Re: [PATCH v7 0/6] arm64: support FEAT_BBM level 2 and large block mapping when rodata=full

[Ryan Roberts <ryan.roberts@arm.com>]

On 03/09/2025 01:50, Yang Shi wrote:
>>>>
>>>>
>>>> I am wondering whether we can just have a warn_on_once or something for the
>>>> case
>>>> when we fail to allocate a pagetable page. Or, Ryan had
>>>> suggested in an off-the-list conversation that we can maintain a cache of PTE
>>>> tables for every PMD block mapping, which will give us
>>>> the same memory consumption as we do today, but not sure if this is worth it.
>>>> x86 can already handle splitting but due to the callchains
>>>> I have described above, it has the same problem, and the code has been working
>>>> for years :)
>>> I think it's preferable to avoid having to keep a cache of pgtable memory if we
>>> can...
>>
>> Yes, I agree. We simply don't know how many pages we need to cache, and it
>> still can't guarantee 100% allocation success.
> 
> This is wrong... We can know how many pages will be needed for splitting linear
> mapping to PTEs for the worst case once linear mapping is finalized. But it may
> require a few hundred megabytes memory to guarantee allocation success. I don't
> think it is worth for such rare corner case.

Indeed, we know exactly how much memory we need for pgtables to map the linear
map by pte - that's exactly what we are doing today. So we _could_ keep a cache.
We would still get the benefit of improved performance but we would lose the
benefit of reduced memory.

I think we need to solve the vm_reset_perms() problem somehow, before we can
enable this.

Thanks,
Ryan

> 
> Thanks,
> Yang
> 
>>
>> Thanks,
>> Yang
>>
>>>
>>> Thanks,
>>> Ryan
>>>
>>>
>>
>