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

[Axel Rasmussen <axelrasmussen@google.com>]

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On Mon, Mar 6, 2023 at 5:30 PM Nadav Amit <namit@vmware.com> wrote:

>
>
> > On Mar 6, 2023, at 5:19 PM, Peter Xu <peterx@redhat.com> wrote:
> >
> > !! External Email
> >
> > 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?
>

My assumption is that __u64 is at least 64 bits wide on all platforms, and
so it is sufficient. I believe the standard allows unsigned long to be
32-bits, so __u64 may be overkill on some platforms, but to me the cost is
small enough I'd prefer to avoid defining a second almost-identical
structure.

Then again though as I say below, I don't feel strongly about this refactor.


> >
> > [...]
> >
> >> 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)?
>

I don't feel strongly, I'm fine with dropping this patch. I'll make that
change in a v4 (I think there will be some conflicts to resolve in the
patches after this one, so I'll post a new version to avoid troubling
anyone).


>
> Just in case you missed, it is __always_inline, so I presume that from a
> generated code point-of-view it is the same.
>
> Having said that, small assignments to local start, let and range variables
> would make the code easier to read and reduce the change-set.
>
>

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