linux-kernel - Re: [PATCH mm-unstable v2 1/3] mm/contig_alloc: support __GFP

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Message-ID: <ZtXvtnc2Cf8xWU7d@google.com>
Date: Mon, 2 Sep 2024 11:02:46 -0600
From: Yu Zhao <yuzhao@...gle.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Muchun Song <muchun.song@...ux.dev>, Zi Yan <ziy@...dia.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH mm-unstable v2 1/3] mm/contig_alloc: support __GFP_COMP

On Thu, Aug 22, 2024 at 07:42:20PM +0100, Matthew Wilcox wrote:
> On Thu, Aug 22, 2024 at 11:23:04AM -0600, Yu Zhao wrote:
> > Andrew, could you patch up the line above? This is what it's supposed
> > to check:
> > 
> > diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> > index 59266df56aeb..03ba9563c6db 100644
> > --- a/include/linux/gfp.h
> > +++ b/include/linux/gfp.h
> > @@ -452,7 +452,7 @@ static inline struct folio *folio_alloc_gigantic_noprof(int order, gfp_t gfp,
> >  {
> >  	struct page *page;
> >  
> > -	if (WARN_ON(!order || !(gfp | __GFP_COMP)))
> > +	if (WARN_ON(!order || !(gfp & __GFP_COMP)))
> >  		return NULL;
> 
> I don't think we should do this at all.  Just this should be enough:
> 
> 	gfp |= __GFP_COMP;
> 
> same as folio_alloc() (or now folio_alloc_noprof()).
> Do we really caree if somebody tries to allocate a gigantic page with an
> order of 0?

If this ever happens, I'd bet it's a bug.

> It's weird, but would work, so I don't see the need for the
> warning.

So the warning could catch that, but if we think it's verbose, then
please fold the following in:

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index a951de920e20..b43934d79dd9 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -452,10 +452,7 @@ static inline struct folio *folio_alloc_gigantic_noprof(int order, gfp_t gfp,
 {
 	struct page *page;
 
-	if (WARN_ON(!order || !(gfp & __GFP_COMP)))
-		return NULL;
-
-	page = alloc_contig_pages_noprof(1 << order, gfp, nid, node);
+	page = alloc_contig_pages_noprof(1 << order, gfp | __GFP_COMP, nid, node);
 
 	return page ? page_folio(page) : NULL;
 }