linux-kernel - Re: [PATCH v1 01/10] zram: avoid calling of zram_meta_free under init

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Date:	Fri, 23 Jan 2015 10:15:55 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Dan Streetman <ddstreet@...e.org>,
	Seth Jennings <sjennings@...iantweb.net>,
	Nitin Gupta <ngupta@...are.org>,
	Juneho Choi <juno.choi@....com>, Gunho Lee <gunho.lee@....com>,
	Luigi Semenzato <semenzato@...gle.com>,
	Jerome Marchand <jmarchan@...hat.com>
Subject: Re: [PATCH v1 01/10] zram: avoid calling of zram_meta_free under
 init_lock

On Fri, Jan 23, 2015 at 10:03:36AM +0900, Minchan Kim wrote:
> Hello,
> 
> On Wed, Jan 21, 2015 at 11:21:53PM +0900, Sergey Senozhatsky wrote:
> > On (01/21/15 15:14), Minchan Kim wrote:
> > > We don't need to call zram_meta_free under init_lock.
> > > What we need to prevent race is setting NULL into zram->meta
> > > (ie, init_done). This patch does it.
> > > 
> > > Signed-off-by: Minchan Kim <minchan@...nel.org>
> > > ---
> > >  drivers/block/zram/zram_drv.c | 5 +++--
> > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > > index 9250b3f..7e03d86 100644
> > > --- a/drivers/block/zram/zram_drv.c
> > > +++ b/drivers/block/zram/zram_drv.c
> > > @@ -719,6 +719,8 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
> > >  	}
> > >  
> > >  	meta = zram->meta;
> > > +	zram->meta = NULL;
> > > +
> > >  	/* Free all pages that are still in this zram device */
> > >  	for (index = 0; index < zram->disksize >> PAGE_SHIFT; index++) {
> > >  		unsigned long handle = meta->table[index].handle;
> > > @@ -731,8 +733,6 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
> > >  	zcomp_destroy(zram->comp);
> > >  	zram->max_comp_streams = 1;
> > >  
> > > -	zram_meta_free(zram->meta);
> > > -	zram->meta = NULL;
> > >  	/* Reset stats */
> > >  	memset(&zram->stats, 0, sizeof(zram->stats));
> > >  
> > > @@ -741,6 +741,7 @@ static void zram_reset_device(struct zram *zram, bool reset_capacity)
> > >  		set_capacity(zram->disk, 0);
> > >  
> > >  	up_write(&zram->init_lock);
> > > +	zram_meta_free(meta);
> > 
> > Hello,
> > 
> > since we detached ->meta from zram, this one doesn't really need
> > ->init_lock protection:
> > 
> > 	/* Free all pages that are still in this zram device */
> > 	for (index = 0; index < zram->disksize >> PAGE_SHIFT; index++) {
> > 		unsigned long handle = meta->table[index].handle;
> > 		if (!handle)
> > 			continue;
> > 
> > 		zs_free(meta->mem_pool, handle);
> > 	}
> > 
> > 
> > 	-ss
> 
> Good catch.
> 
> As well, we could move zcomp_destroy and memset(&zram->stats)
> out of the lock but zram_rw_page, ZRAM_ATTR_RO, disksize_show
> and orig_data_size_show have a race bug which access stats
> out of the lock so that it could show the stale vaule.
> Although it's not a significant, there is no reason to hesitate the fix. :)

Argh, sent wrong version.
zram->stats should be protected by the lock but other stat show
functions don't hold a lock so it's racy.

> 
> I will fix it. Thanks!
> 
> 
> > 
> > >  	/*
> > >  	 * Revalidate disk out of the init_lock to avoid lockdep splat.
> > > -- 
> > > 1.9.3
> > > 
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