linux-kernel - Re: [PATCH v1 12/16] arm64/mm: Support huge pte-mapped pages in vmap

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Message-ID: <d6af21f1-e539-4fa6-a3f8-c4859b1325c5@arm.com>
Date: Thu, 13 Feb 2025 09:09:52 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Anshuman Khandual <anshuman.khandual@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Muchun Song <muchun.song@...ux.dev>,
 Pasha Tatashin <pasha.tatashin@...een.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Uladzislau Rezki <urezki@...il.com>, Christoph Hellwig <hch@...radead.org>,
 Mark Rutland <mark.rutland@....com>, Ard Biesheuvel <ardb@...nel.org>,
 Dev Jain <dev.jain@....com>, Alexandre Ghiti <alexghiti@...osinc.com>,
 Steve Capper <steve.capper@...aro.org>, Kevin Brodsky <kevin.brodsky@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 12/16] arm64/mm: Support huge pte-mapped pages in vmap


>>>> +#define arch_vmap_pte_range_map_size arch_vmap_pte_range_map_size
>>>> +static inline unsigned long arch_vmap_pte_range_map_size(unsigned long addr,
>>>> +						unsigned long end, u64 pfn,
>>>> +						unsigned int max_page_shift)
>>>> +{
>>>> +	if (max_page_shift < CONT_PTE_SHIFT)
>>>> +		return PAGE_SIZE;
>>>> +
>>>> +	if (end - addr < CONT_PTE_SIZE)
>>>> +		return PAGE_SIZE;
>>>> +
>>>> +	if (!IS_ALIGNED(addr, CONT_PTE_SIZE))
>>>> +		return PAGE_SIZE;
>>>> +
>>>> +	if (!IS_ALIGNED(PFN_PHYS(pfn), CONT_PTE_SIZE))
>>>> +		return PAGE_SIZE;
>>>> +
>>>> +	return CONT_PTE_SIZE;
>>>
>>> A small nit:
>>>
>>> Should the rationale behind picking CONT_PTE_SIZE be added here as an in code
>>> comment or something in the function - just to make things bit clear.
>>
>> I'm not sure what other size we would pick?
> 
> The suggestion was to add a small comment in the above helper function explaining
> the rationale for various conditions in there while returning either PAGE_SIZE or
> CONT_PTE_SIZE to improve readability etc.

OK I've added the following:

/*
 * If the block is at least CONT_PTE_SIZE in size, and is naturally
 * aligned in both virtual and physical space, then we can pte-map the
 * block using the PTE_CONT bit for more efficient use of the TLB.
 */

Thanks,
Ryan