Re: [5.4 PATCH] mm/gup: Do not force a COW break on file-backed memory

[Linus Torvalds]

On Wed, Dec 1, 2021 at 8:11 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> The other patch we've been kicking around (and works) is:
>
>  static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned
> int flags)
>  {
> -       return is_cow_mapping(vma->vm_flags) && (flags & FOLL_GET);
> +       return is_cow_mapping(vma->vm_flags) &&
> +               (!(vma->vm_flags & VM_DENYWRITE)) && (flags & FOLL_GET);
>  }

That patch makes no sense to me.

It may "work", but it doesn't actually do anything sensible or really
fix the problem that I can tell.

I suspect a real fix would be bigger and more invasive.

If the answer is not to backport all the other changes (and they were
_really_ invasive), I think one answer may be to simply move the
"should_force_cow_break()" down to below the point where you've looked
up the page.

Then you can actually look at "is this a file mapped page", and say
"if so, that's ok, we can return it as-is".

Otherwise, you do something like

        foll_flags |= FOLL_WRITE;
        free_page(page);
        goto repeat;

to repeat the loop (now with FOLL_WRITE).

So the patch is bigger and more involved, because you would have done
the page lookup (for reading) and now notice "Oh, I need it for
writing instead" so you need to undo and re-do).

But at least - unlike backporting everything else - it would be
limited to that one __get_user_pages() function.

Hmm?

(And you'd need to handle that follow_hugetlb_page() case too), not
just the follow_page_mask() one)

             Linus