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Message-ID: <aQmplrpNjvCVjWb_@kernel.org>
Date: Tue, 4 Nov 2025 09:21:58 +0200
From: Mike Rapoport <rppt@...nel.org>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: Peter Xu <peterx@...hat.com>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>,
David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.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>,
Hugh Dickins <hughd@...gle.com>, Michal Hocko <mhocko@...e.com>,
Ujwal Kundur <ujwal.kundur@...il.com>,
Oscar Salvador <osalvador@...e.de>,
Suren Baghdasaryan <surenb@...gle.com>,
Andrea Arcangeli <aarcange@...hat.com>, conduct@...nel.org
Subject: Re: [PATCH v4 0/4] mm/userfaultfd: modulize memory types
On Mon, Nov 03, 2025 at 10:27:05PM +0100, David Hildenbrand (Red Hat) wrote:
>
> And maybe that's the main problem here: Liam talks about general uffd
> cleanups while you are focused on supporting guest_memfd minor mode "as
> simple as possible" (as you write below).
Hijacking for the technical part for a moment ;-)
It seems that "as simple as possible" can even avoid data members in struct
vm_uffd_ops, e.g something along these lines:
diff --git a/include/linux/mm.h b/include/linux/mm.h
index d16b33bacc32..840986780cb5 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -605,6 +605,8 @@ struct vm_fault {
*/
};
+struct vm_uffd_ops;
+
/*
* These are the virtual MM functions - opening of an area, closing and
* unmapping it (needed to keep files on disk up-to-date etc), pointer
@@ -690,6 +692,9 @@ struct vm_operations_struct {
struct page *(*find_normal_page)(struct vm_area_struct *vma,
unsigned long addr);
#endif /* CONFIG_FIND_NORMAL_PAGE */
+#ifdef CONFIG_USERFAULTFD
+ const struct vm_uffd_ops *uffd_ops;
+#endif
};
#ifdef CONFIG_NUMA_BALANCING
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index c0e716aec26a..aac7ac616636 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -111,6 +111,11 @@ static inline uffd_flags_t uffd_flags_set_mode(uffd_flags_t flags, enum mfill_at
/* Flags controlling behavior. These behavior changes are mode-independent. */
#define MFILL_ATOMIC_WP MFILL_ATOMIC_FLAG(0)
+struct vm_uffd_ops {
+ int (*minor_get_folio)(struct inode *inode, pgoff_t pgoff,
+ struct folio **folio);
+};
+
extern int mfill_atomic_install_pte(pmd_t *dst_pmd,
struct vm_area_struct *dst_vma,
unsigned long dst_addr, struct page *page,
diff --git a/mm/shmem.c b/mm/shmem.c
index b9081b817d28..b4318ad3bdf9 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3260,6 +3260,17 @@ int shmem_mfill_atomic_pte(pmd_t *dst_pmd,
shmem_inode_unacct_blocks(inode, 1);
return ret;
}
+
+static int shmem_uffd_minor_get_folio(struct inode *inode, pgoff_t pgoff,
+ struct folio **folio)
+{
+ return shmem_get_folio(inode, pgoff, 0, folio, SGP_NOALLOC);
+}
+
+static const struct vm_uffd_ops shmem_uffd_ops = {
+ .minor_get_folio = shmem_uffd_minor_get_folio,
+};
+
#endif /* CONFIG_USERFAULTFD */
#ifdef CONFIG_TMPFS
@@ -5292,6 +5303,9 @@ static const struct vm_operations_struct shmem_vm_ops = {
.set_policy = shmem_set_policy,
.get_policy = shmem_get_policy,
#endif
+#ifdef CONFIG_USERFAULTFD
+ .uffd_ops = &shmem_uffd_ops,
+#endif
};
static const struct vm_operations_struct shmem_anon_vm_ops = {
@@ -5301,6 +5315,9 @@ static const struct vm_operations_struct shmem_anon_vm_ops = {
.set_policy = shmem_set_policy,
.get_policy = shmem_get_policy,
#endif
+#ifdef CONFIG_USERFAULTFD
+ .uffd_ops = &shmem_uffd_ops,
+#endif
};
int shmem_init_fs_context(struct fs_context *fc)
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index af61b95c89e4..6b30a8f39f4d 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -20,6 +20,20 @@
#include "internal.h"
#include "swap.h"
+static const struct vm_uffd_ops anon_uffd_ops = {
+};
+
+static inline const struct vm_uffd_ops *vma_get_uffd_ops(struct vm_area_struct *vma)
+{
+ if (vma->vm_ops && vma->vm_ops->uffd_ops)
+ return vma->vm_ops->uffd_ops;
+
+ if (vma_is_anonymous(vma))
+ return &anon_uffd_ops;
+
+ return NULL;
+}
+
static __always_inline
bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long dst_end)
{
@@ -382,13 +396,14 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
unsigned long dst_addr,
uffd_flags_t flags)
{
+ const struct vm_uffd_ops *uffd_ops = vma_get_uffd_ops(dst_vma);
struct inode *inode = file_inode(dst_vma->vm_file);
pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
struct folio *folio;
struct page *page;
int ret;
- ret = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);
+ ret = uffd_ops->minor_get_folio(inode, pgoff, &folio);
/* Our caller expects us to return -EFAULT if we failed to find folio */
if (ret == -ENOENT)
ret = -EFAULT;
@@ -707,6 +722,7 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
unsigned long src_addr, dst_addr;
long copied;
struct folio *folio;
+ const struct vm_uffd_ops *uffd_ops;
/*
* Sanitize the command parameters:
@@ -766,10 +782,11 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
return mfill_atomic_hugetlb(ctx, dst_vma, dst_start,
src_start, len, flags);
- if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
+ uffd_ops = vma_get_uffd_ops(dst_vma);
+ if (!uffd_ops)
goto out_unlock;
- if (!vma_is_shmem(dst_vma) &&
- uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
+ if (uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE) &&
+ !uffd_ops->minor_get_folio)
goto out_unlock;
while (src_addr < src_start + len) {
--
Sincerely yours,
Mike.
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