Re: [PATCH v5 08/10] userfaultfd: add UFFDIO_CONTINUE ioctl

[Axel Rasmussen <axelrasmussen@google.com>]

"

On Wed, Feb 10, 2021 at 1:22 PM Axel Rasmussen <axelrasmussen@google.com> wrote:
>
> This ioctl is how userspace ought to resolve "minor" userfaults. The
> idea is, userspace is notified that a minor fault has occurred. It might
> change the contents of the page using its second non-UFFD mapping, or
> not. Then, it calls UFFDIO_CONTINUE to tell the kernel "I have ensured
> the page contents are correct, carry on setting up the mapping".
>
> Note that it doesn't make much sense to use UFFDIO_{COPY,ZEROPAGE} for
> MINOR registered VMAs. ZEROPAGE maps the VMA to the zero page; but in
> the minor fault case, we already have some pre-existing underlying page.
> Likewise, UFFDIO_COPY isn't useful if we have a second non-UFFD mapping.
> We'd just use memcpy() or similar instead.
>
> It turns out hugetlb_mcopy_atomic_pte() already does very close to what
> we want, if an existing page is provided via `struct page **pagep`. We
> already special-case the behavior a bit for the UFFDIO_ZEROPAGE case, so
> just extend that design: add an enum for the three modes of operation,
> and make the small adjustments needed for the MCOPY_ATOMIC_CONTINUE
> case. (Basically, look up the existing page, and avoid adding the
> existing page to the page cache or calling set_page_huge_active() on
> it.)
>
> Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
> ---
>  fs/userfaultfd.c                 | 67 ++++++++++++++++++++++++++++++++
>  include/linux/hugetlb.h          |  3 ++
>  include/linux/userfaultfd_k.h    | 18 +++++++++
>  include/uapi/linux/userfaultfd.h | 21 +++++++++-
>  mm/hugetlb.c                     | 39 ++++++++++++-------
>  mm/userfaultfd.c                 | 37 +++++++++++-------
>  6 files changed, 156 insertions(+), 29 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index b351a8552140..5e20f425efd7 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1527,6 +1527,10 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
>                 if (!(uffdio_register.mode & UFFDIO_REGISTER_MODE_WP))
>                         ioctls_out &= ~((__u64)1 << _UFFDIO_WRITEPROTECT);
>
> +               /* CONTINUE ioctl is only supported for MINOR ranges. */
> +               if (!(uffdio_register.mode & UFFDIO_REGISTER_MODE_MINOR))
> +                       ioctls_out &= ~((__u64)1 << _UFFDIO_CONTINUE);
> +
>                 /*
>                  * Now that we scanned all vmas we can already tell
>                  * userland which ioctls methods are guaranteed to
> @@ -1880,6 +1884,66 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
>         return ret;
>  }
>
> +static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
> +{
> +       __s64 ret;
> +       struct uffdio_continue uffdio_continue;
> +       struct uffdio_continue __user *user_uffdio_continue;
> +       struct userfaultfd_wake_range range;
> +
> +       user_uffdio_continue = (struct uffdio_continue __user *)arg;
> +
> +       ret = -EAGAIN;
> +       if (READ_ONCE(ctx->mmap_changing))
> +               goto out;
> +
> +       ret = -EFAULT;
> +       if (copy_from_user(&uffdio_continue, user_uffdio_continue,
> +                          /* don't copy the output fields */
> +                          sizeof(uffdio_continue) - (sizeof(__s64))))
> +               goto out;
> +
> +       ret = validate_range(ctx->mm, &uffdio_continue.range.start,
> +                            uffdio_continue.range.len);
> +       if (ret)
> +               goto out;
> +
> +       ret = -EINVAL;
> +       /* double check for wraparound just in case. */
> +       if (uffdio_continue.range.start + uffdio_continue.range.len <=
> +           uffdio_continue.range.start) {
> +               goto out;
> +       }
> +       if (uffdio_continue.mode & ~UFFDIO_CONTINUE_MODE_DONTWAKE)
> +               goto out;
> +
> +       if (mmget_not_zero(ctx->mm)) {
> +               ret = mcopy_continue(ctx->mm, uffdio_continue.range.start,
> +                                    uffdio_continue.range.len,
> +                                    &ctx->mmap_changing);
> +               mmput(ctx->mm);
> +       } else {
> +               return -ESRCH;
> +       }
> +
> +       if (unlikely(put_user(ret, &user_uffdio_continue->mapped)))
> +               return -EFAULT;
> +       if (ret < 0)
> +               goto out;
> +
> +       /* len == 0 would wake all */
> +       BUG_ON(!ret);
> +       range.len = ret;
> +       if (!(uffdio_continue.mode & UFFDIO_CONTINUE_MODE_DONTWAKE)) {
> +               range.start = uffdio_continue.range.start;
> +               wake_userfault(ctx, &range);
> +       }
> +       ret = range.len == uffdio_continue.range.len ? 0 : -EAGAIN;
> +
> +out:
> +       return ret;
> +}
> +
>  static inline unsigned int uffd_ctx_features(__u64 user_features)
>  {
>         /*
> @@ -1964,6 +2028,9 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd,
>         case UFFDIO_WRITEPROTECT:
>                 ret = userfaultfd_writeprotect(ctx, arg);
>                 break;
