Re: [PATCH v10 11/41] KVM: guest_memfd: Ensure pages are not in use before conversion
From: "David Hildenbrand (Arm)" <david@kernel.org>
Date: 2026-08-10 09:20:09
Also in:
kvm, linux-coco, linux-doc, linux-kselftest, linux-mm, lkml
On 8/7/26 23:52, Ackerley Tng via B4 Relay wrote:
From: Ackerley Tng <redacted> When converting memory to private in guest_memfd, it is necessary to ensure that the pages are not currently being accessed by any other part of the kernel or userspace to avoid any current user writing to guest private memory. guest_memfd checks for unexpected refcounts to determine whether a page is still in use. The only expected refcounts after unmapping the range requested for conversion are those that are held by guest_memfd itself. Update the kvm_memory_attributes2 structure to include an error_offset field. This allows KVM to report the exact offset where a conversion failed to userspace. If the safety check fails, return -EAGAIN and copy the error_offset back to userspace so that it can potentially retry the operation or handle the failure gracefully. Update documentation to document the error_offset field and the possible -EAGAIN error. Suggested-by: David Hildenbrand <david@kernel.org> Co-developed-by: Vishal Annapurve <redacted> Signed-off-by: Vishal Annapurve <redacted> Reviewed-by: Fuad Tabba <redacted> Tested-by: Shivank Garg <redacted> Signed-off-by: Ackerley Tng <redacted> ---
[...]
quoted hunk ↗ jump to hunk
#define KVM_MEMORY_ATTRIBUTE_PRIVATE (1ULL << 3)diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c index 3783e63476569..13c3989136f67 100644 --- a/virt/kvm/guest_memfd.c +++ b/virt/kvm/guest_memfd.c@@ -524,8 +524,42 @@ static int kvm_gmem_mas_preallocate(struct ma_state *mas, u64 attributes, return mas_preallocate(mas, xa_mk_value(attributes), GFP_KERNEL); } +static bool kvm_gmem_is_safe_for_conversion(struct inode *inode, pgoff_t start, + size_t nr_pages, pgoff_t *err_index)
I would focus on the "to_private" aspect or abstract it to "kvm_gmem_mem_has_unexpected_refs" or sth like that.
+{
+ struct address_space *mapping = inode->i_mapping;
+ const int filemap_get_folios_refcount = 1;
+ pgoff_t last = start + nr_pages - 1;
+ struct folio_batch fbatch;
+ bool safe = true;
+ pgoff_t next;
+ int i;
+
+ folio_batch_init(&fbatch);
+
+ next = start;
+ while (safe && filemap_get_folios(mapping, &next, last, &fbatch)) {
+ for (i = 0; i < folio_batch_count(&fbatch); ++i) {
+ struct folio *folio = fbatch.folios[i];
+
+ if (folio_ref_count(folio) !=
+ folio_nr_pages(folio) + filemap_get_folios_refcount) {I'd rather add a comment than have this filemap_get_folios_refcount. /* * We expect one reference per folio-page in the pagecache and one * reference from filemap_get_folios(). */ if (folio_ref_count(folio) != folio_nr_pages(folio) + 1)
quoted hunk ↗ jump to hunk
+ safe = false; + *err_index = max(start, folio->index); + break; + } + } + + folio_batch_release(&fbatch); + cond_resched(); + } + + return safe; +} + static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start, - size_t nr_pages, uint64_t attrs) + size_t nr_pages, uint64_t attrs, + pgoff_t *err_index) { bool to_private = attrs & KVM_MEMORY_ATTRIBUTE_PRIVATE; struct address_space *mapping = inode->i_mapping;@@ -542,8 +576,21 @@ static int __kvm_gmem_set_attributes(struct inode *inode, pgoff_t start, mas_init(&mas, mt, start); r = kvm_gmem_mas_preallocate(&mas, attrs, start, nr_pages); - if (r) + if (r) { + *err_index = start; goto out; + } + + if (to_private) {
I'd add a comment here for the "why are we unmapping". -- Cheers, David