On 10/06/2025 23:25, Peter Xu wrote:
On Fri, Apr 04, 2025 at 03:43:50PM +0000, Nikita Kalyazin wrote:
Add support for sending a pagefault event if userfaultfd is registered.
Only page minor event is currently supported.
Signed-off-by: Nikita Kalyazin <kalya...@amazon.com>
---
virt/kvm/guest_memfd.c | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index fbf89e643add..096d89e7282d 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -4,6 +4,9 @@
#include <linux/kvm_host.h>
#include <linux/pagemap.h>
#include <linux/anon_inodes.h>
+#ifdef CONFIG_KVM_PRIVATE_MEM
+#include <linux/userfaultfd_k.h>
+#endif /* CONFIG_KVM_PRIVATE_MEM */
#include "kvm_mm.h"
@@ -380,6 +383,13 @@ static vm_fault_t kvm_gmem_fault(struct vm_fault *vmf)
kvm_gmem_mark_prepared(folio);
}
+ if (userfaultfd_minor(vmf->vma) &&
+ !(vmf->flags & FAULT_FLAG_USERFAULT_CONTINUE)) {
+ folio_unlock(folio);
+ filemap_invalidate_unlock_shared(inode->i_mapping);
+ return handle_userfault(vmf, VM_UFFD_MINOR);
+ }
+
Hmm, does guest-memfd (when with your current approach) at least needs to
define the new can_userfault() hook?
Meanwhile, we have some hard-coded lines so far, like:
mfill_atomic():
if (!vma_is_shmem(dst_vma) &&
uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
goto out_unlock;
I thought it would fail guest-memfd already on a CONTINUE request, and it
doesn't seem to be touched yet in this series.
I'm not yet sure how the test worked out without hitting things like it.
Highly likely I missed something. Some explanations would be welcomed..
Yes, I realised that I'd failed to post this part soon after I sent the
series, but I refrained from sending a new version because the upstream
consensus was to review/merge the mmap support in guest_memfd [1] before
continuing to build on top of it. This is the missed part I planned to
include in the next version. Sorry for the confusion.
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 64551e8a55fb..080437fa7eab 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -221,8 +221,10 @@ static inline bool vma_can_userfault(struct
vm_area_struct *vma,
if (vm_flags & VM_DROPPABLE)
return false;
- if (!vma->vm_ops->can_userfault ||
- !vma->vm_ops->can_userfault(vma, VM_UFFD_MINOR))
+ if ((vm_flags & VM_UFFD_MINOR) &&
+ (!vma->vm_ops ||
+ !vma->vm_ops->can_userfault ||
+ !vma->vm_ops->can_userfault(vma, VM_UFFD_MINOR)))
return false;
/*
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 0aa82c968e16..638360a78561 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -788,7 +788,9 @@ static __always_inline ssize_t mfill_atomic(struct
userfaultfd_ctx *ctx,
return mfill_atomic_hugetlb(ctx, dst_vma, dst_start,
src_start, len, flags);
- can_userfault = dst_vma->vm_ops->can_userfault &&
+ can_userfault =
+ dst_vma->vm_ops &&
+ dst_vma->vm_ops->can_userfault &&
dst_vma->vm_ops->can_userfault(dst_vma, __VM_UFFD_FLAGS);
if (!vma_is_anonymous(dst_vma) && !can_userfault)
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 91ee5dd91c31..202b12dc4b6f 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -420,8 +420,15 @@ static vm_fault_t kvm_gmem_fault(struct vm_fault *vmf)
return ret;
}
+static bool kvm_gmem_can_userfault(struct vm_area_struct *vma,
+ unsigned long vm_flags)
+{
+ return vm_flags & VM_UFFD_MINOR;
+}
+
static const struct vm_operations_struct kvm_gmem_vm_ops = {
- .fault = kvm_gmem_fault,
+ .fault = kvm_gmem_fault,
+ .can_userfault = kvm_gmem_can_userfault,
};
static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
[1] https://lore.kernel.org/kvm/20250605153800.557144-1-ta...@google.com/
Thanks,
vmf->page = folio_file_page(folio, vmf->pgoff);
out_folio:
--
2.47.1
--
Peter Xu
