Re: [PATCH 4/6] x86, mm: make split_mem_range() more easy to read

[Wei Yang]

On Sun, Mar 24, 2019 at 03:29:04PM +0100, Thomas Gleixner wrote:
>Wei,
>
>On Tue, 12 Feb 2019, Wei Yang wrote:
>> 
>> This patch changes the implementation from the first perception to the
>> second to reduce one different handling on end_pfn. After doing so, the
>> code is easier to read.
>
>It's maybe slightly easier to read, but it's still completely unreadable
>garbage.
>
>  Not your fault, it was garbage before.
>
>But refining garbage still results in garbage. Just the smell is slightly
>different.
>
>Why?
>
> 1) Doing all the calculations PFN based is just a pointless
>    indirection. Just look at all the rounding magic and back and forth
>    conversions.
>    
>    All of that can be done purely address/size based which makes the code
>    truly readable.
>
> 2) The 5(3) sections are more or less copied code with a few changes of
>    constants, except for the first section (A) which has an extra quirk
>    for 32bit. Plus all the 64bit vs. 32bit #ifdeffery which is not making
>    it any better.
>
>    This copied mess can be avoided by using helper functions and proper
>    loops.
>
> 3) During the bootmem phase the code tries to preserve large mappings
>    _AFTER_ splitting them up and then it tries to merge the resulting
>    overlaps.
>
>    This is completely backwards because the expansion of the range can be
>    tried right away when then mapping of a large page is attempted. Surely
>    not with the current mess, but with a proper loop based approach it can
>    be done nicely.
>
>    Btw, there is a bug in that expansion code which could result in
>    undoing the enforced 4K mapping of the lower 2M/4M range on 32bit. It's
>    probably not an issue in practice because the low range is usually not
>    contiguous. But it works by chance not by design.

Hi, Thomas

I want to confirm with you, this bug is in adjust_range_page_size_mask(),
right?

-- 
Wei Yang
Help you, Help me