linux-kernel - Re: [PATCH] slub: Clear __GFP_COMP flag when allocating 0 order page

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Date: Fri, 12 Apr 2024 10:01:29 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: "Christoph Lameter (Ampere)" <cl@...ux.com>,
 Haifeng Xu <haifeng.xu@...pee.com>, Matthew Wilcox <willy@...radead.org>
Cc: penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
 akpm@...ux-foundation.org, roman.gushchin@...ux.dev, 42.hyeyoo@...il.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] slub: Clear __GFP_COMP flag when allocating 0 order page

On 4/11/24 6:51 PM, Christoph Lameter (Ampere) wrote:
> On Thu, 11 Apr 2024, Haifeng Xu wrote:
> 
>> @@ -1875,6 +1875,13 @@ static inline struct slab *alloc_slab_page(gfp_t flags, int node,
>> 	struct slab *slab;
>> 	unsigned int order = oo_order(oo);
>>
>> +	/*
>> +	 * If fallback to the minimum order allocation and the order is 0,
>> +	 * clear the __GFP_COMP flag.
>> +	 */
>> +	if (order == 0)
>> +		flags = flags & ~__GFP_COMP;
> 
> 
> This would be better placed in allocate_slab() when the need for a
> fallback to a lower order is detected after the first call to alloc_slab_page().

Yeah. Although I don't really see the harm of __GFP_COMP with order-0 in the
first place, if the only issue is that the error output might be confusing.
I'd also hope we should eventually get rid of those odd non-__GFP_COMP
high-order allocations and then can remove the flag.