On 10/18/2025 4:11 AM, Ackerley Tng wrote:
[...]
+static int kvm_gmem_init_inode(struct inode *inode, loff_t size, u64 flags)
+{
+ struct gmem_inode *gi = GMEM_I(inode);
+ MA_STATE(mas, &gi->attributes, 0, (size >> PAGE_SHIFT) - 1);
+ u64 attrs;
+ int r;
+
+ inode->i_op = &kvm_gmem_iops;
+ inode->i_mapping->a_ops = &kvm_gmem_aops;
+ inode->i_mode |= S_IFREG;
+ inode->i_size = size;
+ mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
+ mapping_set_inaccessible(inode->i_mapping);
+ /* Unmovable mappings are supposed to be marked unevictable as well. */
AS_UNMOVABLE has been removed and got merged into AS_INACCESSIBLE, not sure if
it's better to use "Inaccessible" instead of "Unmovable"
+ WARN_ON_ONCE(!mapping_unevictable(inode->i_mapping));
+
+ gi->flags = flags;
+
+ mt_set_external_lock(&gi->attributes,
+ &inode->i_mapping->invalidate_lock);
+
+ /*
+ * Store default attributes for the entire gmem instance. Ensuring every
+ * index is represented in the maple tree at all times simplifies the
+ * conversion and merging logic.
+ */
+ attrs = gi->flags & GUEST_MEMFD_FLAG_INIT_SHARED ? 0 :
KVM_MEMORY_ATTRIBUTE_PRIVATE;
+
+ /*
+ * Acquire the invalidation lock purely to make lockdep happy. There
+ * should be no races at this time since the inode hasn't yet been fully
+ * created.
+ */
+ filemap_invalidate_lock(inode->i_mapping);
+ r = mas_store_gfp(&mas, xa_mk_value(attrs), GFP_KERNEL);
+ filemap_invalidate_unlock(inode->i_mapping);
+
+ return r;
+}
+
[...]
@@ -925,13 +986,39 @@ static struct inode *kvm_gmem_alloc_inode(struct
super_block *sb)
mpol_shared_policy_init(&gi->policy, NULL);
+ /*
+ * Memory attributes are protected the filemap invalidation lock, but
^
protected by
+ * the lock structure isn't available at this time. Immediately mark
+ * maple tree as using external locking so that accessing the tree
+ * before its fully initialized results in NULL pointer dereferences
+ * and not more subtle bugs.
+ */
+ mt_init_flags(&gi->attributes, MT_FLAGS_LOCK_EXTERN);
+
gi->flags = 0;
return &gi->vfs_inode;
}
static void kvm_gmem_destroy_inode(struct inode *inode)
{
- mpol_free_shared_policy(&GMEM_I(inode)->policy);
+ struct gmem_inode *gi = GMEM_I(inode);
+
+ mpol_free_shared_policy(&gi->policy);
+
+ /*
+ * Note! Checking for an empty tree is functionally necessary to avoid
+ * explosions if the tree hasn't been initialized, i.e. if the inode is
It makes sense to skip __mt_destroy() when mtree is empty.
But what explosions it could trigger if mtree is empty?
It seems __mt_destroy() can handle the case if the external lock is not set.
+ * being destroyed before guest_memfd can set the external lock.
+ */
+ if (!mtree_empty(&gi->attributes)) {
+ /*
+ * Acquire the invalidation lock purely to make lockdep happy,
+ * the inode is unreachable at this point.
+ */
+ filemap_invalidate_lock(inode->i_mapping);
+ __mt_destroy(&gi->attributes);
+ filemap_invalidate_unlock(inode->i_mapping);
+ }
}
static void kvm_gmem_free_inode(struct inode *inode)
--
2.51.0.858.gf9c4a03a3a-goog
