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

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Message-ID: <0a60d99b-fa3b-482b-8171-ac63dcdf3168@redhat.com>
Date: Tue, 30 Sep 2025 11:23:19 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Cc: 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 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);


-- 
Cheers

David / dhildenb