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

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Date:	Tue, 18 Nov 2014 15:23:36 -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 Fri, 14 Nov 2014 09:49:07 +0900 Minchan Kim <minchan@...nel.org> wrote:

> When I tested zram, I found processes got segfaulted.
> The reason was zram_rw_page doesn't make the page dirty
> again when swap write failed, and even it doesn't return
> error by [1].
> 
> If error by zram internal happens, zram_rw_page should return
> non-zero without calling page_endio.
> It causes resubmit the IO with bio so that it ends up calling
> bio->bi_end_io.
> 
> The reason is zram could be used for a block device for FS and
> swap, which they uses different bio complete callback, which
> works differently. So, we should rely on the bio I/O complete
> handler rather than zram_bvec_rw itself in case of I/O fail.
> 
> This patch fixes the segfault issue as well one [1]'s
> mentioned
> 
> ...
>
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -978,12 +978,10 @@ static int zram_rw_page(struct block_device *bdev, sector_t sector,
>  out_unlock:
>  	up_read(&zram->init_lock);
>  out:
> -	page_endio(page, rw, err);
> +	if (unlikely(err))
> +		return err;
>  
> -	/*
> -	 * 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.

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