linux-kernel - Re: [PATCH v3 4/5] mm: userfaultfd: don't separate addr + len arguments

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Date:   Mon, 6 Mar 2023 20:19:56 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        Muchun Song <muchun.song@...ux.dev>,
        Nadav Amit <namit@...are.com>, Shuah Khan <shuah@...nel.org>,
        James Houghton <jthoughton@...gle.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v3 4/5] mm: userfaultfd: don't separate addr + len
 arguments

On Mon, Mar 06, 2023 at 02:50:23PM -0800, Axel Rasmussen wrote:
> We have a lot of functions which take an address + length pair,
> currently passed as separate arguments. However, in our userspace API we
> already have struct uffdio_range, which is exactly this pair, and this
> is what we get from userspace when ioctls are called.
> 
> Instead of splitting the struct up into two separate arguments, just
> plumb the struct through to the functions which use it (once we get to
> the mfill_atomic_pte level, we're dealing with single (huge)pages, so we
> don't need both parts).
> 
> Relatedly, for waking, just re-use this existing structure instead of
> defining a new "struct uffdio_wake_range".
> 
> Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>
> ---
>  fs/userfaultfd.c              | 107 +++++++++++++---------------------
>  include/linux/userfaultfd_k.h |  17 +++---
>  mm/userfaultfd.c              |  92 ++++++++++++++---------------
>  3 files changed, 96 insertions(+), 120 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index b8e328123b71..984b63b0fc75 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -95,11 +95,6 @@ struct userfaultfd_wait_queue {
>  	bool waken;
>  };
>  
> -struct userfaultfd_wake_range {
> -	unsigned long start;
> -	unsigned long len;
> -};

Would there still be a difference on e.g. 32 bits systems?

[...]

>  static __always_inline int validate_range(struct mm_struct *mm,
> -					  __u64 start, __u64 len)
> +					  const struct uffdio_range *range)
>  {
>  	__u64 task_size = mm->task_size;
>  
> -	if (start & ~PAGE_MASK)
> +	if (range->start & ~PAGE_MASK)
>  		return -EINVAL;
> -	if (len & ~PAGE_MASK)
> +	if (range->len & ~PAGE_MASK)
>  		return -EINVAL;
> -	if (!len)
> +	if (!range->len)
>  		return -EINVAL;
> -	if (start < mmap_min_addr)
> +	if (range->start < mmap_min_addr)
>  		return -EINVAL;
> -	if (start >= task_size)
> +	if (range->start >= task_size)
>  		return -EINVAL;
> -	if (len > task_size - start)
> +	if (range->len > task_size - range->start)
>  		return -EINVAL;
>  	return 0;
>  }

Personally I don't like a lot on such a change. :( It avoids one parameter
being passed over but it can add a lot indirections.

Do you strongly suggest this?  Shall we move on without this so to not
block the last patch (which I assume is the one you're looking for)?

Thanks,

-- 
Peter Xu