> +       case UFFDIO_CONTINUE:
> +               ret = userfaultfd_continue(ctx, arg);
> +               break;
>         }
>         return ret;
>  }
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index aa9e1d6de831..3d01d228fc78 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -11,6 +11,7 @@
>  #include <linux/kref.h>
>  #include <linux/pgtable.h>
>  #include <linux/gfp.h>
> +#include <linux/userfaultfd_k.h>
>
>  struct ctl_table;
>  struct user_struct;
> @@ -139,6 +140,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm, pte_t *dst_pte,
>                                 struct vm_area_struct *dst_vma,
>                                 unsigned long dst_addr,
>                                 unsigned long src_addr,
> +                               enum mcopy_atomic_mode mode,
>                                 struct page **pagep);
>  #endif /* CONFIG_USERFAULTFD */
>  bool hugetlb_reserve_pages(struct inode *inode, long from, long to,
> @@ -317,6 +319,7 @@ static inline int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
>                                                 struct vm_area_struct *dst_vma,
>                                                 unsigned long dst_addr,
>                                                 unsigned long src_addr,
> +                                               enum mcopy_atomic_mode mode,
>                                                 struct page **pagep)
>  {
>         BUG();
> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index e060d5f77cc5..794d1538b8ba 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -37,6 +37,22 @@ extern int sysctl_unprivileged_userfaultfd;
>
>  extern vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason);
>
> +/*
> + * The mode of operation for __mcopy_atomic and its helpers.
> + *
> + * This is almost an implementation detail (mcopy_atomic below doesn't take this
> + * as a parameter), but it's exposed here because memory-kind-specific
> + * implementations (e.g. hugetlbfs) need to know the mode of operation.
> + */
> +enum mcopy_atomic_mode {
> +       /* A normal copy_from_user into the destination range. */
> +       MCOPY_ATOMIC_NORMAL,
> +       /* Don't copy; map the destination range to the zero page. */
> +       MCOPY_ATOMIC_ZEROPAGE,
> +       /* Just install pte(s) with the existing page(s) in the page cache. */
> +       MCOPY_ATOMIC_CONTINUE,
> +};
> +
>  extern ssize_t mcopy_atomic(struct mm_struct *dst_mm, unsigned long dst_start,
>                             unsigned long src_start, unsigned long len,
>                             bool *mmap_changing, __u64 mode);
> @@ -44,6 +60,8 @@ extern ssize_t mfill_zeropage(struct mm_struct *dst_mm,
>                               unsigned long dst_start,
>                               unsigned long len,
>                               bool *mmap_changing);
> +extern ssize_t mcopy_continue(struct mm_struct *dst_mm, unsigned long dst_start,
> +                             unsigned long len, bool *mmap_changing);
>  extern int mwriteprotect_range(struct mm_struct *dst_mm,
>                                unsigned long start, unsigned long len,
>                                bool enable_wp, bool *mmap_changing);
> diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
> index f24dd4fcbad9..bafbeb1a2624 100644
> --- a/include/uapi/linux/userfaultfd.h
> +++ b/include/uapi/linux/userfaultfd.h
> @@ -40,10 +40,12 @@
>         ((__u64)1 << _UFFDIO_WAKE |             \
>          (__u64)1 << _UFFDIO_COPY |             \
>          (__u64)1 << _UFFDIO_ZEROPAGE |         \
> -        (__u64)1 << _UFFDIO_WRITEPROTECT)
> +        (__u64)1 << _UFFDIO_WRITEPROTECT |     \
> +        (__u64)1 << _UFFDIO_CONTINUE)
>  #define UFFD_API_RANGE_IOCTLS_BASIC            \
>         ((__u64)1 << _UFFDIO_WAKE |             \
> -        (__u64)1 << _UFFDIO_COPY)
> +        (__u64)1 << _UFFDIO_COPY |             \
> +        (__u64)1 << _UFFDIO_CONTINUE)
>
>  /*
>   * Valid ioctl command number range with this API is from 0x00 to
> @@ -59,6 +61,7 @@
>  #define _UFFDIO_COPY                   (0x03)
>  #define _UFFDIO_ZEROPAGE               (0x04)
>  #define _UFFDIO_WRITEPROTECT           (0x06)
> +#define _UFFDIO_CONTINUE               (0x07)
>  #define _UFFDIO_API                    (0x3F)
>
>  /* userfaultfd ioctl ids */
> @@ -77,6 +80,8 @@
>                                       struct uffdio_zeropage)
>  #define UFFDIO_WRITEPROTECT    _IOWR(UFFDIO, _UFFDIO_WRITEPROTECT, \
>                                       struct uffdio_writeprotect)
> +#define UFFDIO_CONTINUE                _IOR(UFFDIO, _UFFDIO_CONTINUE,  \
> +                                    struct uffdio_continue)
>
>  /* read() structure */
>  struct uffd_msg {
> @@ -268,6 +273,18 @@ struct uffdio_writeprotect {
>         __u64 mode;
>  };
>
> +struct uffdio_continue {
> +       struct uffdio_range range;
> +#define UFFDIO_CONTINUE_MODE_DONTWAKE          ((__u64)1<<0)
> +       __u64 mode;
> +
> +       /*
> +        * Fields below here are written by the ioctl and must be at the end:
> +        * the copy_from_user will not read past here.
