Re: [PATCH v5 bpf-next 7/7] bpf, x86_64: use bpf_prog_pack allocator

[Song Liu]

> On Jan 21, 2022, at 10:29 AM, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
> 
> On Fri, Jan 21, 2022 at 9:53 AM Song Liu <songliubraving@xxxxxx> wrote:
>>> 
>>> the header->size could be just below 2MB.
>>> I don't think kzalloc() can handle that.
>> 
>> Technically, kzalloc can handle 2MB allocation via:
>>  kzalloc() => kmalloc() => kmalloc_large() => kmalloc_order()
>> 
>> But this would fail when the memory is fragmented. I guess we should use
>> kvmalloc() instead?
> 
> Contiguous 2MB allocation?

Yeah, I tried that both kzalloc() and kvmalloc() could get 2MB memory.
I think kzalloc will fail when the memory is fragmented, but I haven't 
confirmed it yet.

> 
>>> 
>>>> +                               if (!tmp_header) {
>>>> +                                       bpf_jit_binary_free_pack(header);
>>>> +                                       header = NULL;
>>>> +                                       prog = orig_prog;
>>>> +                                       goto out_addrs;
>>>> +                               }
>>>> +                               tmp_header->size = header->size;
>>>> +                               tmp_image = (void *)tmp_header + ((void *)image - (void *)header);
>>> 
>>> Why is 'tmp_image' needed at all?
>>> The above math can be done where necessary.
>> 
>> We pass both image and tmp_image to do_jit(), as it needs both of them.
>> I think maintaining a tmp_image variable makes the logic cleaner. We can
>> remove it from x64_jit_data, I guess.
> 
> I'd remove from x64_jit_data. The recompute is cheap.

Will do. 

> 
> Speaking of tmp_header name... would be great to come up
> with something more descriptive.
> Here both tmp_header/tmp_image and header/image are used at the same time.
> My initial confusion with the patch was due to the name 'tmp'.
> The "tmp" prefix implies that the tmp_image will be used first
> and then it will become an image.
> But it's not the case.
> Maybe call it 'rw_header' and add a comment that 'header/image'
> are not writeable directly ?
> Or call it 'poke_header' ?
> Other ideas?

I think rw_header/rw_image is good. poke_header is confusing, as we will
text_poke "header". 

Thanks,
Song