linux-kernel - Re: [PATCH v5 3/3] userfaultfd: use per-vma locks in userfaultfd operations

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Message-ID: <20240213033307.zbhrpjigco7vl56z@revolver>
Date: Mon, 12 Feb 2024 22:33:07 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lokesh Gidra <lokeshgidra@...gle.com>
Cc: akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        selinux@...r.kernel.org, surenb@...gle.com, kernel-team@...roid.com,
        aarcange@...hat.com, peterx@...hat.com, david@...hat.com,
        axelrasmussen@...gle.com, bgeffon@...gle.com, willy@...radead.org,
        jannh@...gle.com, kaleshsingh@...gle.com, ngeoffray@...gle.com,
        timmurray@...gle.com, rppt@...nel.org
Subject: Re: [PATCH v5 3/3] userfaultfd: use per-vma locks in userfaultfd
 operations

* Lokesh Gidra <lokeshgidra@...gle.com> [240212 19:19]:
> All userfaultfd operations, except write-protect, opportunistically use
> per-vma locks to lock vmas. On failure, attempt again inside mmap_lock
> critical section.
> 
> Write-protect operation requires mmap_lock as it iterates over multiple
> vmas.
> 
> Signed-off-by: Lokesh Gidra <lokeshgidra@...gle.com>
> ---
>  fs/userfaultfd.c              |  13 +-
>  include/linux/userfaultfd_k.h |   5 +-
>  mm/userfaultfd.c              | 392 ++++++++++++++++++++++++++--------
>  3 files changed, 312 insertions(+), 98 deletions(-)
> 
..

> +
> +static __always_inline
> +struct vm_area_struct *find_vma_and_prepare_anon(struct mm_struct *mm,
> +						 unsigned long addr)
> +{
> +	struct vm_area_struct *vma;
> +
> +	mmap_assert_locked(mm);
> +	vma = vma_lookup(mm, addr);
> +	if (!vma)
> +		vma = ERR_PTR(-ENOENT);
> +	else if (!(vma->vm_flags & VM_SHARED) && anon_vma_prepare(vma))
> +		vma = ERR_PTR(-ENOMEM);

Nit: I just noticed that the code below says anon_vma_prepare() is unlikely.

..

> +static struct vm_area_struct *lock_mm_and_find_dst_vma(struct mm_struct *dst_mm,
> +						       unsigned long dst_start,
> +						       unsigned long len)
> +{
> +	struct vm_area_struct *dst_vma;
> +	int err;
> +
> +	mmap_read_lock(dst_mm);
> +	dst_vma = find_vma_and_prepare_anon(dst_mm, dst_start);
> +	if (IS_ERR(dst_vma)) {
> +		err = PTR_ERR(dst_vma);

It's sort of odd you decode then re-encode this error, but it's correct
the way you have it written.  You could just encode ENOENT instead?

> +		goto out_unlock;
> +	}
> +
> +	if (validate_dst_vma(dst_vma, dst_start + len))
> +		return dst_vma;
> +
> +	err = -ENOENT;
> +out_unlock:
> +	mmap_read_unlock(dst_mm);
> +	return ERR_PTR(err);
>  }
> +#endif
>  
..

> +static __always_inline
> +long find_vmas_mm_locked(struct mm_struct *mm,

int would probably do?
> +			 unsigned long dst_start,
> +			 unsigned long src_start,
> +			 struct vm_area_struct **dst_vmap,
> +			 struct vm_area_struct **src_vmap)
> +{
> +	struct vm_area_struct *vma;
> +
> +	mmap_assert_locked(mm);
> +	vma = find_vma_and_prepare_anon(mm, dst_start);
> +	if (IS_ERR(vma))
> +		return PTR_ERR(vma);
> +
> +	*dst_vmap = vma;
> +	/* Skip finding src_vma if src_start is in dst_vma */
> +	if (src_start >= vma->vm_start && src_start < vma->vm_end)
> +		goto out_success;
> +
> +	vma = vma_lookup(mm, src_start);
> +	if (!vma)
> +		return -ENOENT;
> +out_success:
> +	*src_vmap = vma;
> +	return 0;
> +}
> +
> +#ifdef CONFIG_PER_VMA_LOCK
> +static long find_and_lock_vmas(struct mm_struct *mm,

This could also be an int return type, I must be missing something?

..

> +	*src_vmap = lock_vma_under_rcu(mm, src_start);
> +	if (likely(*src_vmap))
> +		return 0;
> +
> +	/* Undo any locking and retry in mmap_lock critical section */
> +	vma_end_read(*dst_vmap);
> +
> +	mmap_read_lock(mm);
> +	err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
> +	if (!err) {
> +		/*
> +		 * See comment in lock_vma() as to why not using
> +		 * vma_start_read() here.
> +		 */
> +		down_read(&(*dst_vmap)->vm_lock->lock);
> +		if (*dst_vmap != *src_vmap)
> +			down_read(&(*src_vmap)->vm_lock->lock);
> +	}
> +	mmap_read_unlock(mm);
> +	return err;
> +}
> +#else
> +static long lock_mm_and_find_vmas(struct mm_struct *mm,
> +				  unsigned long dst_start,
> +				  unsigned long src_start,
> +				  struct vm_area_struct **dst_vmap,
> +				  struct vm_area_struct **src_vmap)
> +{
> +	long err;
> +
> +	mmap_read_lock(mm);
> +	err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
> +	if (err)
> +		mmap_read_unlock(mm);
> +	return err;
>  }
> +#endif

This section is much easier to understand.  Thanks.

Thanks,
Liam