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[54.240.197.234]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e3ebcb958sm23845574f8f.21.2026.10.27.08.47.07 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Tue, 27 Oct 2026 08:47:09 -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 07/14] KVM: guest_memfd: Update exports when pages are allocated Date: Tue, 27 Oct 2026 15:46:42 +0000 Message-ID: <20261027154649.91571-8-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: linux-kernel@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 An export maps only the pages that have memory when it looks them up, and a device cannot fault in the others, so guest_memfd must call update() when a page gets memory. Lookups treat a folio that was never zeroed as a hole. Zero folios in one helper, under the invalidate lock, from the host fault, KVM's get_pfn() and fallocate(), which did not zero at all, and update the exports for each folio zeroed. Mark zeroed folios dirty: lookups hold no reference, and memory-failure handling evicts clean folios that nothing references. In PFN mode, update the block that get_pfn() reports when it fills a hole, since the provider may back all of it. Assisted-by: Claude:claude-opus-5-5 Signed-off-by: Fred Griffoul --- include/linux/guest_memfd.h | 26 ++++++------ virt/kvm/guest_memfd.c | 85 +++++++++++++++++++++++++++++++------ 2 files changed, 85 insertions(+), 26 deletions(-) diff --git a/include/linux/guest_memfd.h b/include/linux/guest_memfd.h index 937693ae9573..f8e89723dd68 100644 --- a/include/linux/guest_memfd.h +++ b/include/linux/guest_memfd.h @@ -90,13 +90,14 @@ static inline int kvm_gmem_revoke(struct inode *inode, pgoff_t start, size_t nr_ * nor fault them. It attaches to the file, looks up the frame behind each * shared page without taking a reference, and guest_memfd calls its update() * whenever what a lookup returns for a range may have changed: conversions, - * punch holes, memory failure and a provider's kvm_gmem_revoke(). update() - * makes the consumer's mappings of the range match what a lookup returns now: - * a frame, or a hole for a private page or a page without a frame. It may be - * called for pages that did not change, and for ranges that do not follow the - * blocks that lookups report: the consumer may then remap whole blocks, also - * outside the range, e.g. to merge them into larger mappings. A consumer that - * can fault may instead only remove its mappings of the range. + * punch holes, memory failure, a provider's kvm_gmem_revoke(), fallocate() and + * pages getting their first frame. update() makes the consumer's mappings of + * the range match what a lookup returns now: a frame, or a hole for a private + * page or a page without a frame. It may be called for pages that did not + * change, and for ranges that do not follow the blocks that lookups report: + * the consumer may then remap whole blocks, also outside the range, e.g. to + * merge them into larger mappings. A consumer that can fault may instead only + * remove its mappings of the range. * * Until update() returns, the frames it replaces stay valid, so a consumer * may switch each mapping to its new target in one step; once it returns, @@ -105,11 +106,12 @@ static inline int kvm_gmem_revoke(struct inode *inode, pgoff_t start, size_t nr_ * beforehand. * * guest_memfd calls update() with its invalidate lock held, exclusive when it - * takes frames away or pages convert and shared on memory failure, possibly - * with a folio of the file locked; so update() may only call - * kvm_gmem_export_lookup_pfn(). Under the shared lock, updates may run at the - * same time, also for overlapping ranges. The consumer holds a lock across a - * lookup and the mapping it installs, which update() also takes. Lock order: + * takes frames away or pages convert, and shared when pages only gain frames + * and on memory failure, possibly with a folio of the file locked; so update() + * may only call kvm_gmem_export_lookup_pfn(). Under the shared lock, updates + * may run at the same time, also for overlapping ranges. The consumer holds a + * lock across a lookup and the mapping it installs, which update() also takes. + * Lock order: * * PFN mode provider lock (across kvm_gmem_revoke()) * guest_memfd invalidate lock diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index 1c5826c5384e..42bb5f263e23 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c @@ -286,7 +286,8 @@ static bool kvm_gmem_range_has_attributes(struct inode *inode, * There is no backing storage for the memory, so the folio will remain * up-to-date until it's removed. * - * Ignore accessed, referenced, and dirty flags. The