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

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Message-ID: <3b3be4db-4f8e-4d81-8e52-dadad23dcd24@os.amperecomputing.com>
Date: Wed, 23 Jul 2025 13:51:54 -0700
From: Yang Shi <yang@...amperecomputing.com>
To: Dev Jain <dev.jain@....com>, Ryan Roberts <ryan.roberts@....com>,
 will@...nel.org, catalin.marinas@....com, Miko.Lenczewski@....com,
 scott@...amperecomputing.com, cl@...two.org
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] arm64: mm: support large block mapping when
 rodata=full



On 7/23/25 10:38 AM, Dev Jain wrote:
>
> On 23/06/25 6:56 pm, Ryan Roberts wrote:
>> [...]
>>
>>>> +
>>>> +int split_leaf_mapping(unsigned long addr)
>>> Thanks for coming up with the code. It does help to understand your 
>>> idea. Now I
>>> see why you suggested "split_mapping(start); split_mapping(end);" 
>>> model. It does
>>> make the implementation easier because we don't need a loop anymore. 
>>> But this
>>> may have a couple of problems:
>>>    1. We need walk the page table twice instead of once. It sounds 
>>> expensive.
>> Yes we need to walk twice. That may be more expensive or less expensive,
>> depending on the size of the range that you are splitting. If the 
>> range is large
>> then your approach loops through every leaf mapping between the start 
>> and end
>> which will be more expensive than just doing 2 walks. If the range is 
>> small then
>> your approach can avoid the second walk, but at the expense of all 
>> the extra
>> loop overhead.
>>
>> My suggestion requires 5 loads (assuming the maximum of 5 levels of 
>> lookup).
>> Personally I think this is probably acceptable? Perhaps we need some 
>> other
>> voices here.
>
> Hello all,
>
> I am starting to implement vmalloc-huge by default with BBML2 no-abort 
> on arm64.
> I see that there is some disagreement related to the way the splitting 
> needs to
> be implemented - I skimmed through the discussions and it will require 
> some work
> to understand what is going on :) hopefully I'll be back soon to give 
> some of
> my opinions.

Hi Dev,

Thanks for the heads up.

In the last email I suggested skip the leaf mappings in the split range 
in order to reduce page table walk overhead for split_mapping(start, 
end). In this way we can achieve:
     - reuse the most split code for repainting (just need 
NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS flag for repainting to split page 
table to PTEs)
     - just walk page table once
     - have similar page table walk overhead with 
split_mapping(start)/split_mapping(end) if the split range is large

I'm basically done on a new spin to implement it and solve all the 
review comments from v4. I should be able to post the new spin by the 
end of this week.

Regards,
Yang

>
>>
>>