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Message-ID: <20250915161815.40729-3-kalyazin@amazon.com>
Date: Mon, 15 Sep 2025 16:18:39 +0000
From: "Kalyazin, Nikita" <kalyazin@...zon.co.uk>
To: "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
"david@...hat.com" <david@...hat.com>, "pbonzini@...hat.com"
<pbonzini@...hat.com>, "seanjc@...gle.com" <seanjc@...gle.com>,
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<brauner@...nel.org>
CC: "peterx@...hat.com" <peterx@...hat.com>, "lorenzo.stoakes@...cle.com"
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"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-kernel@...r.kernel.org"
<linux-kernel@...r.kernel.org>, "linux-fsdevel@...r.kernel.org"
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<roypat@...zon.co.uk>, "Thomson, Jack" <jackabt@...zon.co.uk>, "Manwaring,
Derek" <derekmn@...zon.com>, "Cali, Marco" <xmarcalx@...zon.co.uk>,
"Kalyazin, Nikita" <kalyazin@...zon.co.uk>
Subject: [RFC PATCH v6 2/2] userfaulfd: add minor mode for guestmem
From: Nikita Kalyazin <kalyazin@...zon.com>
UserfaultFD support in guestmem enables use cases like restoring a
guest_memfd-backed VM from a memory snapshot in Firecracker [1] where an
external process is responsible for supplying the content of the guest
memory or live migration of guest_memfd-backed VMs.
[1] https://github.com/firecracker-microvm/firecracker/blob/main/docs/snapshotting/handling-page-faults-on-snapshot-resume.md
Signed-off-by: Nikita Kalyazin <kalyazin@...zon.com>
---
Documentation/admin-guide/mm/userfaultfd.rst | 4 +++-
fs/userfaultfd.c | 3 ++-
include/linux/userfaultfd_k.h | 8 +++++---
include/uapi/linux/userfaultfd.h | 8 +++++++-
mm/userfaultfd.c | 14 +++++++++++---
5 files changed, 28 insertions(+), 9 deletions(-)
diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst
index e5cc8848dcb3..ca8c5954ffdb 100644
--- a/Documentation/admin-guide/mm/userfaultfd.rst
+++ b/Documentation/admin-guide/mm/userfaultfd.rst
@@ -111,7 +111,9 @@ events, except page fault notifications, may be generated:
- ``UFFD_FEATURE_MINOR_HUGETLBFS`` indicates that the kernel supports
``UFFDIO_REGISTER_MODE_MINOR`` registration for hugetlbfs virtual memory
areas. ``UFFD_FEATURE_MINOR_SHMEM`` is the analogous feature indicating
- support for shmem virtual memory areas.
+ support for shmem virtual memory areas. ``UFFD_FEATURE_MINOR_GUESTMEM``
+ is the analogous feature indicating support for guestmem-backed memory
+ areas.
- ``UFFD_FEATURE_MOVE`` indicates that the kernel supports moving an
existing page contents from userspace.
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 54c6cc7fe9c6..e4e80f1072a6 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1978,7 +1978,8 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
uffdio_api.features = UFFD_API_FEATURES;
#ifndef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
uffdio_api.features &=
- ~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM);
+ ~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM |
+ UFFD_FEATURE_MINOR_GUESTMEM);
#endif
#ifndef CONFIG_HAVE_ARCH_USERFAULTFD_WP
uffdio_api.features &= ~UFFD_FEATURE_PAGEFAULT_FLAG_WP;
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index c0e716aec26a..37bd4e71b611 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -14,6 +14,7 @@
#include <linux/userfaultfd.h> /* linux/include/uapi/linux/userfaultfd.h */
#include <linux/fcntl.h>
+#include <linux/guestmem.h>
#include <linux/mm.h>
#include <linux/swap.h>
#include <linux/swapops.h>
@@ -218,7 +219,8 @@ static inline bool vma_can_userfault(struct vm_area_struct *vma,
return false;
if ((vm_flags & VM_UFFD_MINOR) &&
- (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma)))
+ (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma) &&
+ !guestmem_vma_is_guestmem(vma)))
return false;
/*
@@ -238,9 +240,9 @@ static inline bool vma_can_userfault(struct vm_area_struct *vma,
return false;
#endif
- /* By default, allow any of anon|shmem|hugetlb */
+ /* By default, allow any of anon|shmem|hugetlb|guestmem */
return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||
- vma_is_shmem(vma);
+ vma_is_shmem(vma) || guestmem_vma_is_guestmem(vma);
}
static inline bool vma_has_uffd_without_event_remap(struct vm_area_struct *vma)
diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index 2841e4ea8f2c..0fe9fbd29772 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -42,7 +42,8 @@
UFFD_FEATURE_WP_UNPOPULATED | \
UFFD_FEATURE_POISON | \
UFFD_FEATURE_WP_ASYNC | \
- UFFD_FEATURE_MOVE)
+ UFFD_FEATURE_MOVE | \
+ UFFD_FEATURE_MINOR_GUESTMEM)
#define UFFD_API_IOCTLS \
((__u64)1 << _UFFDIO_REGISTER | \
(__u64)1 << _UFFDIO_UNREGISTER | \
@@ -230,6 +231,10 @@ struct uffdio_api {
*
* UFFD_FEATURE_MOVE indicates that the kernel supports moving an
* existing page contents from userspace.
+ *
+ * UFFD_FEATURE_MINOR_GUESTMEM indicates the same support as
+ * UFFD_FEATURE_MINOR_HUGETLBFS, but for guestmem-backed pages
+ * instead.
*/
#define UFFD_FEATURE_PAGEFAULT_FLAG_WP (1<<0)
#define UFFD_FEATURE_EVENT_FORK (1<<1)
@@ -248,6 +253,7 @@ struct uffdio_api {
#define UFFD_FEATURE_POISON (1<<14)
#define UFFD_FEATURE_WP_ASYNC (1<<15)
#define UFFD_FEATURE_MOVE (1<<16)
+#define UFFD_FEATURE_MINOR_GUESTMEM (1<<17)
__u64 features;
__u64 ioctls;
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 45e6290e2e8b..304e5d7dbb70 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -388,7 +388,14 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,
struct page *page;
int ret;
- ret = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);
+ if (guestmem_vma_is_guestmem(dst_vma)) {
+ ret = 0;
+ folio = guestmem_grab_folio(inode->i_mapping, pgoff);
+ if (IS_ERR(folio))
+ ret = PTR_ERR(folio);
+ } else {
+ ret = shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);
+ }
/* Our caller expects us to return -EFAULT if we failed to find folio */
if (ret == -ENOENT)
ret = -EFAULT;
@@ -766,9 +773,10 @@ 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))
+ if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma)
+ && !guestmem_vma_is_guestmem(dst_vma))
goto out_unlock;
- if (!vma_is_shmem(dst_vma) &&
+ if (!vma_is_shmem(dst_vma) && !guestmem_vma_is_guestmem(dst_vma) &&
uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
goto out_unlock;
--
2.50.1
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