On 2021/12/29 0:14, Dave Hansen wrote:
On 12/28/21 2:26 AM, Kefeng Wang wrote:
There are some disadvantages about this feature[2], one of the main
concerns is the possible memory fragmentation/waste in some scenarios,
also archs must ensure that any arch specific vmalloc allocations that
require PAGE_SIZE mappings(eg, module alloc with STRICT_MODULE_RWX)
use the VM_NO_HUGE_VMAP flag to inhibit larger mappings.
That just says that x86 *needs* PAGE_SIZE allocations. But, what
happens if VM_NO_HUGE_VMAP is not passed (like it was in v1)? Will the
subsequent permission changes just fragment the 2M mapping?
Yes, without VM_NO_HUGE_VMAP, it could fragment the 2M mapping.
When module alloc with STRICT_MODULE_RWX on x86, it calls
__change_page_attr()
from set_memory_ro/rw/nx which will split large page, so there is no
need to make
module alloc with HUGE_VMALLOC.
This all sounds very fragile to me. Every time a new architecture would
get added for huge vmalloc() support, the developer needs to know to go
find that architecture's module_alloc() and add this flag. They next
guy is going to forget, just like you did.
Considering that this is not a hot path, a weak function would be a nice
choice:
/* vmalloc() flags used for all module allocations. */
unsigned long __weak arch_module_vm_flags()
{
/*
* Modules use a single, large vmalloc(). Different
* permissions are applied later and will fragment
* huge mappings. Avoid using huge pages for modules.
*/
return VM_NO_HUGE_VMAP;
For x86, it only fragment, but for arm64, due to apply_to_page_range() in
set_memory_*, without this flag maybe crash. Whatever, we need this
flag for module.
}
Stick that in some the common module code, next to:
void * __weak module_alloc(unsigned long size)
{
return __vmalloc_node_range(size, 1, VMALLOC_START, VMALLOC_END,
...
Then, put arch_module_vm_flags() in *all* of the module_alloc()
implementations, including the generic one. That way (even with a new
architecture) whoever copies-and-pastes their module_alloc()
implementation is likely to get it right. The next guy who just does a
"select HAVE_ARCH_HUGE_VMALLOC" will hopefully just work.
OK， Let me check the VM_FLUSH_RESET_PERMS and try about this way.
Thanks.
VM_FLUSH_RESET_PERMS could probably be dealt with in the same way.
.
