From: Linus Torvalds <torvalds@linux-foundation.org>
To: Peter Xu <peterx@redhat.com>
Cc: Linux-MM <linux-mm@kvack.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Marty Mcfadden <mcfadden8@llnl.gov>,
"Maya B . Gokhale" <gokhale2@llnl.gov>,
Andrea Arcangeli <aarcange@redhat.com>,
Jann Horn <jannh@google.com>, Christoph Hellwig <hch@lst.de>,
Oleg Nesterov <oleg@redhat.com>,
Kirill Shutemov <kirill@shutemov.name>, Jan Kara <jack@suse.cz>
Subject: Re: [PATCH v3] mm/gup: Allow real explicit breaking of COW
Date: Tue, 11 Aug 2020 12:24:20 -0700 [thread overview]
Message-ID: <CAHk-=whQM=m5td5tfbuxh1f_Gxjsa74XV962BYkjrbeDMAhBpA@mail.gmail.com> (raw)
In-Reply-To: <20200811183950.10603-1-peterx@redhat.com>
On Tue, Aug 11, 2020 at 11:40 AM Peter Xu <peterx@redhat.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
next prev parent reply other threads:[~2020-08-11 19:24 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-11 18:39 [PATCH v3] mm/gup: Allow real explicit breaking of COW Peter Xu
2020-08-11 19:07 ` Jann Horn
2020-08-11 20:02 ` Peter Xu
2020-08-11 20:22 ` Jann Horn
2020-08-11 21:23 ` Peter Xu
2020-08-11 19:24 ` Linus Torvalds [this message]
2020-08-11 20:06 ` Linus Torvalds
2020-08-11 20:46 ` Linus Torvalds
2020-08-11 21:42 ` Peter Xu
2020-08-11 23:10 ` Linus Torvalds
2020-08-20 21:54 ` Peter Xu
2020-08-20 22:01 ` Linus Torvalds
2020-08-21 2:34 ` Peter Xu
2020-08-21 10:13 ` Jan Kara
2020-08-21 12:27 ` Linus Torvalds
2020-08-21 15:47 ` Jan Kara
2020-08-21 17:00 ` Linus Torvalds
2020-08-21 18:08 ` Peter Xu
2020-08-21 18:23 ` Linus Torvalds
2020-08-21 19:05 ` Linus Torvalds
2020-08-21 19:06 ` Linus Torvalds
2020-08-21 19:31 ` Peter Xu
2020-08-21 19:42 ` Linus Torvalds
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