Re: [PATCH] mm/memory_hotplug: fix kfree() of bootmem memory

[David Hildenbrand]

On 08.02.22 02:59, Miaohe Lin wrote:
> Hi:
> On 2022/2/7 22:33, David Hildenbrand wrote:
>> On 07.02.22 14:56, Miaohe Lin wrote:
>>> We can't use kfree() to release the resource as it might come from bootmem.
>>> Use release_mem_region() instead.
>>
>> How can this happen? release_mem_region() is called either from
>> __add_memory() or from add_memory_driver_managed(), where we allocated
>> the region via register_memory_resource(). Both functions shouldn't ever
>> be called before the buddy is up an running.
>>
>> Do you have a backtrace of an actual instance of this issue? Or was this
>> identified as possibly broken by code inspection?
>>
> 
> This is identified as possibly broken by code inspection. IIUC, alloc_resource
> is always used to allocate the resource. It has the below logic:
> 
>   if (bootmem_resource_free) {
> 	res = bootmem_resource_free;
> 	bootmem_resource_free = res->sibling;
>   }
> 
> where bootmem_resource_free is used to reusing the resource entries allocated by boot
> mem after the system is up:
> 
> /*
>  * For memory hotplug, there is no way to free resource entries allocated
>  * by boot mem after the system is up. So for reusing the resource entry
>  * we need to remember the resource.
>  */
> static struct resource *bootmem_resource_free;
> 
> So I think register_memory_resource() can reuse the resource allocated by bootmem.
> Or am I miss anything?

I think you're right, if we did a previous free_resource() of a resource allocated
during boot we could end up reusing that here. My best guess is that this never
really happens.

Wow, that's ugly. It affects essentially anybody reserving+freeing a resource.

E.g., dax/kmem.c similarly does a release_resource(res)+kfree(res)


We could either

a) Expose free_resource() and replace all kfree(res) instances by it

b) Just simplify that. I don't think we care about saving a couple of 
   bytes in corner cases. I might be wrong (IIRC primarily ppc64 really 
   succeeds in unplugging boot memory)


diff --git a/kernel/resource.c b/kernel/resource.c
index 9c08d6e9eef2..fe91a72fd951 100644
--- a/kernel/resource.c
+++ b/kernel/resource.c
@@ -56,14 +56,6 @@ struct resource_constraint {
 
 static DEFINE_RWLOCK(resource_lock);
 
-/*
- * For memory hotplug, there is no way to free resource entries allocated
- * by boot mem after the system is up. So for reusing the resource entry
- * we need to remember the resource.
- */
-static struct resource *bootmem_resource_free;
-static DEFINE_SPINLOCK(bootmem_resource_lock);
-
 static struct resource *next_resource(struct resource *p)
 {
        if (p->child)
@@ -160,36 +152,19 @@ __initcall(ioresources_init);
 
 static void free_resource(struct resource *res)
 {
-       if (!res)
-               return;
-
-       if (!PageSlab(virt_to_head_page(res))) {
-               spin_lock(&bootmem_resource_lock);
-               res->sibling = bootmem_resource_free;
-               bootmem_resource_free = res;
-               spin_unlock(&bootmem_resource_lock);
-       } else {
+       /*
+        * If the resource was allocated using memblock early during boot
+        * we'll leak it here: we can only return full pages back to the
+        * buddy and trying to be smart and reusing them eventually in
+        * alloc_resource() overcomplicates resource handling.
+        */
+       if (res && PageSlab(virt_to_head_page(res)))
                kfree(res);
-       }
 }
 
 static struct resource *alloc_resource(gfp_t flags)
 {
-       struct resource *res = NULL;
-
-       spin_lock(&bootmem_resource_lock);
-       if (bootmem_resource_free) {
-               res = bootmem_resource_free;
-               bootmem_resource_free = res->sibling;
-       }
-       spin_unlock(&bootmem_resource_lock);
-
-       if (res)
-               memset(res, 0, sizeof(struct resource));
-       else
-               res = kzalloc(sizeof(struct resource), flags);
-
-       return res;
+       return kzalloc(sizeof(struct resource), flags);
 }
 
 /* Return the conflict entry if you can't request it */


-- 
Thanks,

David / dhildenb