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Message-ID: <20251117114631.2029447-4-rppt@kernel.org>
Date: Mon, 17 Nov 2025 13:46:30 +0200
From: Mike Rapoport <rppt@...nel.org>
To: linux-mm@...ck.org
Cc: Andrea Arcangeli <aarcange@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Baolin Wang <baolin.wang@...ux.alibaba.com>,
David Hildenbrand <david@...hat.com>,
Hugh Dickins <hughd@...gle.com>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
Michal Hocko <mhocko@...e.com>,
Mike Rapoport <rppt@...nel.org>,
Nikita Kalyazin <kalyazin@...zon.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Peter Xu <peterx@...hat.com>,
Sean Christopherson <seanjc@...gle.com>,
Shuah Khan <shuah@...nel.org>,
Suren Baghdasaryan <surenb@...gle.com>,
Vlastimil Babka <vbabka@...e.cz>,
linux-kernel@...r.kernel.org,
kvm@...r.kernel.org,
linux-kselftest@...r.kernel.org
Subject: [RFC PATCH 3/4] userfaultfd, guest_memfd: support userfault minor mode in guest_memfd
From: "Mike Rapoport (Microsoft)" <rppt@...nel.org>
* Export handle_userfault() for KVM module so that fault() handler in
guest_memfd would be able to notify userspace about page faults in its
address space.
* Implement get_pagecache_folio() for guest_memfd.
* And finally, introduce UFFD_FEATURE_MINOR_GENERIC that will allow
using userfaultfd minor mode with memory types other than shmem and
hugetlb provided they are allowed to call handle_userfault() and
implement get_pagecache_folio().
Signed-off-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
---
fs/userfaultfd.c | 4 +++-
include/uapi/linux/userfaultfd.h | 8 +++++++-
virt/kvm/guest_memfd.c | 30 ++++++++++++++++++++++++++++++
3 files changed, 40 insertions(+), 2 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 54c6cc7fe9c6..964fa2662d5c 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -537,6 +537,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
out:
return ret;
}
+EXPORT_SYMBOL_FOR_MODULES(handle_userfault, "kvm");
static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
struct userfaultfd_wait_queue *ewq)
@@ -1978,7 +1979,8 @@ 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_GENERIC);
#endif
#ifndef CONFIG_HAVE_ARCH_USERFAULTFD_WP
uffdio_api.features &= ~UFFD_FEATURE_PAGEFAULT_FLAG_WP;
diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index 2841e4ea8f2c..c5cbd4a5a26e 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_GENERIC)
#define UFFD_API_IOCTLS \
((__u64)1 << _UFFDIO_REGISTER | \
(__u64)1 << _UFFDIO_UNREGISTER | \
@@ -210,6 +211,10 @@ struct uffdio_api {
* UFFD_FEATURE_MINOR_SHMEM indicates the same support as
* UFFD_FEATURE_MINOR_HUGETLBFS, but for shmem-backed pages instead.
*
+ * UFFD_FEATURE_MINOR_GENERIC indicates that minor faults can be
+ * intercepted for file-backed memory in case subsystem backing this
+ * memory supports it.
+ *
* UFFD_FEATURE_EXACT_ADDRESS indicates that the exact address of page
* faults would be provided and the offset within the page would not be
* masked.
@@ -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_GENERIC (1<<17)
__u64 features;
__u64 ioctls;
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index fbca8c0972da..5e3c63307fdf 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -4,6 +4,7 @@
#include <linux/kvm_host.h>
#include <linux/pagemap.h>
#include <linux/anon_inodes.h>
+#include <linux/userfaultfd_k.h>
#include "kvm_mm.h"
@@ -369,6 +370,12 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
return vmf_error(err);
}
+ if (userfaultfd_minor(vmf->vma)) {
+ folio_unlock(folio);
+ folio_put(folio);
+ return handle_userfault(vmf, VM_UFFD_MINOR);
+ }
+
if (WARN_ON_ONCE(folio_test_large(folio))) {
ret = VM_FAULT_SIGBUS;
goto out_folio;
@@ -390,8 +397,31 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf)
return ret;
}
+#ifdef CONFIG_USERFAULTFD
+static struct folio *kvm_gmem_get_pagecache_folio(struct vm_area_struct *vma,
+ pgoff_t pgoff)
+{
+ struct inode *inode = file_inode(vma->vm_file);
+ struct folio *folio;
+
+ folio = kvm_gmem_get_folio(inode, pgoff);
+ if (IS_ERR_OR_NULL(folio))
+ return folio;
+
+ if (!folio_test_uptodate(folio)) {
+ clear_highpage(folio_page(folio, 0));
+ kvm_gmem_mark_prepared(folio);
+ }
+
+ return folio;
+}
+#endif
+
static const struct vm_operations_struct kvm_gmem_vm_ops = {
.fault = kvm_gmem_fault_user_mapping,
+#ifdef CONFIG_USERFAULTFD
+ .get_pagecache_folio = kvm_gmem_get_pagecache_folio,
+#endif
};
static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
--
2.50.1
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