linux-kernel - Re: [PATCH v3 1/4] mm: Introduce vm_uffd

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Message-ID: <aNwmE11LirPtEuGW@x1.local>
Date: Tue, 30 Sep 2025 14:48:51 -0400
From: Peter Xu <peterx@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	James Houghton <jthoughton@...gle.com>,
	Nikita Kalyazin <kalyazin@...zon.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Ujwal Kundur <ujwal.kundur@...il.com>,
	Mike Rapoport <rppt@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Michal Hocko <mhocko@...e.com>, Muchun Song <muchun.song@...ux.dev>,
	Oscar Salvador <osalvador@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v3 1/4] mm: Introduce vm_uffd_ops API

On Tue, Sep 30, 2025 at 11:36:53AM +0200, David Hildenbrand wrote:
> > +/* VMA userfaultfd operations */
> > +struct vm_uffd_ops {
> > +	/**
> > +	 * @uffd_features: features supported in bitmask.
> > +	 *
> > +	 * When the ops is defined, the driver must set non-zero features
> > +	 * to be a subset (or all) of: VM_UFFD_MISSING|WP|MINOR.
> > +	 *
> > +	 * NOTE: VM_UFFD_MISSING is still only supported under mm/ so far.
> > +	 */
> > +	unsigned long uffd_features;
> 
> This variable name is a bit confusing , because it's all about vma flags,
> not uffd features. Just reading the variable, I would rather connect it to
> things like UFFD_FEATURE_WP_UNPOPULATED.
> 
> As currently used for VM flags, maybe you should call this
> 
> 	unsigned long uffd_vm_flags;
> 
> or sth like that.

Indeed it's slightly confusing.  However uffd_vm_flags is confusing in
another way, where it seems to imply some flags similar to vm_flags that is
prone to change.

How about uffd_vm_flags_supported / uffd_modes_supported?

> 
> I briefly wondered whether we could use actual UFFD_FEATURE_* here, but they
> are rather unsuited for this case here (e.g., different feature flags for
> hugetlb support/shmem support etc).
> 
> But reading "uffd_ioctls" below, can't we derive the suitable vma flags from
> the supported ioctls?
> 
> _UFFDIO_COPY | _UFDIO_ZEROPAGE -> VM_UFFD_MISSING
> _UFFDIO_WRITEPROTECT -> VM_UFFD_WP
> _UFFDIO_CONTINUE -> VM_UFFD_MINOR

Yes we can deduce that, but it'll be unclear then when one stares at a
bunch of ioctls and cannot easily digest the modes the memory type
supports.  Here, the modes should be the most straightforward way to
describe the capability of a memory type.

If hugetlbfs supported ZEROPAGE, then we can deduce the ioctls the other
way round, and we can drop the uffd_ioctls.  However we need the ioctls now
for hugetlbfs to make everything generic.

Do you mind I still keep it as-is?  So far that's still the clearest I can
think of.  It's only set when some support is added to a memory type, so
it's a one-time shot.

Thanks,

-- 
Peter Xu