linux-kernel - Re: [PATCH mm-unstable] ext4: Convert mext_page_double_lock() to mext_folio_double

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Date:   Tue, 6 Dec 2022 20:53:45 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Cc:     linux-mm@...ck.org, tytso@....edu, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH mm-unstable] ext4: Convert mext_page_double_lock() to
 mext_folio_double_lock()

On Tue, Dec 06, 2022 at 08:50:02PM +0000, Matthew Wilcox wrote:
> >  	flags = memalloc_nofs_save();
> > -	page[0] = grab_cache_page_write_begin(mapping[0], index1);
> > -	if (!page[0]) {
> > +	folio[0] = __filemap_get_folio(mapping[0], index1, fgp_flags,
> > +			mapping_gfp_mask(mapping[0]));
> 
> one
> 
> > +	if (!folio[0]) {
> >  		memalloc_nofs_restore(flags);
> >  		return -ENOMEM;
> >  	}
> >  
> > -	page[1] = grab_cache_page_write_begin(mapping[1], index2);
> > +	folio[1] = __filemap_get_folio(mapping[1], index2, fgp_flags,
> > +			mapping_gfp_mask(mapping[1]));
> 
> two

*facepalm*.  Those don't contain calls to compound_head(), they contain
calls to folio_file_page(), which is still a moderately-expensive
unnecessary conversion, but a conversion in the other direction.