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

On 04/09/2025 14:14, Ryan Roberts wrote:

On 03/09/2025 01:50, Yang Shi wrote:

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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.

Sorry I realise this was not very clear... I am saying I think we need to fix it
somehow. A cache would likely work. But I'd prefer to avoid it if we can find a
better solution.

Thanks,
Ryan

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Thanks,
Yang

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Thanks,
Yang

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Thanks,
Ryan