Re: [PATCH 5/6] userfaultfd: decouple fault reason from VMA flags

[Mike Rapoport <rppt@kernel.org>]

On Thu, Aug 27, 2026 at 10:10:24AM +0200, David Hildenbrand (Arm) wrote:
> On 8/27/26 09:49, Mike Rapoport wrote:
> > On Mon, Aug 24, 2026 at 04:46:14PM +0200, David Hildenbrand (Arm) wrote:
> >> On 8/23/26 14:17, Mike Rapoport (Microsoft) wrote:
> >>> Introduce enum uffd_reason to define reasons for user faults rather than
> >>> overload VM_UFFD_* VMA flags for that.
> >>>
> >>> Using a dedicated enum makes the code clearer and decoupling the fault
> >>> reason from VMA flags clears the way for moving the uffd mode bits out
> >>> of VMA namespace.
> >>>
> >>> No functional change.
> >>>
> >>> Assisted-by: copilot:claude-opus-4.6
> >>> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> >>> ---
> >>>  include/linux/userfaultfd_k.h    | 16 ++++++++++++++--
> >>>  include/uapi/linux/userfaultfd.h |  6 +++---
> >>>  mm/huge_memory.c                 |  6 +++---
> >>>  mm/hugetlb.c                     | 10 +++++-----
> >>>  mm/memory.c                      | 10 +++++-----
> >>>  mm/shmem.c                       |  4 ++--
> >>>  mm/userfaultfd.c                 | 30 +++++++++++++++---------------
> >>>  7 files changed, 47 insertions(+), 35 deletions(-)
> >>>
> >>> diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
> >>> index 45355bdb4ec7..f401623f315d 100644
> >>> --- a/include/linux/userfaultfd_k.h
> >>> +++ b/include/linux/userfaultfd_k.h
> >>> @@ -9,6 +9,18 @@
> >>>  #ifndef _LINUX_USERFAULTFD_K_H
> >>>  #define _LINUX_USERFAULTFD_K_H
> >>>  
> >>> +#include <linux/bits.h>
> >>> +
> >>> +/* Fault reason #PF handler passes to handle_userfault() */
> >>> +enum uf_reason {
> >>
> >> Can we just call this "userfault_reason" or "uffd_reason" ? Maybe the latter is
> >> actually what we want?
> > 
> > userfault_reason sounds better to me.
> > 
> > It describes what kind of user fault we are handling and the 'fd' part has
> > nothing to do with it. 
> > We do use uffd as a short name for the subsystem, but still most if not all
> > userfaultfd "external" APIs use userfault_ prefix.
> > 
> > uf_ was an attempt to make it wee shorter :)
> 
> Yeah, I got that; while uffd is a known acronym, the uf_ not so much (and also I
> wouldn't suggest it to become a thing, lol :) )
> 
> I've been wondering for a while whether it really should be called
> 
> 	handle_userfault()
> 
> And not instead
> 
> 	handle_userfaultfd()

The 'fd' part here sounds really weird :)
 
> Or maybe even better
> 
> 	handle_uffd_fault()

That's somehow tautological, but maybe using uffd_ as prefix would make it
a "subsystem namespace", so tautology won't be as blunt:

	uffd_handle_fault()
 
> And then have
> 
> 	uffd_fault_reason

Could work, yes. No strong feelings between this one and userfault_reason.
 
> ... but just a thought.
> 
> -- 
> Cheers,
> 
> David

-- 
Sincerely yours,
Mike.