linux-kernel - Re: [PATCH 04/12] mm: Allow compound zone device pages

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Message-ID: <ZuBMlUK-v24-9m3G@casper.infradead.org>
Date: Tue, 10 Sep 2024 14:41:41 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Alistair Popple <apopple@...dia.com>
Cc: dan.j.williams@...el.com, linux-mm@...ck.org, vishal.l.verma@...el.com,
	dave.jiang@...el.com, logang@...tatee.com, bhelgaas@...gle.com,
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	peterx@...hat.com, linux-doc@...r.kernel.org,
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	jhubbard@...dia.com, hch@....de, david@...morbit.com,
	Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH 04/12] mm: Allow compound zone device pages

On Tue, Sep 10, 2024 at 04:57:41PM +1000, Alistair Popple wrote:
> 
> Matthew Wilcox <willy@...radead.org> writes:
> 
> > On Tue, Sep 10, 2024 at 02:14:29PM +1000, Alistair Popple wrote:
> >> @@ -337,6 +341,7 @@ struct folio {
> >>  	/* private: */
> >>  				};
> >>  	/* public: */
> >> +			struct dev_pagemap *pgmap;
> >
> > Shouldn't that be indented by one more tab stop?
> >
> > And for ease of reading, perhaps it should be placed either immediately
> > before or after 'struct list_head lru;'?
> >
> >> +++ b/include/linux/mmzone.h
> >> @@ -1134,6 +1134,12 @@ static inline bool is_zone_device_page(const struct page *page)
> >>  	return page_zonenum(page) == ZONE_DEVICE;
> >>  }
> >>  
> >> +static inline struct dev_pagemap *page_dev_pagemap(const struct page *page)
> >> +{
> >> +	WARN_ON(!is_zone_device_page(page));
> >> +	return page_folio(page)->pgmap;
> >> +}
> >
> > I haven't read to the end yet, but presumably we'll eventually want:
> >
> > static inline struct dev_pagemap *folio_dev_pagemap(const struct folio *folio)
> > {
> > 	WARN_ON(!folio_is_zone_device(folio))
> > 	return folio->pgmap;
> > }
> >
> > and since we'll want it eventually, maybe now is the time to add it,
> > and make page_dev_pagemap() simply call it?
> 
> Sounds reasonable. I had open-coded folio->pgmap where it's needed
> because at those points it's "obviously" a ZONE_DEVICE folio. Will add
> it.

Oh, if it's obvious then just do the dereference.