On 17.11.25 12:46, Mike Rapoport wrote:
From: "Mike Rapoport (Microsoft)" <[email protected]>
* 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) <[email protected]>
---
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);
+ }
Staring at things like VM_FAULT_NEEDDSYNC, I'm wondering whether we could have a
new return value from ->fault that would indicate that
handle_userfault(vmf, VM_UFFD_MINOR) should be called.
Maybe some VM_FAULT_UFFD_MINOR or simply VM_FAULT_USERFAULTFD and we
can just derive that it is VM_UFFD_MINOR.
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 4f66a3206a63c..2cf17da880f0e 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1601,6 +1601,8 @@ typedef __bitwise unsigned int vm_fault_t;
* fsync() to complete (for synchronous page faults
* in DAX)
* @VM_FAULT_COMPLETED: ->fault completed, meanwhile mmap lock
released
+ * @VM_FAULT_USERFAULTFD: ->fault did not modify page tables and needs
+ * handle_userfault() to complete
* @VM_FAULT_HINDEX_MASK: mask HINDEX value
*
*/
@@ -1618,6 +1620,7 @@ enum vm_fault_reason {
VM_FAULT_DONE_COW = (__force vm_fault_t)0x001000,
VM_FAULT_NEEDDSYNC = (__force vm_fault_t)0x002000,
VM_FAULT_COMPLETED = (__force vm_fault_t)0x004000,
+ VM_FAULT_USERFAULTFD = (__force vm_fault_t)0x006000,
VM_FAULT_HINDEX_MASK = (__force vm_fault_t)0x0f0000,
};
@@ -1642,6 +1645,7 @@ enum vm_fault_reason {
{ VM_FAULT_FALLBACK, "FALLBACK" }, \
{ VM_FAULT_DONE_COW, "DONE_COW" }, \
{ VM_FAULT_NEEDDSYNC, "NEEDDSYNC" }, \
+ { VM_FAULT_USERFAULTFD, "USERFAULTFD" },\
{ VM_FAULT_COMPLETED, "COMPLETED" }
struct vm_special_mapping {
IIUC, we have exactly two invocations of ->fault(vmf) in memory.c where
we would have to handle it IIUC. And the return value would never leave
the core.
That way, we wouldn't have to export handle_userfault().
Just a thought ...
--
Cheers
David
