linux-kernel - Re: [PATCH v3 0/4] mm/userfaultfd: modulize memory types

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Message-ID: <urtzlj3wwjmmgdon3goi2qopxlxrajewdxhhchzw72exz4of7l@vlmbjw3i3xth>
Date: Tue, 30 Sep 2025 15:49:34 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Peter Xu <peterx@...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>,
        David Hildenbrand <david@...hat.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>, 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 0/4] mm/userfaultfd: modulize memory types

* Peter Xu <peterx@...hat.com> [250926 17:17]:
> [based on latest akpm/mm-new of Sep 26th, commit e612c80ae0aeb]
> 
> v3 changelog:
> - Fixed checkpatch issues on spaces or typedef
> - Dropped uffd_copy() API
> - Refined commit messages here and there to reflect the removal of uffd_copy()
> 
> v1: https://lore.kernel.org/r/20250620190342.1780170-1-peterx@redhat.com
> v2: https://lore.kernel.org/r/20250627154655.2085903-1-peterx@redhat.com
> 
> This series is an alternative proposal of what Nikita proposed here on the
> initial three patches:
> 
>   https://lore.kernel.org/r/20250404154352.23078-1-kalyazin@amazon.com
> 
> This is not yet relevant to any guest-memfd support, but paving way for it.

It would be much easier to review this with the guest-memfd support in
this patch set.  Any chance of including the target user?

> Here, the major goal is to make kernel modules be able to opt-in with any
> form of userfaultfd supports, like guest-memfd.  This alternative option
> should hopefully be cleaner, and avoid leaking userfault details into
> vm_ops.fault().
> 
> It also means this series does not depend on anything.  It's a pure
> refactoring of userfaultfd internals to provide a generic API, so that
> other types of files, especially RAM based, can support userfaultfd without
> touching mm/ at all.
> 
> To achieve it, this series introduced a file operation called vm_uffd_ops.
> The ops needs to be provided when a file type supports any of userfaultfd.
> 
> With that, I moved both hugetlbfs and shmem over, whenever possible.  So
> far due to concerns on exposing an uffd_copy() API, the MISSING faults are
> still separately processed and can only be done within mm/.  Hugetlbfs kept
> its special paths untouched.
> 
> An example of shmem uffd_ops:
> 
> static const vm_uffd_ops shmem_uffd_ops = {
> 	.uffd_features	= 	__VM_UFFD_FLAGS,
> 	.uffd_ioctls	= 	BIT(_UFFDIO_COPY) |
> 				BIT(_UFFDIO_ZEROPAGE) |
> 				BIT(_UFFDIO_WRITEPROTECT) |
> 				BIT(_UFFDIO_CONTINUE) |
> 				BIT(_UFFDIO_POISON),
> 	.uffd_get_folio	=	shmem_uffd_get_folio,
> };
> 
> No functional change expected at all after the whole series applied.  There
> might be some slightly stricter check on uffd ops here and there in the
> last patch, but that really shouldn't stand out anywhere to anyone.
> 
> For testing: besides the cross-compilation tests, I did also try with
> uffd-stress in a VM to measure any perf difference before/after the change;
> The static call becomes a pointer now.  I really cannot measure anything
> different, which is more or less expected.
> 
> Comments welcomed, thanks.
> 
> Peter Xu (4):
>   mm: Introduce vm_uffd_ops API
>   mm/shmem: Support vm_uffd_ops API
>   mm/hugetlb: Support vm_uffd_ops API
>   mm: Apply vm_uffd_ops API to core mm
> 
>  include/linux/mm.h            |   9 +++
>  include/linux/userfaultfd_k.h |  83 ++++++++++++++++-----------
>  mm/hugetlb.c                  |  19 +++++++
>  mm/shmem.c                    |  25 +++++++++
>  mm/userfaultfd.c              | 102 ++++++++++++++++++++++++++--------
>  5 files changed, 181 insertions(+), 57 deletions(-)
> 
> -- 
> 2.50.1
>