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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.

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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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