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Date:   Tue, 19 Oct 2021 12:36:42 -0700
From:   Luis Chamberlain <mcgrof@kernel.org>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>, tj@kernel.org,
        gregkh@linuxfoundation.org, akpm@linux-foundation.org,
        minchan@kernel.org, jeyu@kernel.org, shuah@kernel.org,
        bvanassche@acm.org, dan.j.williams@intel.com, joe@perches.com,
        tglx@linutronix.de, keescook@chromium.org, rostedt@goodmis.org,
        linux-spdx@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v8 11/12] zram: fix crashes with cpu hotplug multistate
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Sender: Luis Chamberlain <mcgrof@infradead.org>
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On Wed, Oct 20, 2021 at 12:29:53AM +0800, Ming Lei wrote:
> On Tue, Oct 19, 2021 at 08:28:21AM -0700, Luis Chamberlain wrote:
> > On Tue, Oct 19, 2021 at 10:34:41AM +0800, Ming Lei wrote:
> > > Please try the following patch against upstream(linus or next) tree(basically
> > > fold revised 2 and 3 of V1, and cover two issues: not fail zram_remove in
> > > module_exit(), race between zram_remove() and disksize_store()), and see if
> > > everything is fine for you:
> > 
> > Page fault ...
> > 
> > [   18.284256] zram: Removed device: zram0
> > [   18.312974] BUG: unable to handle page fault for address:
> > ffffad86de903008
> > [   18.313707] #PF: supervisor read access in kernel mode
> > [   18.314248] #PF: error_code(0x0000) - not-present page
> > [   18.314797] PGD 100000067 P4D 100000067 PUD 10031e067 PMD 136a28067
> 
> That is another race between zram_reset_device() and disksize_store(),
> which is supposed to be covered by ->init_lock, and follows the delta fix
> against the last patch I posted, and the whole patch can be found in the
> github link:
> 
> https://github.com/ming1/linux/commit/fa6045b1371eb301f392ac84adaf3ad53bb16894
> 
> 
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index d0cae7a42f4d..a14ba3d350ea 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -1704,12 +1704,12 @@ static void zram_reset_device(struct zram *zram)
>  	set_capacity_and_notify(zram->disk, 0);
>  	part_stat_set_all(zram->disk->part0, 0);
>  
> -	up_write(&zram->init_lock);
>  	/* I/O operation under all of CPU are done so let's free */
>  	zram_meta_free(zram, disksize);
>  	memset(&zram->stats, 0, sizeof(zram->stats));
>  	zcomp_destroy(comp);
>  	reset_bdev(zram);
> +	up_write(&zram->init_lock);
>  }
>  
>  static ssize_t disksize_store(struct device *dev,

With this, it still ends up in a state where we loop and can't get out of:

zram: Can't change algorithm for initialized device

  Luis