linux-kernel - Re: [RFC PATCH v1 04/57] mm/page_alloc: Make page_frag

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Message-ID: <1db4bb59-bd68-4aff-883b-1af40ca123b2@suse.cz>
Date: Thu, 14 Nov 2024 10:43:25 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Ryan Roberts <ryan.roberts@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Ard Biesheuvel <ardb@...nel.org>, Catalin Marinas <catalin.marinas@....com>,
 David Hildenbrand <david@...hat.com>, Greg Marsden
 <greg.marsden@...cle.com>, Ivan Ivanov <ivan.ivanov@...e.com>,
 Kalesh Singh <kaleshsingh@...gle.com>, Marc Zyngier <maz@...nel.org>,
 Mark Rutland <mark.rutland@....com>, Matthias Brugger <mbrugger@...e.com>,
 Miroslav Benes <mbenes@...e.cz>, Will Deacon <will@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [RFC PATCH v1 04/57] mm/page_alloc: Make page_frag_cache
 boot-time page size compatible

On 11/14/24 10:36, Ryan Roberts wrote:
> On 14/11/2024 08:23, Vlastimil Babka wrote:
>> On 10/14/24 12:58, Ryan Roberts wrote:
>>> "struct page_frag_cache" has some optimizations that depend on page
>>> size. Let's refactor it a bit so that those optimizations can be
>>> determined at run-time for the case where page size is a boot-time
>>> parameter. For compile-time page size, the compiler should dead code
>>> strip and the result is very similar to before.
>>>
>>> One wrinkle is that we don't know if we need the size member until
>>> runtime. So remove the ifdeffery and always define offset as u32 (needed
>>> if PAGE_SIZE is >= 64K) and size as u16 (only used when PAGE_SIZE <=
>>> 32K). We move the members around a bit so that the overall size of the
>>> struct remains the same; 24 bytes for 64-bit and 16 bytes on 32 bit.
>>>
>>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>> 
>> Looks ok, but ideally the PAGE_FRAG_CACHE_MAX_ORDER #define should also be
>> replaced by some variable that's populated just once. It can be static local
>> to page_alloc.c as nothing else seems to use it.
> 
> I can certainly do that, but wouldn't that be penalizing a compile-time page
> size configuration? My current change means that PAGE_FRAG_CACHE_MAX_ORDER still
> resolves to a compile-time constant in that situation and the compiler can
> eliminate conditional branches it knows will never be taken. Or perhaps you're

Ah, I see.

> suggesting I conditionally make it a variable if PAGE_SIZE_MIN != PAGE_SIZE_MAX?

Given the only place it's being used, it shouldn't be worth it after all.

You can add for this patch:

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> Thanks,
> Ryan