Re: [PATCH bpf 2/2] bpf: use vmalloc with VM_ALLOW_HUGE_VMAP for bpf_prog_pack

[Song Liu <songliubraving@fb.com>]

> On Apr 8, 2022, at 10:29 PM, Christoph Hellwig <hch@infradead.org> wrote:
> 
> On Fri, Apr 08, 2022 at 03:34:43PM -0700, Song Liu wrote:
>> +static void *bpf_prog_pack_vmalloc(unsigned long size)
>> +{
>> +#if defined(MODULES_VADDR)
>> +	unsigned long start = MODULES_VADDR;
>> +	unsigned long end = MODULES_END;
>> +#else
>> +	unsigned long start = VMALLOC_START;
>> +	unsigned long end = VMALLOC_END;
>> +#endif
>> +
>> +	return __vmalloc_node_range(size, PAGE_SIZE, start, end, GFP_KERNEL, PAGE_KERNEL,
>> +				    VM_DEFER_KMEMLEAK | VM_ALLOW_HUGE_VMAP,
>> +				    NUMA_NO_NODE, __builtin_return_address(0));
>> +}
> 
> Instead of having this magic in bpf I think a module_alloc_large would
> seems like the better interface here.

AFAICT, modules allocate a large piece of memory and put both text and
data on it, so modules cannot really use huge pages yet. 

OTOH, it is probably beneficial for the modules to use something 
similar to bpf_prog_pack, i.e., put text from multiple modules to a 
single huge page. Of course, this requires non-trivial work in both 
mm code and module code.

Given that 1) modules cannot use huge pages yet, and 2) module may
use differently (with sharing), I think adding module_alloc_large()
doesn't add much value at the moment. So we can just keep this logic
in BPF for now. 

Does this make sense?

Thanks,
Song