[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <c23f7ecf-78f6-4ec8-8f00-b92e98fa95a3@os.amperecomputing.com>
Date: Tue, 5 Aug 2025 17:10:01 -0700
From: Yang Shi <yang@...amperecomputing.com>
To: Dev Jain <dev.jain@....com>, ryan.roberts@....com, will@...nel.org,
catalin.marinas@....com, akpm@...ux-foundation.org, 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
>>
>> --- snip ---
>>
>> I'm afraid I'll have to agree with Ryan :) Looking at the signature
>> of split_kernel_pgtable_mapping,
>> one would expect that this function splits all block mappings in this
>> region. But that is just a
>> nit; it does not seem right to me that we are iterating over the
>> entire space when we know *exactly* where
>> we have to make the split, just to save on pgd/p4d/pud loads - the
>> effect of which is probably cancelled
>> out by unnecessary iterations your approach takes to skip the
>> intermediate blocks.
>
> The implementation is aimed to reuse the split code for repainting. We
> have to split all leaf mappings down to PTEs for repainting.
>
> Now Ryan suggested use pgtable walk API for repainting, it made the
> duplicate code problem gone. We had some discussion in the other series.
>
>>
>> If we are concerned that most change_memory_common() operations are
>> for a single page, then
>> we can do something like:
>>
>> unsigned long size = end - start;
>> bool end_split, start_split = false;
>>
>> if (start not aligned to block mapping)
>> start_split = split(start);
>>
>> /*
>> * split the end only if the start wasn't split, or
>> * if it cannot be guaranteed that start and end lie
>> * on the same contig block
>> */
>> if (!start_split || (size > CONT_PTE_SIZE))
>> end_split = split(end);
>
> Thanks for the suggestion. It works for some cases, but I don't think
> it can work if the range is cross page table IIUC. For example, start
> is in a PMD, but end is in another PMD.
I think we have to call split_mapping(end) if size is greater than page
size, i.e.
split_mapping(start);
if (size > PAGE_SIZE)
split_mapping(end);
This can avoid walking page table twice for page size range, which
should be the most case in the current kernel.
Thanks,
Yang
>
> Regards,
> Yang
>
>>
>>
>>
>
Powered by blists - more mailing lists