> +        */
> +       __s64 mapped;
> +};
> +
>  /*
>   * Flags for the userfaultfd(2) system call itself.
>   */
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 2331281cf133..7ddf28730361 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -39,7 +39,6 @@
>  #include <linux/hugetlb.h>
>  #include <linux/hugetlb_cgroup.h>
>  #include <linux/node.h>
> -#include <linux/userfaultfd_k.h>
>  #include <linux/page_owner.h>
>  #include "internal.h"
>
> @@ -4648,8 +4647,10 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
>                             struct vm_area_struct *dst_vma,
>                             unsigned long dst_addr,
>                             unsigned long src_addr,
> +                           enum mcopy_atomic_mode mode,
>                             struct page **pagep)
>  {
> +       bool is_continue = (mode == MCOPY_ATOMIC_CONTINUE);
>         struct address_space *mapping;
>         pgoff_t idx;
>         unsigned long size;
> @@ -4659,8 +4660,18 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
>         spinlock_t *ptl;
>         int ret;
>         struct page *page;
> +       int writable;
>
> -       if (!*pagep) {
> +       mapping = dst_vma->vm_file->f_mapping;
> +       idx = vma_hugecache_offset(h, dst_vma, dst_addr);
> +
> +       if (is_continue) {
> +               ret = -EFAULT;
> +               page = find_lock_page(mapping, idx);
> +               *pagep = NULL;
> +               if (!page)
> +                       goto out;
> +       } else if (!*pagep) {
>                 ret = -ENOMEM;
>                 page = alloc_huge_page(dst_vma, dst_addr, 0);
>                 if (IS_ERR(page))
> @@ -4689,13 +4700,8 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
>          */
>         __SetPageUptodate(page);
>
> -       mapping = dst_vma->vm_file->f_mapping;
> -       idx = vma_hugecache_offset(h, dst_vma, dst_addr);
> -
> -       /*
> -        * If shared, add to page cache
> -        */
> -       if (vm_shared) {
> +       /* Add shared, newly allocated pages to the page cache. */
> +       if (vm_shared && !is_continue) {
>                 size = i_size_read(mapping->host) >> huge_page_shift(h);
>                 ret = -EFAULT;
>                 if (idx >= size)
> @@ -4740,8 +4746,14 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
>                 hugepage_add_new_anon_rmap(page, dst_vma, dst_addr);
>         }
>
> -       _dst_pte = make_huge_pte(dst_vma, page, dst_vma->vm_flags & VM_WRITE);
> -       if (dst_vma->vm_flags & VM_WRITE)
> +       /* For CONTINUE on a non-shared VMA, don't set VM_WRITE for CoW. */
> +       if (is_continue && !vm_shared)
> +               writable = 0;
> +       else
> +               writable = dst_vma->vm_flags & VM_WRITE;
> +
> +       _dst_pte = make_huge_pte(dst_vma, page, writable);
> +       if (writable)
>                 _dst_pte = huge_pte_mkdirty(_dst_pte);
>         _dst_pte = pte_mkyoung(_dst_pte);
>
> @@ -4755,8 +4767,9 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
>         update_mmu_cache(dst_vma, dst_addr, dst_pte);
>
>         spin_unlock(ptl);
> -       SetHPageMigratable(page);
> -       if (vm_shared)
> +       if (!is_continue)
> +               SetHPageMigratable(page);
> +       if (vm_shared || is_continue)
>                 unlock_page(page);
>         ret = 0;
>  out:

Apologies, I noticed a mistake here. Just like the
"unlock_page(page);" line above, I think we need to change to "if
(vm_shared || is_continue)" in the "out_release_unlock:" case as well.
I'll send this change in a v5.1 or a v6, depending on what other
comments this series gets in the next few days.

> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index b2ce61c1b50d..ce6cb4760d2c 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -207,7 +207,7 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
>                                               unsigned long dst_start,
>                                               unsigned long src_start,
>                                               unsigned long len,
> -                                             bool zeropage)
> +                                             enum mcopy_atomic_mode mode)
>  {
>         int vm_alloc_shared = dst_vma->vm_flags & VM_SHARED;
>         int vm_shared = dst_vma->vm_flags & VM_SHARED;
> @@ -227,7 +227,7 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
>          * by THP.  Since we can not reliably insert a zero page, this
>          * feature is not supported.
>          */
> -       if (zeropage) {
> +       if (mode == MCOPY_ATOMIC_ZEROPAGE) {
>                 mmap_read_unlock(dst_mm);
>                 return -EINVAL;
>         }
> @@ -273,8 +273,6 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
>         }
>
>         while (src_addr < src_start + len) {
> -               pte_t dst_pteval;
> -
>                 BUG_ON(dst_addr >= dst_start + len);
>
>                 /*
> @@ -297,16 +295,16 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
>                         goto out_unlock;
>                 }
>
> -               err = -EEXIST;
> -               dst_pteval = huge_ptep_get(dst_pte);
> -               if (!huge_pte_none(dst_pteval)) {
> +               if (mode != MCOPY_ATOMIC_CONTINUE &&
> +                   !huge_pte_none(huge_ptep_get(dst_pte))) {
> +                       err = -EEXIST;
>                         mutex_unlock(&hugetlb_fault_mutex_table[hash]);
>                         i_mmap_unlock_read(mapping);
>                         goto out_unlock;
>                 }
>
>                 err = hugetlb_mcopy_atomic_pte(dst_mm, dst_pte, dst_vma,
> -                                               dst_addr, src_addr, &page);
> +                                              dst_addr, src_addr, mode, &page);
>
>                 mutex_unlock(&hugetlb_fault_mutex_table[hash]);
>                 i_mmap_unlock_read(mapping);
> @@ -408,7 +406,7 @@ extern ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
>                                       unsigned long dst_start,
>                                       unsigned long src_start,
>                                       unsigned long len,
> -                                     bool zeropage);
> +                                     enum mcopy_atomic_mode mode);
>  #endif /* CONFIG_HUGETLB_PAGE */
>
>  static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
> @@ -458,7 +456,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
>                                               unsigned long dst_start,
>                                               unsigned long src_start,
>                                               unsigned long len,
> -                                             bool zeropage,
> +                                             enum mcopy_atomic_mode mcopy_mode,
>                                               bool *mmap_changing,
>                                               __u64 mode)
>  {
> @@ -469,6 +467,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
>         long copied;
>         struct page *page;
>         bool wp_copy;
> +       bool zeropage = (mcopy_mode == MCOPY_ATOMIC_ZEROPAGE);
>
>         /*
>          * Sanitize the command parameters:
> @@ -527,10 +526,12 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
>          */
>         if (is_vm_hugetlb_page(dst_vma))
>                 return  __mcopy_atomic_hugetlb(dst_mm, dst_vma, dst_start,
> -                                               src_start, len, zeropage);
> +                                               src_start, len, mcopy_mode);
>
>         if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
>                 goto out_unlock;
> +       if (mcopy_mode == MCOPY_ATOMIC_CONTINUE)
> +               goto out_unlock;
>
>         /*
>          * Ensure the dst_vma has a anon_vma or this page
> @@ -626,14 +627,22 @@ ssize_t mcopy_atomic(struct mm_struct *dst_mm, unsigned long dst_start,
>                      unsigned long src_start, unsigned long len,
>                      bool *mmap_changing, __u64 mode)
>  {
> -       return __mcopy_atomic(dst_mm, dst_start, src_start, len, false,
> -                             mmap_changing, mode);
> +       return __mcopy_atomic(dst_mm, dst_start, src_start, len,
> +                             MCOPY_ATOMIC_NORMAL, mmap_changing, mode);
>  }
>
>  ssize_t mfill_zeropage(struct mm_struct *dst_mm, unsigned long start,
>                        unsigned long len, bool *mmap_changing)
>  {
> -       return __mcopy_atomic(dst_mm, start, 0, len, true, mmap_changing, 0);
> +       return __mcopy_atomic(dst_mm, start, 0, len, MCOPY_ATOMIC_ZEROPAGE,
> +                             mmap_changing, 0);
> +}
> +
> +ssize_t mcopy_continue(struct mm_struct *dst_mm, unsigned long start,
> +                      unsigned long len, bool *mmap_changing)
> +{
> +       return __mcopy_atomic(dst_mm, start, 0, len, MCOPY_ATOMIC_CONTINUE,
> +                             mmap_changing, 0);
>  }
>
>  int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
> --
> 2.30.0.478.g8a0d178c01-goog
>