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Message-ID: <007c08e4-ce70-4c30-b3b1-e25e02dfe29d@kernel.org>
Date: Tue, 18 Nov 2025 17:41:13 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org
Cc: Andrea Arcangeli <aarcange@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Baolin Wang <baolin.wang@...ux.alibaba.com>, Hugh Dickins
<hughd@...gle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Michal Hocko
<mhocko@...e.com>, 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: Re: [RFC PATCH 3/4] userfaultfd, guest_memfd: support userfault minor
mode in guest_memfd
On 17.11.25 12:46, Mike Rapoport wrote:
> 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);
> + }
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
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