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

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Date:	Thu, 20 Nov 2014 08:32:32 +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

Hello,

On Wed, Nov 19, 2014 at 01:15:35PM -0800, Andrew Morton wrote:
> 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

It's not bdev_read_page but upper functions.
(ie, do_mpage_readpage, swap_readpage, __mpage_writepage, __swap_writepage)

> how the bio I/O complete handler (which one?) gets involved.

bio->bi_end_io.

> 
> 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.

Indeed. I was terrible.

Hope this is better.

>From 635f22cac08f76b909ccc13b2de0b5d437eef5aa Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan@...nel.org>
Date: Wed, 19 Nov 2014 08:57:49 +0900
Subject: [PATCH] zram: add comment

Signed-off-by: Minchan Kim <minchan@...nel.org>
---
 drivers/block/zram/zram_drv.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index decca6f161b8..6f1ceb14e64b 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -975,6 +975,14 @@ 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(ie, non-zero) without
+	 * calling page_endio.
+	 * It causes resubmit the I/O with bio request by upper functions
+	 * of rw_page(e.g., swap_readpage, __swap_writepage) and
+	 * bio->bi_end_io does things to handle the error
+	 * (e.g., SetPageError, set_page_dirty and extra works).
+	 */
 	if (err == 0)
 		page_endio(page, rw, 0);
 	return err;
-- 
2.0.0

> 
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-- 
Kind regards,
Minchan Kim
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