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

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Date: Fri, 12 Apr 2024 13:17:52 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: "Christoph Lameter (Ampere)" <cl@...ux.com>,
	Haifeng Xu <haifeng.xu@...pee.com>, 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 Fri, Apr 12, 2024 at 10:01:29AM +0200, Vlastimil Babka wrote:
> 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.

The patch seems pointless to me.  I wouldn't clear the flag.  If
somebody finds it confusing, that's really just their expectations being
wrong.  folio_alloc() sets __GFP_COMP on all allocations, whether or not
they're order 0.