On Sun, Jun 19, 2022 at 04:34:48PM -0700, Nadav Amit wrote:
> From: Nadav Amit <namit@xxxxxxxxxx>
>
> When userfaultfd provides a zeropage in response to ioctl, it provides a
> readonly alias to the zero page. If the page is later written (which is
> the likely scenario), page-fault occurs and the page-fault allocator
> allocates a page and rewires the page-tables.
>
> This is an expensive flow for cases in which a page is likely be written
> to. Users can use the copy ioctl to initialize zero page (by copying
> zeros), but this is also wasteful.
>
> Allow userfaultfd users to efficiently map initialized zero-pages that
> are writable. Introduce UFFDIO_ZEROPAGE_MODE_WRITE_LIKELY, which, when
> provided would map a clear page instead of an alias to the zero page.
>
> For consistency, introduce also UFFDIO_ZEROPAGE_MODE_ACCESS_LIKELY.
>
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
> Cc: Peter Xu <peterx@xxxxxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
> ---
> fs/userfaultfd.c | 14 +++++++++++--
> include/uapi/linux/userfaultfd.h | 2 ++
> mm/userfaultfd.c | 36 ++++++++++++++++++++++++++++++++
> 3 files changed, 50 insertions(+), 2 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index a56983b594d5..ff073de78ea8 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1770,6 +1770,8 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
> struct uffdio_zeropage uffdio_zeropage;
> struct uffdio_zeropage __user *user_uffdio_zeropage;
> struct userfaultfd_wake_range range;
> + bool mode_dontwake, mode_access_likely, mode_write_likely;
> + uffd_flags_t uffd_flags;
>
> user_uffdio_zeropage = (struct uffdio_zeropage __user *) arg;
>
> @@ -1788,8 +1790,16 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
> if (ret)
> goto out;
> ret = -EINVAL;
> - if (uffdio_zeropage.mode & ~UFFDIO_ZEROPAGE_MODE_DONTWAKE)
> - goto out;
> +
> + mode_dontwake = uffdio_zeropage.mode & UFFDIO_ZEROPAGE_MODE_DONTWAKE;
> + mode_access_likely = uffdio_zeropage.mode & UFFDIO_ZEROPAGE_MODE_ACCESS_LIKELY;
> + mode_write_likely = uffdio_zeropage.mode & UFFDIO_ZEROPAGE_MODE_WRITE_LIKELY;
> +
> + if (mode_dontwake)
> + return -EINVAL;
Hmm.. Why?
Note that the above uffdio_zeropage.mode check was for invalid mode flags
only, and I think that should be kept, but still I don't see why we want to
fail UFFDIO_ZEROPAGE_MODE_DONTWAKE users.
> +
> + uffd_flags = (mode_access_likely ? UFFD_FLAGS_ACCESS_LIKELY : 0) |
> + (mode_write_likely ? UFFD_FLAGS_WRITE_LIKELY : 0);
>
> if (mmget_not_zero(ctx->mm)) {
> ret = mfill_zeropage(ctx->mm, uffdio_zeropage.range.start,
> diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> index 6ad93a13282e..b586b7c1e265 100644
> --- a/include/uapi/linux/userfaultfd.h
> +++ b/include/uapi/linux/userfaultfd.h
> @@ -286,6 +286,8 @@ struct uffdio_copy {
> struct uffdio_zeropage {
> struct uffdio_range range;
> #define UFFDIO_ZEROPAGE_MODE_DONTWAKE ((__u64)1<<0)
> +#define UFFDIO_ZEROPAGE_MODE_ACCESS_LIKELY ((__u64)1<<2)
> +#define UFFDIO_ZEROPAGE_MODE_WRITE_LIKELY ((__u64)1<<3)
> __u64 mode;
>
> /*
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 3172158d8faa..5dfbb1e80369 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -249,6 +249,38 @@ static int mfill_zeropage_pte(struct mm_struct *dst_mm,
> return ret;
> }
>
> +static int mfill_clearpage_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
> + struct vm_area_struct *dst_vma,
> + unsigned long dst_addr,
> + uffd_flags_t uffd_flags)
> +{
> + struct page *page;
> + int ret;
> +
> + ret = -ENOMEM;
> + page = alloc_zeroed_user_highpage_movable(dst_vma, dst_addr);
> + if (!page)
> + goto out;
> +
> + /* The PTE is not marked as dirty unconditionally */
> + SetPageDirty(page);
> + __SetPageUptodate(page);
> +
> + ret = -ENOMEM;
Nit: can drop this since ret will always be -ENOMEM here..
Thanks,
> + if (mem_cgroup_charge(page_folio(page), dst_vma->vm_mm, GFP_KERNEL))
> + goto out_release;
> +
> + ret = mfill_atomic_install_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
> + page, true, uffd_flags);
> + if (ret)
> + goto out_release;
> +out:
> + return ret;
> +out_release:
> + put_page(page);
> + goto out;
> +}
> +
> /* Handles UFFDIO_CONTINUE for all shmem VMAs (shared or private). */
> static int mcontinue_atomic_pte(struct mm_struct *dst_mm,
> pmd_t *dst_pmd,
> @@ -511,6 +543,10 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
> err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
> dst_addr, src_addr, page,
> uffd_flags);
> + else if (!(uffd_flags & UFFD_FLAGS_WP) &&
> + (uffd_flags & UFFD_FLAGS_WRITE_LIKELY))
> + err = mfill_clearpage_pte(dst_mm, dst_pmd, dst_vma,
> + dst_addr, uffd_flags);
> else
> err = mfill_zeropage_pte(dst_mm, dst_pmd,
> dst_vma, dst_addr, uffd_flags);
> --
> 2.25.1
>
--
Peter Xu
