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

[Song Liu <songliubraving@fb.com>]

Hi Peter, 

> On Aug 22, 2022, at 9:56 AM, Song Liu <songliubraving@fb.com> 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
> 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?
> 
> There are a couple benefits from a second tree:
> 
> 1. It allows text allocations to go below PAGE_SIZE granularity, while 
>   data allocations would still use PAGE_SIZE granularity, which is the
>   same as current code. 
> 2. Text allocate requires mapping one vm_struct to many vmap_area. Putting
>   text allocations in a separate tree make it easier to handle this. 
>   (Well, I haven't finished this logic yet). 
> 3. A separate tree makes it easier to use text tail page, 
>   [_etext, roundup(_etext, PMD_SIZE)], for modules and BPF programs. 
> 
> Does this make sense? Do you see other downsides with a second tree?

Did these make sense? Do you have future comments that I would address in 
future versions?

Thanks,
Song