wrote:
>
> From: Peter Xu <peterx@xxxxxxxxxx>
>
> Introduce "pud_t pud" in the function, so the code won't dereference *pudp
> multiple time.  Not only because that looks less straightforward, but also
> because if the dereference really happened, it's not clear whether there
> can be race to see different *pudp values if it's being modified at the
> same time.
>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>  mm/gup.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 6c0d82fa8cc7..97e87b7a15c3 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -753,26 +753,27 @@ static struct page *follow_pud_mask(struct vm_area_struct *vma,
>                                     unsigned int flags,
>                                     struct follow_page_context *ctx)
>  {
> -       pud_t *pud;
> +       pud_t *pudp, pud;
>         spinlock_t *ptl;
>         struct page *page;
>         struct mm_struct *mm = vma->vm_mm;
>
> -       pud = pud_offset(p4dp, address);
> -       if (pud_none(*pud))
> +       pudp = pud_offset(p4dp, address);
> +       pud = *pudp;

I think you might want a READ_ONCE() on this so that the compiler
doesn't actually read the pud multiple times.

> +       if (pud_none(pud))
>                 return no_page_table(vma, flags, address);
> -       if (pud_devmap(*pud)) {
> -               ptl = pud_lock(mm, pud);
> -               page = follow_devmap_pud(vma, address, pud, flags, &ctx->pgmap);
> +       if (pud_devmap(pud)) {
> +               ptl = pud_lock(mm, pudp);
> +               page = follow_devmap_pud(vma, address, pudp, flags, &ctx->pgmap);
>                 spin_unlock(ptl);
>                 if (page)
>                         return page;
>                 return no_page_table(vma, flags, address);
>         }
> -       if (unlikely(pud_bad(*pud)))
> +       if (unlikely(pud_bad(pud)))
>                 return no_page_table(vma, flags, address);

Not your change, but reading this, it's not clear to me that
`pud_present(*pudp)` (and non-leaf) would necessarily be true at this
point -- like, I would prefer to see `!pud_present(pud)` instead of
`pud_bad()`. Thank you for adding that in the next patch. :)

Feel free to add:

Acked-by: James Houghton <jthoughton@xxxxxxxxxx>