Re: [PATCH 3/4] arm64: mm: support large block mapping when rodata=full

[Yang Shi <yang@os.amperecomputing.com>]

>>
>> --- snip ---
>>
>> I'm afraid I'll have to agree with Ryan :) Looking at the signature 
>> of split_kernel_pgtable_mapping,
>> one would expect that this function splits all block mappings in this 
>> region. But that is just a
>> nit; it does not seem right to me that we are iterating over the 
>> entire space when we know *exactly* where
>> we have to make the split, just to save on pgd/p4d/pud loads - the 
>> effect of which is probably cancelled
>> out by unnecessary iterations your approach takes to skip the 
>> intermediate blocks.
>
> The implementation is aimed to reuse the split code for repainting. We 
> have to split all leaf mappings down to PTEs for repainting.
>
> Now Ryan suggested use pgtable walk API for repainting, it made the 
> duplicate code problem gone. We had some discussion in the other series.
>
>>
>> If we are concerned that most change_memory_common() operations are 
>> for a single page, then
>> we can do something like:
>>
>> unsigned long size = end - start;
>> bool end_split, start_split = false;
>>
>> if (start not aligned to block mapping)
>>     start_split = split(start);
>>
>> /*
>>  * split the end only if the start wasn't split, or
>>  * if it cannot be guaranteed that start and end lie
>>  * on the same contig block
>>  */
>> if (!start_split || (size > CONT_PTE_SIZE))
>>     end_split = split(end);
>
> Thanks for the suggestion. It works for some cases, but I don't think 
> it can work if the range is cross page table IIUC. For example, start 
> is in a PMD, but end is in another PMD.

I think we have to call split_mapping(end) if size is greater than page 
size, i.e.

split_mapping(start);
if (size > PAGE_SIZE)
     split_mapping(end);

This can avoid walking page table twice for page size range, which 
should be the most case in the current kernel.

Thanks,
Yang

>
> Regards,
> Yang
>
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