linux-kernel - Re: [PATCH 6/6] fs: add CONFIG_BUFFER

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Message-ID: <ZLlId9+kXl5Tb7wj@casper.infradead.org>
Date:   Thu, 20 Jul 2023 15:45:11 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Jens Axboe <axboe@...nel.dk>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <christian@...uner.io>,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] fs: add CONFIG_BUFFER_HEAD

On Thu, Jul 20, 2023 at 04:04:52PM +0200, Christoph Hellwig wrote:
> @@ -400,7 +391,8 @@ static int blkdev_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
>  	iomap->type = IOMAP_MAPPED;
>  	iomap->addr = iomap->offset;
>  	iomap->length = isize - iomap->offset;
> -	iomap->flags |= IOMAP_F_BUFFER_HEAD;
> +	if (IS_ENABLED(CONFIG_BUFFER_HEAD))
> +		iomap->flags |= IOMAP_F_BUFFER_HEAD;

Wouldn't it be simpler to do ...

+#ifdef CONFIG_BUFFER_HEAD
 #define IOMAP_F_BUFFER_HEAD     (1U << 4)
+#else
+#define IOMAP_F_BUFFER_HEAD	0
+#endif

in include/linux/iomap.h ?

> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 0607790827b48a..6dc585c010c020 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -41,6 +41,12 @@ static inline struct iomap_page *to_iomap_page(struct folio *folio)
>  	return NULL;
>  }
>  
> +static inline bool iomap_use_buffer_heads(const struct iomap *iomap)
> +{
> +	return IS_ENABLED(CONFIG_BUFFER_HEAD) &&
> +		(iomap->flags & IOMAP_F_BUFFER_HEAD);
> +}

... because this function then goes away.

> @@ -675,7 +681,7 @@ static int iomap_write_begin(struct iomap_iter *iter, loff_t pos,
>  
>  	if (srcmap->type == IOMAP_INLINE)
>  		status = iomap_write_begin_inline(iter, folio);
> -	else if (srcmap->flags & IOMAP_F_BUFFER_HEAD)
> +	else if (iomap_use_buffer_heads(srcmap))

... as this will be optimised away.