linux-kernel - Re: [PATCH v11 16/33] mm/util: Add folio_mapping() and folio_file

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Date:   Wed, 16 Jun 2021 11:10:09 +0100
From:   David Howells <dhowells@...hat.com>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     dhowells@...hat.com, akpm@...ux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        Jeff Layton <jlayton@...nel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        William Kucharski <william.kucharski@...cle.com>
Subject: Re: [PATCH v11 16/33] mm/util: Add folio_mapping() and folio_file_mapping()

Matthew Wilcox (Oracle) <willy@...radead.org> wrote:

> These are the folio equivalent of page_mapping() and page_file_mapping().
> Add an out-of-line page_mapping() wrapper around folio_mapping()
> in order to prevent the page_folio() call from bloating every caller
> of page_mapping().  Adjust page_file_mapping() and page_mapping_file()

Confusingly named, but not the fault of this patch.

> to use folios internally.  Rename __page_file_mapping() to
> swapcache_mapping() and change it to take a folio.
> 
> This ends up saving 122 bytes of text overall.  folio_mapping() is
> 45 bytes shorter than page_mapping() was, but the new page_mapping()
> wrapper is 30 bytes.  The major reduction is a few bytes less in dozens
> of nfs functions (which call page_file_mapping()).  Most of these appear
> to be a slight change in gcc's register allocation decisions, which allow:
> 
>    48 8b 56 08         mov    0x8(%rsi),%rdx
>    48 8d 42 ff         lea    -0x1(%rdx),%rax
>    83 e2 01            and    $0x1,%edx
>    48 0f 44 c6         cmove  %rsi,%rax
> 
> to become:
> 
>    48 8b 46 08         mov    0x8(%rsi),%rax
>    48 8d 78 ff         lea    -0x1(%rax),%rdi
>    a8 01               test   $0x1,%al
>    48 0f 44 fe         cmove  %rsi,%rdi
> 
> for a reduction of a single byte.  Once the NFS client is converted to
> use folios, this entire sequence will disappear.
> 
> Also add folio_mapping() documentation.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> Reviewed-by: Christoph Hellwig <hch@....de>
> Acked-by: Jeff Layton <jlayton@...nel.org>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> Reviewed-by: William Kucharski <william.kucharski@...cle.com>

Reviewed-by: David Howells <dhowells@...hat.com>