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

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Message-ID: <452be681-f28b-4b5b-848d-c967672b4f5e@arm.com>
Date: Wed, 23 Jul 2025 23:08:23 +0530
From: Dev Jain <dev.jain@....com>
To: Ryan Roberts <ryan.roberts@....com>,
 Yang Shi <yang@...amperecomputing.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 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.

>
>