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

Wei Yang
2019-03-28T03:36:13Z

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