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

Peter Xu
2023-03-07T01:20:05Z

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@google.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