Re: [RFC PATCH 01/14] userfaultfd: set dirty and young on writeprotect

David Hildenbrand
2022-07-20T09:43:08Z

On 18.07.22 14:01, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
>
> When userfaultfd makes a PTE writable, it can now change the PTE
> directly, in some cases, without going triggering a page-fault first.
> Yet, doing so might leave the PTE that was write-unprotected as old and
> clean. At least on x86, this would cause a >500 cycles overhead when the
> PTE is first accessed.
>
> Use MM_CP_WILL_NEED to set the PTE as young and dirty when userfaultfd
> gets a hint that the page is likely to be used. Avoid changing the PTE
> to young and dirty in other cases to avoid excessive writeback and
> messing with the page reclamation logic.
>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Will Deacon <will@kernel.org>
> Cc: Yu Zhao <yuzhao@google.com>
> Cc: Nick Piggin <npiggin@gmail.com>
> ---
> include/linux/mm.h | 2 ++
> mm/mprotect.c | 9 ++++++++-
> mm/userfaultfd.c | 8 ++++++--
> 3 files changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 9cc02a7e503b..4afd75ce5875 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1988,6 +1988,8 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma,
> /* Whether this change is for write protecting */
> #define MM_CP_UFFD_WP (1UL << 2) /* do wp */
> #define MM_CP_UFFD_WP_RESOLVE (1UL << 3) /* Resolve wp */
> +/* Whether to try to mark entries as dirty as they are to be written */
> +#define MM_CP_WILL_NEED (1UL << 4)
> #define MM_CP_UFFD_WP_ALL (MM_CP_UFFD_WP | \
> MM_CP_UFFD_WP_RESOLVE)
>
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 996a97e213ad..34c2dfb68c42 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -82,6 +82,7 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
> bool prot_numa = cp_flags & MM_CP_PROT_NUMA;
> bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
> bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
> + bool will_need = cp_flags & MM_CP_WILL_NEED;
>
> tlb_change_page_size(tlb, PAGE_SIZE);
>
> @@ -172,6 +173,9 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
> ptent = pte_clear_uffd_wp(ptent);
> }
>
> + if (will_need)
> + ptent = pte_mkyoung(ptent);
> +
> /*
> * In some writable, shared mappings, we might want
> * to catch actual write access -- see
> @@ -187,8 +191,11 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
> */
> if ((cp_flags & MM_CP_TRY_CHANGE_WRITABLE) &&
> !pte_write(ptent) &&

Why would we want to check if we can set something writable if it
already *is* writable? That doesn't make sense to me.

> - can_change_pte_writable(vma, addr, ptent))
> + can_change_pte_writable(vma, addr, ptent)) {
> ptent = pte_mkwrite(ptent);
> + if (will_need)
> + ptent = pte_mkdirty(ptent);
> + }

--
Thanks,

David / dhildenb