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

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Message-ID: <c77aa9e4-2209-4dbd-904e-daace1c929b6@arm.com>
Date: Thu, 24 Jul 2025 17:13:37 +0530
From: Dev Jain <dev.jain@....com>
To: Yang Shi <yang@...amperecomputing.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 24/07/25 2:21 am, Yang Shi wrote:
>
>
> 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.

Great! As Catalin notes on my huge-perm change series, that series 
doesn't have any user so it does not make sense for that to go in 
without your

series - can you merge that series into your work for the new version?


>
> Regards,
> Yang
>
>>
>>>
>>>
>