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

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Message-ID: <34ad8349-c4c3-4370-a662-324fa91b32b1@redhat.com>
Date: Wed, 8 Jan 2025 22:12:36 +0100
From: David Hildenbrand <david@...hat.com>
To: Matthew Wilcox <willy@...radead.org>
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 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.

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