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[54.240.197.234]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e3ebcb958sm23845574f8f.21.2026.10.27.08.47.22 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Tue, 27 Oct 2026 08:47:24 -0700 (PDT) From: Fred Griffoul To: Ackerley Tng , Paolo Bonzini , Sean Christopherson , Jason Gunthorpe , Kevin Tian Cc: Alex Williamson , Alexander Graf , Alexander Viro , Alexey Kardashevskiy , Andrew Morton , "Aneesh Kumar K . V" , Ashish Kalra , Baolin Wang , Baolu Lu , Bill Wendling , Borislav Petkov , Catalin Marinas , Christian Brauner , =?UTF-8?q?Christian=20K=C3=B6nig?= , Connor Williamson , Dave Hansen , Dave Jiang , David Hildenbrand , David Rientjes , David Woodhouse , Erdem Aktas , Frank van der Linden , Gregory Price , "H . Peter Anvin" , Hugh Dickins , Ingo Molnar , Jacob Pan , James Houghton , Jan Kara , Jianxiong Gao , Joerg Roedel , Joey Gouly , John Starks , Jonathan Corbet , Justin Stitt , Lorenzo Stoakes , Marc Zyngier , Michael Roth , Mike Rapoport , Nathan Chancellor , Nick Desaulniers , Nicolin Chen , Oliver Upton , Pratyush Yadav , Randy Dunlap , Robin Murphy , Samiullah Khawaja , Shuah Khan , Steffen Eiden , Steven Price , Suzuki K Poulose , Tarun Sahu , Thomas Gleixner , Tom Lendacky , Tycho Andersen , Vishal Annapurve , Vlastimil Babka , Will Deacon , Xu Yilun , Yan Zhao , Zenghui Yu , x86@kernel.org, Shuah Khan , Fuad Tabba , kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org, llvm@lists.linux.dev, iommu@lists.linux.dev Subject: [RFC PATCH 13/14] samples/kvm: Back absent pages on demand in the sample provider Date: Tue, 27 Oct 2026 15:46:48 +0000 Message-ID: <20261027154649.91571-14-griffoul@gmail.com> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20261027154649.91571-1-griffoul@gmail.com> References: <20261022-gmem-alternative-providers-v2-0-5dd1dc6c660a@google.com> <20261027154649.91571-1-griffoul@gmail.com> X-Mailing-List: kvm@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Fred Griffoul The sample provider fails get_pfn() for an absent page and keeps its frames on a punch hole, so tests cannot exercise a provider that backs a hole when it is asked for a frame, one that backs more than the page asked for, or one that frees the frames that a punch hole gives back. Add MPS_NEW_FILE_FILL: get_pfn() then backs an absent page with the whole 2 MiB block around it, and put_pfn_range() makes the pages absent again. Assisted-by: Claude:claude-opus-5-5 Signed-off-by: Fred Griffoul --- samples/kvm/mem_provider_sample.c | 29 ++++++++++++++++++++++++----- samples/kvm/mem_provider_sample.h | 9 ++++++++- 2 files changed, 32 insertions(+), 6 deletions(-) diff --git a/samples/kvm/mem_provider_sample.c b/samples/kvm/mem_provider_sample.c index 670e36207e59..41e0ce5b6bc6 100644 --- a/samples/kvm/mem_provider_sample.c +++ b/samples/kvm/mem_provider_sample.c @@ -41,6 +41,7 @@ struct mps_info { unsigned long *absent; /* taken away by MPS_SET_PRESENT */ unsigned long *readonly; /* set by MPS_SET_READONLY */ + bool fill; /* MPS_NEW_FILE_FILL */ struct inode *gmem_inode; }; @@ -62,8 +63,15 @@ static void mps_gmem_put_pfn_range(void *provider, pgoff_t index, size_t nr_pages) { struct file *resource_file = provider; + struct mps_info *info = resource_file->private_data; + size_t i; - mps_zero_range(resource_file->private_data, index, nr_pages); + mps_zero_range(info, index, nr_pages); + if (!info->fill || index >= info->npages) + return; + /* Lockless, like get_pfn(): guest_memfd holds its invalidate lock. */ + for (i = index; i < min_t(size_t, index + nr_pages, info->npages); i++) + set_bit(i, info->absent); } /* Byte range [offset, offset + len) of @info as page indices. */ @@ -228,7 +236,9 @@ static bool mps_bits_are(unsigned long *bitmap, unsigned long first, /* * The frame behind @index, and the largest aligned block around it that is * present and has the same attributes: the file's frames are contiguous. - * Lockless: a change is followed by kvm_gmem_revoke() or kvm_gmem_restore(). + * Lockless: a change is followed by kvm_gmem_revoke() or kvm_gmem_restore(), + * except a fill, which only gives frames to holes; guest_memfd tells its users + * about the block. */ static int mps_gmem_get_pfn(void *provider, pgoff_t index, unsigned long *pfn, int *max_order, u32 *attrs) @@ -241,8 +251,16 @@ static int mps_gmem_get_pfn(void *provider, pgoff_t index, unsigned long *pfn, if (index >= info->npages) return -EINVAL; - if (test_bit(index, info->absent)) - return -EFAULT; + if (test_bit(index, info->absent)) { + unsigned long first = ALIGN_DOWN(index, PTRS_PER_PMD); + unsigned long i; + + if (!info->fill) + return -EFAULT; + /* Absent pages are zeroed already. */ + for (i = first; i < min(first + PTRS_PER_PMD, info->npages); i++) + clear_bit(i, info->absent); + } ro = test_bit(index, info->readonly); *pfn = info->base_pfn + index; @@ -307,7 +325,7 @@ static long mps_ctl_new_file(void __user *uarg) if (copy_from_user(&nf, uarg, sizeof(nf))) return -EFAULT; - if (nf.flags || nf.pad) + if ((nf.flags & ~MPS_NEW_FILE_FILL) || nf.pad) return -EINVAL; if (!nf.size || !PAGE_ALIGNED(nf.size)) return -EINVAL; @@ -337,6 +355,7 @@ static long mps_ctl_new_file(void __user *uarg) info->cma_pages = pages; info->base_pfn = page_to_pfn(pages); info->npages = npages; + info->fill = nf.flags & MPS_NEW_FILE_FILL; mutex_init(&info->lock); fd = get_unused_fd_flags(O_CLOEXEC); diff --git a/samples/kvm/mem_provider_sample.h b/samples/kvm/mem_provider_sample.h index 57f494dc46fa..45ec9605b593 100644 --- a/samples/kvm/mem_provider_sample.h +++ b/samples/kvm/mem_provider_sample.h @@ -19,11 +19,18 @@ * Returns the provider fd. */ struct mps_new_file { - __u32 flags; /* Reserved, must be 0 */ + __u32 flags; /* MPS_NEW_FILE_* */ __u32 pad; __u64 size; /* bytes, page aligned */ }; +/* + * get_pfn() backs an absent page, with every page of the 2 MiB block around + * it, instead of failing, and put_pfn_range() takes the pages back: a + * provider that allocates on demand and frees on a punch hole, for tests. + */ +#define MPS_NEW_FILE_FILL (1U << 0) + #define MPS_NEW_FILE \ _IOW(MPS_IOCTL_BASE, 5, struct mps_new_file) -- 2.47.3