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"akpm@linux-foundation.org" , "david@redhat.com" , "pbonzini@redhat.com" , "seanjc@google.com" , "viro@zeniv.linux.org.uk" , "brauner@kernel.org" CC: "peterx@redhat.com" , "lorenzo.stoakes@oracle.com" , "Liam.Howlett@oracle.com" , "willy@infradead.org" , "vbabka@suse.cz" , "rppt@kernel.org" , "surenb@google.com" , "mhocko@suse.com" , "jack@suse.cz" , "linux-mm@kvack.org" , "kvm@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "jthoughton@google.com" , "tabba@google.com" , "vannapurve@google.com" , "Roy, Patrick" , "Thomson, Jack" , "Manwaring, Derek" , "Cali, Marco" , "Kalyazin, Nikita" Subject: [RFC PATCH v6 2/2] userfaulfd: add minor mode for guestmem Thread-Topic: [RFC PATCH v6 2/2] userfaulfd: add minor mode for guestmem Thread-Index: AQHcJlxlJ76MA28SOEel205dRdsXkA== Date: Mon, 15 Sep 2025 16:18:39 +0000 Message-ID: <20250915161815.40729-3-kalyazin@amazon.com> References: <20250915161815.40729-1-kalyazin@amazon.com> In-Reply-To: <20250915161815.40729-1-kalyazin@amazon.com> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: Nikita Kalyazin =0A= =0A= UserfaultFD support in guestmem enables use cases like restoring a=0A= guest_memfd-backed VM from a memory snapshot in Firecracker [1] where an=0A= external process is responsible for supplying the content of the guest=0A= memory or live migration of guest_memfd-backed VMs.=0A= =0A= [1] https://github.com/firecracker-microvm/firecracker/blob/main/docs/snaps= hotting/handling-page-faults-on-snapshot-resume.md=0A= =0A= Signed-off-by: Nikita Kalyazin =0A= ---=0A= Documentation/admin-guide/mm/userfaultfd.rst | 4 +++-=0A= fs/userfaultfd.c | 3 ++-=0A= include/linux/userfaultfd_k.h | 8 +++++---=0A= include/uapi/linux/userfaultfd.h | 8 +++++++-=0A= mm/userfaultfd.c | 14 +++++++++++---=0A= 5 files changed, 28 insertions(+), 9 deletions(-)=0A= =0A= diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/a= dmin-guide/mm/userfaultfd.rst=0A= index e5cc8848dcb3..ca8c5954ffdb 100644=0A= --- a/Documentation/admin-guide/mm/userfaultfd.rst=0A= +++ b/Documentation/admin-guide/mm/userfaultfd.rst=0A= @@ -111,7 +111,9 @@ events, except page fault notifications, may be generat= ed:=0A= - ``UFFD_FEATURE_MINOR_HUGETLBFS`` indicates that the kernel supports=0A= ``UFFDIO_REGISTER_MODE_MINOR`` registration for hugetlbfs virtual memory= =0A= areas. ``UFFD_FEATURE_MINOR_SHMEM`` is the analogous feature indicating= =0A= - support for shmem virtual memory areas.=0A= + support for shmem virtual memory areas. ``UFFD_FEATURE_MINOR_GUESTMEM``= =0A= + is the analogous feature indicating support for guestmem-backed memory= =0A= + areas.=0A= =0A= - ``UFFD_FEATURE_MOVE`` indicates that the kernel supports moving an=0A= existing page contents from userspace.=0A= diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c=0A= index 54c6cc7fe9c6..e4e80f1072a6 100644=0A= --- a/fs/userfaultfd.c=0A= +++ b/fs/userfaultfd.c=0A= @@ -1978,7 +1978,8 @@ static int userfaultfd_api(struct userfaultfd_ctx *ct= x,=0A= uffdio_api.features =3D UFFD_API_FEATURES;=0A= #ifndef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR=0A= uffdio_api.features &=3D=0A= - ~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM);=0A= + ~(UFFD_FEATURE_MINOR_HUGETLBFS | UFFD_FEATURE_MINOR_SHMEM |=0A= + UFFD_FEATURE_MINOR_GUESTMEM);=0A= #endif=0A= #ifndef CONFIG_HAVE_ARCH_USERFAULTFD_WP=0A= uffdio_api.features &=3D ~UFFD_FEATURE_PAGEFAULT_FLAG_WP;=0A= diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h= =0A= index c0e716aec26a..37bd4e71b611 100644=0A= --- a/include/linux/userfaultfd_k.h=0A= +++ b/include/linux/userfaultfd_k.h=0A= @@ -14,6 +14,7 @@=0A= #include /* linux/include/uapi/linux/userfaultfd.h *= /=0A= =0A= #include =0A= +#include =0A= #include =0A= #include =0A= #include =0A= @@ -218,7 +219,8 @@ static inline bool vma_can_userfault(struct vm_area_str= uct *vma,=0A= return false;=0A= =0A= if ((vm_flags & VM_UFFD_MINOR) &&=0A= - (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma)))=0A= + (!is_vm_hugetlb_page(vma) && !vma_is_shmem(vma) &&=0A= + !guestmem_vma_is_guestmem(vma)))=0A= return false;=0A= =0A= /*=0A= @@ -238,9 +240,9 @@ static inline bool vma_can_userfault(struct vm_area_str= uct *vma,=0A= return false;=0A= #endif=0A= =0A= - /* By default, allow any of anon|shmem|hugetlb */=0A= + /* By default, allow any of anon|shmem|hugetlb|guestmem */=0A= return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||=0A= - vma_is_shmem(vma);=0A= + vma_is_shmem(vma) || guestmem_vma_is_guestmem(vma);=0A= }=0A= =0A= static inline bool vma_has_uffd_without_event_remap(struct vm_area_struct = *vma)=0A= diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaul= tfd.h=0A= index 2841e4ea8f2c..0fe9fbd29772 100644=0A= --- a/include/uapi/linux/userfaultfd.h=0A= +++ b/include/uapi/linux/userfaultfd.h=0A= @@ -42,7 +42,8 @@=0A= UFFD_FEATURE_WP_UNPOPULATED | \=0A= UFFD_FEATURE_POISON | \=0A= UFFD_FEATURE_WP_ASYNC | \=0A= - UFFD_FEATURE_MOVE)=0A= + UFFD_FEATURE_MOVE | \=0A= + UFFD_FEATURE_MINOR_GUESTMEM)=0A= #define UFFD_API_IOCTLS \=0A= ((__u64)1 << _UFFDIO_REGISTER | \=0A= (__u64)1 << _UFFDIO_UNREGISTER | \=0A= @@ -230,6 +231,10 @@ struct uffdio_api {=0A= *=0A= * UFFD_FEATURE_MOVE indicates that the kernel supports moving an=0A= * existing page contents from userspace.=0A= + *=0A= + * UFFD_FEATURE_MINOR_GUESTMEM indicates the same support as=0A= + * UFFD_FEATURE_MINOR_HUGETLBFS, but for guestmem-backed pages=0A= + * instead.=0A= */=0A= #define UFFD_FEATURE_PAGEFAULT_FLAG_WP (1<<0)=0A= #define UFFD_FEATURE_EVENT_FORK (1<<1)=0A= @@ -248,6 +253,7 @@ struct uffdio_api {=0A= #define UFFD_FEATURE_POISON (1<<14)=0A= #define UFFD_FEATURE_WP_ASYNC (1<<15)=0A= #define UFFD_FEATURE_MOVE (1<<16)=0A= +#define UFFD_FEATURE_MINOR_GUESTMEM (1<<17)=0A= __u64 features;=0A= =0A= __u64 ioctls;=0A= diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c=0A= index 45e6290e2e8b..304e5d7dbb70 100644=0A= --- a/mm/userfaultfd.c=0A= +++ b/mm/userfaultfd.c=0A= @@ -388,7 +388,14 @@ static int mfill_atomic_pte_continue(pmd_t *dst_pmd,= =0A= struct page *page;=0A= int ret;=0A= =0A= - ret =3D shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);=0A= + if (guestmem_vma_is_guestmem(dst_vma)) {=0A= + ret =3D 0;=0A= + folio =3D guestmem_grab_folio(inode->i_mapping, pgoff);=0A= + if (IS_ERR(folio))=0A= + ret =3D PTR_ERR(folio);=0A= + } else {=0A= + ret =3D shmem_get_folio(inode, pgoff, 0, &folio, SGP_NOALLOC);=0A= + }=0A= /* Our caller expects us to return -EFAULT if we failed to find folio */= =0A= if (ret =3D=3D -ENOENT)=0A= ret =3D -EFAULT;=0A= @@ -766,9 +773,10 @@ static __always_inline ssize_t mfill_atomic(struct use= rfaultfd_ctx *ctx,=0A= return mfill_atomic_hugetlb(ctx, dst_vma, dst_start,=0A= src_start, len, flags);=0A= =0A= - if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))=0A= + if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma)=0A= + && !guestmem_vma_is_guestmem(dst_vma))=0A= goto out_unlock;=0A= - if (!vma_is_shmem(dst_vma) &&=0A= + if (!vma_is_shmem(dst_vma) && !guestmem_vma_is_guestmem(dst_vma) &&=0A= uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))=0A= goto out_unlock;=0A= =0A= -- =0A= 2.50.1=0A= =0A=