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[54.240.197.234]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e3ebcb958sm23845574f8f.21.2026.10.27.08.47.20 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Tue, 27 Oct 2026 08:47:22 -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 12/14] samples/kvm: Add read-only pages and large blocks to the sample provider Date: Tue, 27 Oct 2026 15:46:47 +0000 Message-ID: <20261027154649.91571-13-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 reports every page as writable and of order 0, and does nothing when a page comes back, so tests cannot exercise read-only frames, large mappings, or a device mapping that follows a page made present again. Add MPS_SET_READONLY, report the largest aligned block of pages in the same state, and call kvm_gmem_restore() when pages become present or writable again; making pages read only revokes them. Assisted-by: Claude:claude-opus-5-5 Signed-off-by: Fred Griffoul --- samples/kvm/mem_provider_sample.c | 89 +++++++++++++++++++++++++++---- samples/kvm/mem_provider_sample.h | 9 ++++ 2 files changed, 87 insertions(+), 11 deletions(-) diff --git a/samples/kvm/mem_provider_sample.c b/samples/kvm/mem_provider_sample.c index f30654a60dd8..670e36207e59 100644 --- a/samples/kvm/mem_provider_sample.c +++ b/samples/kvm/mem_provider_sample.c @@ -6,7 +6,8 @@ * * The control device /dev/mem_provider_sample creates provider files. * Each file allocates its own contiguous memory from alloc_contig_pages(). - * Pages can be dynamically marked present or absent. + * Pages can be dynamically marked present or absent, and read only (for + * tests). * See mem_provider_sample.h. * * Confidential VMs are not supported. @@ -39,6 +40,7 @@ struct mps_info { struct page *cma_pages; /* from alloc_contig_pages() */ unsigned long *absent; /* taken away by MPS_SET_PRESENT */ + unsigned long *readonly; /* set by MPS_SET_READONLY */ struct inode *gmem_inode; }; @@ -83,6 +85,9 @@ static void mps_set_present(struct mps_info *info, unsigned long start, mutex_lock(&info->lock); if (present) { bitmap_clear(info->absent, start, end - start); + /* Tell the users of guest_memfd that cannot fault. */ + if (info->gmem_inode) + kvm_gmem_restore(info->gmem_inode, start, end - start); } else { bitmap_set(info->absent, start, end - start); if (info->gmem_inode) { @@ -95,6 +100,31 @@ static void mps_set_present(struct mps_info *info, unsigned long start, mutex_unlock(&info->lock); } +/* + * Change what get_pfn() reports for [start, end). Making it read only takes + * write access away, so revoke it: the users of the guest_memfd map it again + * read only. Making it writable takes nothing away, so only tell the users + * that cannot fault. The pages keep their contents. + */ +static void mps_set_readonly(struct mps_info *info, unsigned long start, + unsigned long end, bool value) +{ + unsigned long pfn = info->base_pfn + start; + + mutex_lock(&info->lock); + if (value) { + bitmap_set(info->readonly, start, end - start); + if (info->gmem_inode) + kvm_gmem_revoke(info->gmem_inode, start, end - start, + &pfn); + } else { + bitmap_clear(info->readonly, start, end - start); + if (info->gmem_inode) + kvm_gmem_restore(info->gmem_inode, start, end - start); + } + mutex_unlock(&info->lock); +} + static long mps_get_stats(struct mps_info *info, void __user *uarg) { struct mps_stats st = {}; @@ -114,11 +144,13 @@ static int mps_file_release(struct inode *inode, struct file *file) if (info->cma_pages) free_contig_range(info->base_pfn, info->npages); bitmap_free(info->absent); + bitmap_free(info->readonly); kfree(info); return 0; } -static long mps_file_set_present(struct mps_info *info, void __user *uarg) +static long mps_file_set_range(struct mps_info *info, void __user *uarg, + unsigned int cmd) { struct mps_set_present sp; unsigned long start, end; @@ -129,9 +161,13 @@ static long mps_file_set_present(struct mps_info *info, void __user *uarg) if (sp.value > 1 || sp.pad) return -EINVAL; ret = mps_range(info, sp.offset, sp.len, &start, &end); - if (!ret) + if (ret) + return ret; + if (cmd == MPS_SET_PRESENT) mps_set_present(info, start, end, sp.value); - return ret; + else + mps_set_readonly(info, start, end, sp.value); + return 0; } static long mps_file_ioctl(struct file *file, unsigned int cmd, @@ -144,7 +180,8 @@ static long mps_file_ioctl(struct file *file, unsigned int cmd, case MPS_GET_STATS: return mps_get_stats(info, uarg); case MPS_SET_PRESENT: - return mps_file_set_present(info, uarg); + case MPS_SET_READONLY: + return mps_file_set_range(info, uarg, cmd); default: return -ENOTTY; } @@ -178,11 +215,28 @@ static void mps_gmem_release(void *provider) fput(resource_file); } +/* Is every bit of @bitmap in [first, first + n) equal to @value? */ +static bool mps_bits_are(unsigned long *bitmap, unsigned long first, + unsigned long n, bool value) +{ + unsigned long end = first + n; + + return (value ? find_next_zero_bit(bitmap, end, first) : + find_next_bit(bitmap, end, first)) >= end; +} + +/* + * 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(). + */ static int mps_gmem_get_pfn(void *provider, pgoff_t index, unsigned long *pfn, int *max_order, u32 *attrs) { struct file *resource_file = provider; struct mps_info *info = resource_file->private_data; + bool ro; + int order; if (index >= info->npages) return -EINVAL; @@ -190,10 +244,22 @@ static int mps_gmem_get_pfn(void *provider, pgoff_t index, unsigned long *pfn, if (test_bit(index, info->absent)) return -EFAULT; + ro = test_bit(index, info->readonly); *pfn = info->base_pfn + index; - if (max_order) - *max_order = 0; - + *attrs = ro ? GUEST_MEMFD_PFN_READONLY : 0; + + if (!max_order) + return 0; + for (order = PUD_ORDER; order; order--) { + unsigned long first = ALIGN_DOWN(index, 1UL << order); + unsigned long n = 1UL << order; + + if (first + n <= info->npages && + mps_bits_are(info->absent, first, n, false) && + mps_bits_are(info->readonly, first, n, ro)) + break; + } + *max_order = order; return 0; } @@ -255,9 +321,10 @@ static long mps_ctl_new_file(void __user *uarg) return -ENOMEM; info->absent = bitmap_zalloc(npages, GFP_KERNEL); - if (!info->absent) { + info->readonly = bitmap_zalloc(npages, GFP_KERNEL); + if (!info->absent || !info->readonly) { ret = -ENOMEM; - goto err_free_info; + goto err_free_absent; } pages = alloc_contig_pages(npages, GFP_KERNEL | __GFP_ZERO, @@ -293,7 +360,7 @@ static long mps_ctl_new_file(void __user *uarg) free_contig_range(page_to_pfn(pages), npages); err_free_absent: bitmap_free(info->absent); -err_free_info: + bitmap_free(info->readonly); kfree(info); return ret; } diff --git a/samples/kvm/mem_provider_sample.h b/samples/kvm/mem_provider_sample.h index b9f10286cb30..57f494dc46fa 100644 --- a/samples/kvm/mem_provider_sample.h +++ b/samples/kvm/mem_provider_sample.h @@ -51,4 +51,13 @@ struct mps_set_present { #define MPS_SET_PRESENT \ _IOW(MPS_IOCTL_BASE, 12, struct mps_set_present) +/* + * ioctl on a provider fd, with the same argument as MPS_SET_PRESENT: report + * [@offset, @offset + @len) as read only (GUEST_MEMFD_PFN_READONLY, @value 1) + * or writable again (@value 0). Every mapping follows. For tests: the + * sample does not really share the pages. + */ +#define MPS_SET_READONLY \ + _IOW(MPS_IOCTL_BASE, 13, struct mps_set_present) + #endif /* _SAMPLES_KVM_MEM_PROVIDER_SAMPLE_H */ -- 2.47.3