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

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Message-ID: <aNwm-uAt98KA9Euh@x1.local>
Date: Tue, 30 Sep 2025 14:52:42 -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 4/4] mm: Apply vm_uffd_ops API to core mm

On Tue, Sep 30, 2025 at 11:23:19AM +0200, David Hildenbrand wrote:
> On 26.09.25 23:16, Peter Xu wrote:
> > Move userfaultfd core to use new vm_uffd_ops API. After this change file
> > systems that implement vm_operations_struct can start using new API for
> > userfaultfd operations.
> > 
> > When at it, moving vma_can_userfault() into mm/userfaultfd.c instead,
> > because it's getting too big.  It's only used in slow paths so it shouldn't
> > be an issue.  Move the pte marker check before wp_async, which might be
> > more intuitive because wp_async depends on pte markers.  That shouldn't
> > cause any functional change though because only one check would take effect
> > depending on whether pte marker was selected in config.
> > 
> > This will also remove quite some hard-coded checks for either shmem or
> > hugetlbfs.  Now all the old checks should still work but with vm_uffd_ops.
> > 
> > Note that anonymous memory will still need to be processed separately
> > because it doesn't have vm_ops at all.
> > 
> > Reviewed-by: James Houghton <jthoughton@...gle.com>
> > Acked-by: Mike Rapoport <rppt@...nel.org>
> > Signed-off-by: Peter Xu <peterx@...hat.com>
> > ---
> 
> [...]
> 
> > +++ b/mm/userfaultfd.c
> > @@ -20,6 +20,43 @@
> >   #include "internal.h"
> >   #include "swap.h"
> > +bool vma_can_userfault(struct vm_area_struct *vma, vm_flags_t vm_flags,
> > +		       bool wp_async)
> > +{
> > +	unsigned long supported;
> > +
> > +	if (vma->vm_flags & VM_DROPPABLE)
> > +		return false;
> > +
> > +	vm_flags &= __VM_UFFD_FLAGS;
> > +
> > +#ifndef CONFIG_PTE_MARKER_UFFD_WP
> 
> While at it, you can turn that into an
> !IS_ENABLED(CONFIG_PTE_MARKER_UFFD_WP) to avoid the ifdef.
> 
> > +	/*
> > +	 * If user requested uffd-wp but not enabled pte markers for
> > +	 * uffd-wp, then any file system (like shmem or hugetlbfs) are not
> > +	 * supported but only anonymous.
> > +	 */
> > +	if ((vm_flags & VM_UFFD_WP) && !vma_is_anonymous(vma))
> > +		return false;
> > +#endif
> > +	/*
> > +	 * If wp async enabled, and WP is the only mode enabled, allow any
> > +	 * memory type.
> > +	 */
> > +	if (wp_async && (vm_flags == VM_UFFD_WP))
> > +		return true;
> 
> 
> > +
> > +	if (vma_is_anonymous(vma))
> > +		/* Anonymous has no page cache, MINOR not supported */
> > +		supported = VM_UFFD_MISSING | VM_UFFD_WP;
> > +	else if (vma_get_uffd_ops(vma))
> > +		supported = vma_get_uffd_ops(vma)->uffd_features;
> > +	else
> > +		return false;
> 
> To avoid the hidde return here, I think you can just do
> 
> 	supported = 0;
> 
> 
> Or even cleaner, just do
> 
> unsigned long supported = 0
> ...
> if (vma_is_anonymous(vma))
> 	supported = ...
> else if (vma_get_uffd_ops(vma))
> 	supported = ...
> return ...
> 
> > +
> > +	return !(vm_flags & (~supported));
> 
> I think this can just be:
> 
> 	return !(vm_flags & ~supported);

Sure thing, I'll apply everything you mentioned above.

Thanks,

-- 
Peter Xu