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

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Date:	Wed, 19 Nov 2014 08:52:01 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.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 Tue, Nov 18, 2014 at 03:23:36PM -0800, Andrew Morton wrote:
> 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.

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;


> 
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-- 
Kind regards,
Minchan Kim
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