memory is + * Ignore accessed and referenced flags; the dirty flag only keeps a zeroed + * folio in the page cache, see kvm_gmem_folio_zero(). The memory is * unevictable and there is no storage to write back to. */ static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index) @@ -430,7 +431,8 @@ static void kvm_gmem_invalidate_end(struct inode *inode, pgoff_t start, /* * What a lookup returns for [start, end) may have changed: exclusive lock when - * frames were taken away or pages converted, at least shared on memory failure. + * frames were taken away or pages converted, at least shared when pages only + * gained frames and on memory failure. */ static void kvm_gmem_update_exports(struct inode *inode, pgoff_t start, pgoff_t end) @@ -590,6 +592,64 @@ int kvm_gmem_revoke(struct inode *inode, pgoff_t start, size_t nr_pages, } EXPORT_SYMBOL_GPL(kvm_gmem_revoke); +/* + * Zero a folio on its first use. Lookups find it from then on, with no + * reference held, so mark it dirty: memory-failure handling drops clean + * folios that nothing references from the page cache (mapping_evict_folio()). + * Called with the folio locked. Returns true if the folio was zeroed. + */ +static bool kvm_gmem_folio_zero(struct folio *folio) +{ + if (folio_test_uptodate(folio)) + return false; + + folio_zero_range(folio, 0, folio_size(folio)); + folio_mark_uptodate(folio); + folio_mark_dirty(folio); + return true; +} + +/* Zero a folio on its first use, and tell the exports. Under the lock. */ +static void kvm_gmem_folio_clear(struct inode *inode, struct folio *folio) +{ + if (kvm_gmem_folio_zero(folio)) + kvm_gmem_update_exports(inode, folio->index, folio_next_index(folio)); +} + +/* + * get_pfn() in PFN mode, which may back a hole: then tell the exports about + * the whole block that get_pfn() reports, as the provider may have backed all + * of it. Called with the invalidate lock held. + */ +static int kvm_gmem_pfn_get(struct inode *inode, pgoff_t index, + kvm_pfn_t *pfn, int *max_order, u32 *attrs) +{ + pgoff_t npages = i_size_read(inode) >> PAGE_SHIFT; + struct gmem_inode *gi = GMEM_I(inode); + bool hole = false; + unsigned long raw; + pgoff_t first; + int order, r; + size_t nr; + + if (!list_empty(&gi->exports)) + hole = gi->provider_ops->lookup_pfn_range(gi->provider, index, + &raw, &nr) == -ENOENT; + + r = gmem_provider_get_pfn(gi, index, pfn, &order, attrs); + if (r) + return r; + + if (hole) { + first = round_down(index, 1UL << order); + kvm_gmem_update_exports(inode, first, + min(first + (1UL << order), npages)); + } + if (max_order) + *max_order = order; + return 0; +} + static long kvm_gmem_allocate(struct inode *inode, loff_t offset, loff_t len) { struct address_space *mapping = inode->i_mapping; @@ -619,7 +679,7 @@ static long kvm_gmem_allocate(struct inode *inode, loff_t offset, loff_t len) break; } - r = gmem_provider_get_pfn(gi, index, &pfn, &max_order, NULL); + r = kvm_gmem_pfn_get(inode, index, &pfn, &max_order, NULL); if (r) break; @@ -647,6 +707,8 @@ static long kvm_gmem_allocate(struct inode *inode, loff_t offset, loff_t len) index = folio_next_index(folio); + /* Zeroed now, so that a device can map it before first use. */ + kvm_gmem_folio_clear(inode, folio); folio_unlock(folio); folio_put(folio); @@ -819,7 +881,7 @@ static vm_fault_t kvm_gmem_pfn_fault(struct vm_fault *vmf) u32 attrs; int r; - r = gmem_provider_get_pfn(GMEM_I(inode), vmf->pgoff, &pfn, NULL, &attrs); + r = kvm_gmem_pfn_get(inode, vmf->pgoff, &pfn, NULL, &attrs); if (r) return vmf_error(r); @@ -873,6 +935,8 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) folio = kvm_gmem_get_folio(inode, vmf->pgoff); if (IS_ERR(folio)) r = PTR_ERR(folio); + else if (!folio_test_large(folio)) + kvm_gmem_folio_clear(inode, folio); } filemap_invalidate_unlock_shared(inode->i_mapping); @@ -891,11 +955,6 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) goto out_folio; } - if (!folio_test_uptodate(folio)) { - clear_highpage(folio_page(folio, 0)); - folio_mark_uptodate(folio); - } - vmf->page = folio_file_page(folio, vmf->pgoff); out_folio: @@ -1887,7 +1946,8 @@ static struct folio *__kvm_gmem_get_pfn(struct file *file, } if (kvm_gmem_is_pfn_mode(gi)) { - r = gmem_provider_get_pfn(gi, index, pfn, max_order, attrs); + r = kvm_gmem_pfn_get(file_inode(file), index, pfn, max_order, + attrs); if (r) return ERR_PTR(r); return NULL; @@ -1903,10 +1963,7 @@ static struct folio *__kvm_gmem_get_pfn(struct file *file, return ERR_PTR(-EHWPOISON); } - if (!folio_test_uptodate(folio)) { - clear_highpage(folio_page(folio, 0)); - folio_mark_uptodate(folio); - } + kvm_gmem_folio_clear(file_inode(file), folio); *pfn = folio_file_pfn(folio, index); if (max_order) -- 2.47.3