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

[Mike Rapoport <rppt@kernel.org>]

On Tue, Aug 25, 2026 at 02:00:10PM +0100, Lorenzo Stoakes (ARM) wrote:
> On Tue, Aug 25, 2026 at 01:37:30PM +0300, Mike Rapoport wrote:
> > On Mon, Aug 24, 2026 at 05:28:29PM +0100, Lorenzo Stoakes (ARM) wrote:
> > > On Sun, Aug 23, 2026 at 03:17:42PM +0300, 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 {
> > > > +	USERFAULT_MISSING	= BIT(0),
> > > > +	USERFAULT_MINOR		= BIT(1),
> > > > +	USERFAULT_RWP		= BIT(2),
> > > > +	USERFAULT_WP		= BIT(3),
> > > > +};
> > >
> > > Hmm your commit message says uffd_reason, uf_reason makes me think of the
> > > character Ulf from House of the Dragon. But not uffd. So as per David let's
> > > rename it :)
> > >
> > > I'm also not sure if an enum is the right thing for flag values?
> >
> > I'll ask LLM why it chose it :)
> 
> I mean you then introduce the same flags again seemingly with different
> names as #define's in the next patch... having several sets of flags with
> subtly different names seems unwise.

The names are important, the values are not. 

There are two cases that currently use the same VMA_UFFD_* flags:
* the way VMA is registered with uffd, i.e. the 'mode' part
* the type of the user fault that the generic #PF handler passes to
  handle_userfault()

They are related, a fault in a VMA that was registered as MISSING will
never pass MINOR to handle_userfault(), but I think it'll be actually
clearer to separate them semantically, so that when you read a call site of
handle_userfault() it is clear what type of the fault it is and when you
parse userfaultfd code you see what modes user wanted for a VMA.
 
> > > Anything that is parameterised by enum uffd_reason that combines flags will
> > > break any switch statement in there and yada yada.
> > >
> > > I wonder if better just as #define's + unsigned long or something?
> > >
> > > Or you could do (and this leads to nicer stuff later):
> > >
> > > 	enum uffd_reason {
> > > 	     USERFAULT_MISSING_BIT = 0,
> > > 	     USERFAULT_MINOR_BIT = 1,
> > > 	     USERFAULT_RWP_BIT = 2,
> > > 	     USERFAULT_WP_BIT = 3,
> > > 	};
> > >
> > > 	#define USERFAULT_MISSING BIT(USERFAULT_MISSING_BIT)
> > > etc.
> >
> > Looks over-engineered to me tbh, if we drop an enum, I'd just
> >
> > #define FLAG (1 << SHIFT)
> >
> > and call it a day.
> >
> > Also see below about aligning with uABI flags.
> 
> See review on 6/6, I'm confused actually why we have several sets of these
> flags...

I can see that ;-)
 
> But in general it seems like these flags (in one form or another) are being
> repeatedly referenced, so it's not really over-engineering I don't think to
> abstract some of that.

Again, the bit numbers do not matter, they are the same because it's easy
to count from 0. I can make one of those count backwards if it helps :)
 
> Maybe can be in wrappers that make it nicer. But really the issue is the
> duplication in modes/reasons/flags...
> 
> >
> > > > @@ -168,9 +168,9 @@ struct uffd_msg {
> > > >
> > > >  /* flags for UFFD_EVENT_PAGEFAULT */
> > > >  #define UFFD_PAGEFAULT_FLAG_WRITE	(1<<0)	/* If this was a write fault */
> > > > -#define UFFD_PAGEFAULT_FLAG_WP		(1<<1)	/* If reason is VM_UFFD_WP */
> > > > -#define UFFD_PAGEFAULT_FLAG_MINOR	(1<<2)	/* If reason is VM_UFFD_MINOR */
> > > > -#define UFFD_PAGEFAULT_FLAG_RWP		(1<<3)	/* If reason is VM_UFFD_RWP */
> > > > +#define UFFD_PAGEFAULT_FLAG_WP		(1<<1)	/* If reason is uffd-wp */
> > > > +#define UFFD_PAGEFAULT_FLAG_MINOR	(1<<2)	/* If reason is uffd-minor */
> > > > +#define UFFD_PAGEFAULT_FLAG_RWP		(1<<3)	/* If reason is uffd-rwp */
> > >
> > > Is it worth retaining the same bit indexes as the reasons?
> > >
> > > Reasons:
> > >
> > > 	Bit number
> > > MINOR	0
> > > RWP	1
> > > WP	2
> > >
> > > Page fault flags:
> > >
> > > 	Bit number
> > > MINOR	2
> > > RWP	3
> > > WP	1
> >
> > If we go this way, than it must be
> >
> > #define USERFAULT_MINOR UFFD_PAGEFAULT_FLAG_MINOR
> >
> > so we won't need to keep them in sync explicitly.
> >
> > With a caveat of USERFAULT_MISSING that is expressed as "no flags in
> > uffd_msg" :)
> 
> Ugh.

Yeah, and the PAGEFAULT_FLAG numbers are set in stone because it's uABI.
 
> > >
> > > With matching flags and unsigned long you could do
> > >
> > > 	msg.arg.pagefault.flags |= reason;
> > >
> > > I think?
> >
> > Almost:
> >
> > 	msg.arg.pagefault.flags |= (reason & ~USERFAULT_MISSING);
> >
> > And define USERFAULT_MISSING as (1 << 0) with a comment why it's fine.
> >
> > I don't feel strongly about it, but my preference is to define reason flags
> > independently of UFFD_PAGEFAULT_FLAGs and keep the ifs here.
> 
> And also modes... Again I think fixing that mess somehow is the better way forward.

Can you elaborate?

> > > > @@ -2793,14 +2793,14 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
> > > >  	 * If VMA has UFFD WP faults enabled and WP fault, wait for userspace to
> > > >  	 * resolve the fault.
> > > >  	 */
> > > > -	if (!pte_write(ptent) && (reason & VM_UFFD_WP))
> > > > +	if (!pte_write(ptent) && (reason & USERFAULT_WP))
> > >
> > > I wonder if you could actually
> > >
> > > You do this quite a lot and they read a bit horribly with the && and & on the
> > > same sight-line. With the changes to the enum proposed above you could do:
> > >
> > > 	if (!pte_write(ptent) && test_bit(reason, USERFAULT_WP_BIT))
> >
> > I find && and & perfectly readable and adding _BIT defines looks really
> > excessive to me.
> 
> Discussed in sub-thread. We'll agree to disagree I suppose.
 
Yes, we will :)

> > > > @@ -2835,7 +2835,7 @@ static inline unsigned int userfaultfd_get_blocking_state(unsigned int flags)
> > > >   * fatal_signal_pending()s, and the mmap_lock must be released before
> > > >   * returning it.
> > > >   */
> > > > -vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
> > > > +vm_fault_t handle_userfault(struct vm_fault *vmf, enum uf_reason reason)
> > >
> > > Hmm what was the 'reason' here before? The flags? Maybe more reason (no pun
> > > intended) to keep the values the same?
> >
> > The 'reason' before was a VM_UFFD_SOMETHING, we really can't keep the
> > values the same, but we surely can keep it unsigned long.
> 
> I notice the 'mode' which is not the same as the 'reason' is an unsigned
> int in 6/6...

Didn't you suggest to make 'reason' an unsigned int as well?

'mode' in 6/6 is an unsigned int because if it were an enum it'd require
#include <linux/userfaultfd_k.h> in mm_types.h, see the commit message
there.

> --
> Cheers, Lorenzo
> 

-- 
Sincerely yours,
Mike.