Date:   Tue, 11 Aug 2020 12:24:20 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marty Mcfadden <mcfadden8@...l.gov>,
        "Maya B . Gokhale" <gokhale2@...l.gov>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jann Horn <jannh@...gle.com>, Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>,
        Kirill Shutemov <kirill@...temov.name>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH v3] mm/gup: Allow real explicit breaking of COW

On Tue, Aug 11, 2020 at 11:40 AM Peter Xu <peterx@...hat.com> wrote:
>
> index 206f52b36ffb..c88f773d03af 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1296,7 +1296,17 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf, pmd_t orig_pmd)
>         if (reuse_swap_page(page, NULL)) {
>                 pmd_t entry;
>                 entry = pmd_mkyoung(orig_pmd);
> -               entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
> +               entry = pmd_mkdirty(entry);
> +               if (pmd_uffd_wp(orig_pmd))
> +                       /*
> +                        * This can happen when an uffd-wp protected page is
> +                        * copied due to enfornced COW.  When it happens, we
> +                        * need to keep the uffd-wp bit even after COW, and
> +                        * make sure write bit is kept cleared.
> +                        */
> +                       entry = pmd_mkuffd_wp(pmd_wrprotect(entry));
> +               else
> +                       entry = maybe_pmd_mkwrite(entry, vma);
>                 if (pmdp_set_access_flags(vma, haddr, vmf->pmd, entry, 1))
>                         update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
>                 unlock_page(page);
> diff --git a/mm/memory.c b/mm/memory.c
> index c39a13b09602..b27b555a9df8 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2706,7 +2706,17 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
>                 flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
>                 entry = mk_pte(new_page, vma->vm_page_prot);
>                 entry = pte_sw_mkyoung(entry);
> -               entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> +               entry = pte_mkdirty(entry);
> +               if (pte_uffd_wp(vmf->orig_pte))
> +                       /*
> +                        * This can happen when an uffd-wp protected page is
> +                        * copied due to enfornced COW.  When it happens, we
> +                        * need to keep the uffd-wp bit even after COW, and
> +                        * make sure write bit is kept cleared.
> +                        */
> +                       entry = pte_mkuffd_wp(pte_wrprotect(entry));
> +               else
> +                       entry = maybe_mkwrite(entry, vma);
>                 /*
>                  * Clear the pte entry and flush it first, before updating the
>                  * pte with the new entry. This will avoid a race condition

I think this needs to be cleaned up some way. I realize it's not an
exact duplicate (pmd vs pte), but this code is illegible.

Maybe just making it a helper inline function (well, two separate
ones) with the comment above the function would resolve my "this is
very ugly" concerns.


> @@ -2900,7 +2910,13 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  {
>         struct vm_area_struct *vma = vmf->vma;
>
> -       if (userfaultfd_pte_wp(vma, *vmf->pte)) {
> +       /*
> +        * Userfaultfd-wp only cares about real writes.  E.g., enforced COW for
> +        * read does not count.  When that happens, we will do the COW with the
> +        * UFFD_WP bit inherited from the original PTE/PMD.
> +        */
> +       if ((vmf->flags & FAULT_FLAG_WRITE) &&
> +           userfaultfd_pte_wp(vma, *vmf->pte)) {
>                 pte_unmap_unlock(vmf->pte, vmf->ptl);
>                 return handle_userfault(vmf, VM_UFFD_WP);
>         }
> @@ -4117,7 +4133,14 @@ static inline vm_fault_t create_huge_pmd(struct vm_fault *vmf)
>  static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf, pmd_t orig_pmd)
>  {
>         if (vma_is_anonymous(vmf->vma)) {
> -               if (userfaultfd_huge_pmd_wp(vmf->vma, orig_pmd))
> +               /*
> +                * Userfaultfd-wp only cares about real writes.  E.g., enforced
> +                * COW for read does not count.  When that happens, we will do
> +                * the COW with the UFFD_WP bit inherited from the original
> +                * PTE/PMD.
> +                */
> +               if ((vmf->flags & FAULT_FLAG_WRITE) &&
> +                   userfaultfd_huge_pmd_wp(vmf->vma, orig_pmd))
>                         return handle_userfault(vmf, VM_UFFD_WP);

Here again the comment placement could be improved. Particularly in
the do_wp_page() case, we have a big and somewhat complex function,
and this duplicated boiler-plate makes me worry.

Making it a helper function with a comment above would again I think
make it more legible.

And I think Jann is on the money wrt the follow_page_pte() issue.

I think you broke COW break there entirely.

That was one of the reasons I did just that "make it use FOLL_WRITE"
originally, because it meant that we couldn't have any subtle places
we'd missed.

Now I wonder if there's any other case of FOLL_WRITE that is missing.

            Linus

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