Re: [PATCH 7/9] userfaultfd: add UFFDIO_CONTINUE ioctl

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On Fri, Jan 15, 2021 at 11:04:49AM -0800, Axel Rasmussen 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@xxxxxxxxxx>
> ---
>  fs/userfaultfd.c                 | 67 +++++++++++++++++++++++++
>  include/linux/userfaultfd_k.h    |  2 +
>  include/uapi/linux/userfaultfd.h | 21 +++++++-
>  mm/hugetlb.c                     | 11 ++--
>  mm/userfaultfd.c                 | 86 ++++++++++++++++++++++++--------
>  5 files changed, 158 insertions(+), 29 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 19d6925be03f..f0eb2d63289f 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1530,6 +1530,10 @@ 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
> @@ -1883,6 +1887,66 @@ 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)
>  {
>  	/*
> @@ -1967,6 +2031,9 @@ 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/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> index ed157804ca02..49d7e7b4581f 100644
> --- a/include/linux/userfaultfd_k.h
> +++ b/include/linux/userfaultfd_k.h
> @@ -41,6 +41,8 @@ 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 1cc2cd8a5279..9a48305f4bdd 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 2b3741d6130c..84392d5fa079 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4666,12 +4666,14 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
>  	spinlock_t *ptl;
>  	int ret;
>  	struct page *page;
> +	bool new_page = false;
>  
>  	if (!*pagep) {
>  		ret = -ENOMEM;
>  		page = alloc_huge_page(dst_vma, dst_addr, 0);
>  		if (IS_ERR(page))
>  			goto out;
> +		new_page = true;
>  
>  		ret = copy_huge_page_from_user(page,
>  						(const void __user *) src_addr,
> @@ -4699,10 +4701,8 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
>  	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 && new_page) {

hugetlb_mcopy_atomic_pte() can be called with *page being set, when
copy_huge_page_from_user(allow_pagefault=true) is called outside of this
function before the retry.

IIUC this change could break that case because here new_page will also be false
then we won't insert page cache (while we should).

So I think instead of the new_page flag we may also want to pass in the new
mcopy_atomic_mode into this function, otherwise I don't see how we can identify
these cases.

>  		size = i_size_read(mapping->host) >> huge_page_shift(h);
>  		ret = -EFAULT;
>  		if (idx >= size)
> @@ -4762,7 +4762,8 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
>  	update_mmu_cache(dst_vma, dst_addr, dst_pte);
>  
>  	spin_unlock(ptl);
> -	set_page_huge_active(page);
> +	if (new_page)
> +		set_page_huge_active(page);

Same here.

>  	if (vm_shared)
>  		unlock_page(page);
>  	ret = 0;
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index b2ce61c1b50d..0ecc50525dd4 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -197,6 +197,16 @@ static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
>  	return pmd_alloc(mm, pud, address);
>  }
>  
> +/* The mode of operation for __mcopy_atomic and its helpers. */
> +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 setup the dst_vma, without modifying the underlying page(s). */
> +	MCOPY_ATOMIC_CONTINUE,
> +};
> +
>  #ifdef CONFIG_HUGETLB_PAGE
>  /*
>   * __mcopy_atomic processing for HUGETLB vmas.  Note that this routine is
> @@ -207,7 +217,7 @@ 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 +237,7 @@ 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 +283,6 @@ 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);
>  
>  		/*
> @@ -297,12 +305,22 @@ 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)) {
> -			mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> -			i_mmap_unlock_read(mapping);
> -			goto out_unlock;
> +		if (mode == MCOPY_ATOMIC_CONTINUE) {
> +			/* hugetlb_mcopy_atomic_pte unlocks the page. */
> +			page = find_lock_page(mapping, idx);

If my above understanding is right, we may also consider to move this
find_lock_page() into hugetlb_mcopy_atomic_pte() directly, then as we pass in
hugetlb_mcopy_atomic_pte(page==NULL, mode==MCOPY_ATOMIC_CONTINUE) we'll fetch
the page cache instead of allocation.

> +			if (!page) {
> +				err = -EFAULT;
> +				mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> +				i_mmap_unlock_read(mapping);
> +				goto out_unlock;
> +			}
> +		} else {
> +			if (!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,
> @@ -408,7 +426,7 @@ 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,
> @@ -417,7 +435,7 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
>  						unsigned long dst_addr,
>  						unsigned long src_addr,
>  						struct page **page,
> -						bool zeropage,
> +						enum mcopy_atomic_mode mode,
>  						bool wp_copy)
>  {
>  	ssize_t err;
> @@ -433,22 +451,38 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
>  	 * and not in the radix tree.
>  	 */
>  	if (!(dst_vma->vm_flags & VM_SHARED)) {
> -		if (!zeropage)
> +		switch (mode) {
> +		case MCOPY_ATOMIC_NORMAL:
>  			err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
>  					       dst_addr, src_addr, page,
>  					       wp_copy);
> -		else
> +			break;
> +		case MCOPY_ATOMIC_ZEROPAGE:
>  			err = mfill_zeropage_pte(dst_mm, dst_pmd,
>  						 dst_vma, dst_addr);
> +			break;
> +		/* It only makes sense to CONTINUE for shared memory. */
> +		case MCOPY_ATOMIC_CONTINUE:
> +			err = -EINVAL;
> +			break;
> +		}
>  	} else {
>  		VM_WARN_ON_ONCE(wp_copy);
> -		if (!zeropage)
> +		switch (mode) {
> +		case MCOPY_ATOMIC_NORMAL:
>  			err = shmem_mcopy_atomic_pte(dst_mm, dst_pmd,
>  						     dst_vma, dst_addr,
>  						     src_addr, page);
> -		else
> +			break;
> +		case MCOPY_ATOMIC_ZEROPAGE:
>  			err = shmem_mfill_zeropage_pte(dst_mm, dst_pmd,
>  						       dst_vma, dst_addr);
> +			break;
> +		case MCOPY_ATOMIC_CONTINUE:
> +			/* FIXME: Add minor fault interception for shmem. */
> +			err = -EINVAL;
> +			break;
> +		}
>  	}
>  
>  	return err;
> @@ -458,7 +492,7 @@ 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)
>  {
> @@ -527,7 +561,7 @@ 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;
> @@ -577,7 +611,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
>  		BUG_ON(pmd_trans_huge(*dst_pmd));
>  
>  		err = mfill_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
> -				       src_addr, &page, zeropage, wp_copy);
> +				       src_addr, &page, mcopy_mode, wp_copy);
>  		cond_resched();
>  
>  		if (unlikely(err == -ENOENT)) {
> @@ -626,14 +660,22 @@ 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,
> -- 
> 2.30.0.284.gd98b1dd5eaa7-goog
> 

-- 
Peter Xu





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