linux-kernel - Re: [PATCH v4 1/4] mm: Introduce vm_uffd

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Message-ID: <5dac6df9-2d47-4322-89eb-65a88d237c35@redhat.com>
Date: Mon, 20 Oct 2025 16:18:27 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Cc: Mike Rapoport <rppt@...nel.org>, Muchun Song <muchun.song@...ux.dev>,
 Nikita Kalyazin <kalyazin@...zon.com>, Vlastimil Babka <vbabka@...e.cz>,
 Axel Rasmussen <axelrasmussen@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 James Houghton <jthoughton@...gle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Hugh Dickins
 <hughd@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Ujwal Kundur <ujwal.kundur@...il.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Oscar Salvador <osalvador@...e.de>, Suren Baghdasaryan <surenb@...gle.com>,
 Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v4 1/4] mm: Introduce vm_uffd_ops API

On 15.10.25 01:14, Peter Xu wrote:
> Currently, most of the userfaultfd features are implemented directly in the
> core mm.  It will invoke VMA specific functions whenever necessary.  So far
> it is fine because it almost only interacts with shmem and hugetlbfs.
> 
> Introduce a generic userfaultfd API extension for vm_operations_struct,
> so that any code that implements vm_operations_struct (including kernel
> modules that can be compiled separately from the kernel core) can support
> userfaults without modifying the core files.
> 
> With this API applied, if a module wants to support userfaultfd, the
> module should only need to properly define vm_uffd_ops and hook it to
> vm_operations_struct, instead of changing anything in core mm.
> 
> This API will not work for anonymous memory. Handling of userfault
> operations for anonymous memory remains unchanged in core mm.
> 
> Due to a security concern while reviewing older versions of this series
> [1], uffd_copy() will be temprorarily removed.  IOW, MISSING-capable memory
> types can only be hard-coded and implemented in mm/.  It would also affect
> UFFDIO_COPY and UFFDIO_ZEROPAGE.  Other functions should still be able to
> be provided from vm_uffd_ops.
> 
> Introduces the API only so that existing userfaultfd users can be moved
> over without breaking them.
> 
> [1] https://lore.kernel.org/all/20250627154655.2085903-1-peterx@redhat.com/
> 
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers

David / dhildenb