linux-kernel - Re: [PATCH] zram: rely on the bi_end_io for zram_rw

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Date:	Wed, 19 Nov 2014 13:15:35 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Nitin Gupta <ngupta@...are.org>,
	Jerome Marchand <jmarchan@...hat.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Karam Lee <karam.lee@....com>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH] zram: rely on the bi_end_io for zram_rw_page fails

On Wed, 19 Nov 2014 08:52:01 +0900 Minchan Kim <minchan@...nel.org> wrote:

> > >  
> > > -	/*
> > > -	 * Return 0 prevents I/O fallback trial caused by rw_page fail
> > > -	 * and upper layer can handle this IO error via page error.
> > > -	 */
> > > +	page_endio(page, rw, 0);
> > >  	return 0;
> > 
> > Losing the comment makes me sad.  The code is somewhat odd-looking.  We
> > should add some words explaining why we're not reporting errors at this
> > point.
> 
> Okay. How about this?
> 
> 
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index decca6f161b8..1d7c90d5e0d0 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -975,6 +975,12 @@ static int zram_rw_page(struct block_device *bdev, sector_t sector,
>  	err = zram_bvec_rw(zram, &bv, index, offset, rw);
>  out_unlock:
>  	up_read(&zram->init_lock);
> +	/*
> +	 * If I/O fails, just return error without calling page_endio.
> +	 * It causes resubmit the I/O with bio request by rw_page fallback
> +	 * and bio I/O complete handler does things to handle the error
> +	 * (e.g., set_page_dirty of swap_writepage fail).
> +	 */
>  	if (err == 0)
>  		page_endio(page, rw, 0);
>  	return err;

I don't understand the comment :( bdev_read_page() doesn't resubmit the
IO if block_device_operations.rw_page() returns zero and it's unclear
how the bio I/O complete handler (which one?) gets involved.

It would help in the comment was more specific.  Instead of using vague
terms like "rw_page fallback" and "bio I/O complete handler", use
actual function names so the reader understand exactly what code we're
referring to.

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