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

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Message-ID: <f0b3bdec-ee11-4648-8a33-d04317f36b93@lucifer.local>
Date: Mon, 13 Jan 2025 17:48:57 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: Matthew Wilcox <willy@...radead.org>,
        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 10:12:36PM +0100, David Hildenbrand wrote:
> On 08.01.25 21:54, Matthew Wilcox wrote:
> > 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.
>
> Yes, that part I remember.
>
> I thought we would be passing in a page into rmap_wrprotect_file_page(), and
> was wondering what we would do to "struct page" that won't be a folio in
> there.

The reason I provide a PFN is that we internally use a PFN for the walk, and
everything else is folio-fied for stuff that isn't necessarily a folio.

However it does seem silly to have to page_to_pfn() a page that we pass in, so I
will update to accept a page and do this bit in the function itself.

>
> Probably, because the "_page" in rmap_wrprotect_file_page() is misleading :)
>
> ... should it be "file_range" ? (but we also pass the pfn ... )
>
> --
> Cheers,
>
> David / dhildenb
>