linux-kernel - Re: [RFC PATCH 3/3] fb_defio: do not use deprecated page->mapping, index fields

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Message-ID: <Z37mE7NdQu-ZgBES@casper.infradead.org>
Date: Wed, 8 Jan 2025 20:54:43 +0000
From: Matthew Wilcox <willy@...radead.org>
To: David Hildenbrand <david@...hat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jaya Kumar <jayalk@...works.biz>, Simona Vetter <simona@...ll.ch>,
	Helge Deller <deller@....de>, linux-fbdev@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [RFC PATCH 3/3] fb_defio: do not use deprecated page->mapping,
 index fields

On Wed, Jan 08, 2025 at 09:14:53PM +0100, David Hildenbrand wrote:
> On 08.01.25 18:32, Matthew Wilcox wrote:
> > On Wed, Jan 08, 2025 at 04:18:42PM +0000, Lorenzo Stoakes wrote:
> > > @@ -280,7 +269,10 @@ static void fb_deferred_io_work(struct work_struct *work)
> > >   		struct folio *folio = page_folio(pageref->page);
> > >   		folio_lock(folio);
> > > -		folio_mkclean(folio);
> > > +		rmap_wrprotect_file_page(fbdefio->mapping,
> > > +					 pageref->offset >> PAGE_SHIFT,
> > > +					 compound_nr(pageref->page),
> > > +					 page_to_pfn(pageref->page));
> > >   		folio_unlock(folio);
> > 
> > Why do we need to lock the folio?  (since this isn't necessarily a
> > folio)
> 
> Can you clarify the "since this isn't necessarily a folio" part ? Do you
> mean in the future, when we split "struct page" and "struct folio"?

Right.  I need to finish the email that explains where I think we're
going in 2025 ...

> Doing an rmap walk on something that won't be a folio is ... sounds odd
> (->wrong :) )

Not necessarily!  We already do that (since 2022) for DAX (see
6a8e0596f004).  rmap lets you find every place that a given range
of a file is mapped into user address spaces; but that file might be a
device file, and so it's not just pagecache but also (in this case)
fb memory, and whatever else device drivers decide to mmap.