Re: [RFC 0/5] vmalloc_exec for modules and BPF programs

[Song Liu <songliubraving@fb.com>]

> On Aug 22, 2022, at 10:42 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> 
> On Mon, Aug 22, 2022 at 04:56:47PM +0000, Song Liu wrote:
>> 
>> 
>>> On Aug 22, 2022, at 9:34 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>>> 
>>> On Mon, Aug 22, 2022 at 03:46:38PM +0000, Song Liu wrote:
>>>> Could you please share your feedback on this? 
>>> 
>>> I've looked at it all of 5 minutes, so perhaps I've missed something.
>>> 
>>> However, I'm a little surprised you went with a second tree instead of
>>> doing the top-down thing for data. The way you did it makes it hard to
>>> have guard pages between text and data.
>> 
>> I didn't realize the importance of the guard pages. But it is not too
> 
> I'm not sure how important it is, just seems like a good idea to trap
> anybody trying to cross that divide. Also, to me it seems like a good
> idea to have a single large contiguous text region instead of splintered
> 2M pages.

A single large contiguous text region is great. However, it is not easy to
keep it contiguous. For example, when we load a big module, and then unload
it. It is not easy to recycle the space. Say we load module-x-v1, which is 
4MB, and uses 2 huge pages. Then we load a small BPF program after it. The 
address space looks like:

MODULE_VADDR to MODULE_VADDR + 4MB:			module-x-v1
MODULE_VADDR + 4MB to MODULE_VADDR + 4MB + 4kB:		bpf_prog_xxxx

When we unload module-x-v1, there will be 4MB hole in the address space. 
If we then load module-x-v2, which is 4.1MB in size, we cannot reuse that
hole, because the module is a little too big for the hole. 

AFAICT, to use the space efficiently, we will have to deal with splintered
2MB pages. 

Does this make sense?

Thanks,
Song

> 
>> hard to do it with this approach. For each 2MB text page, we can reserve
>> 4kB on the beginning and end of it. Would this work?
> 
> Typically a guard page has different protections (as in none what so
> ever) so that every access goes *splat*.