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Date:   Thu, 5 Mar 2020 22:04:14 +0100
From:   Jan Kara <jack@suse.cz>
To:     Jaegeuk Kim <jaegeuk@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, stable@vger.kernel.org,
        Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        Bart Van Assche <bvanassche@acm.org>
Subject: Re: [PATCH] loop: avoid EAGAIN, if offset or block_size are changed
Message-ID: <20200305210414.GA1678@quack2.suse.cz>
References: <20190518004751.18962-1-jaegeuk@kernel.org>
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On Fri 17-05-19 17:47:51, Jaegeuk Kim wrote:
> This patch tries to avoid EAGAIN due to nrpages!=0 that was originally trying
> to drop stale pages resulting in wrong data access.
> 
> Report: https://bugs.chromium.org/p/chromium/issues/detail?id=938958#c38

...

> ---
>  drivers/block/loop.c | 44 +++++++++++++++++---------------------------
>  1 file changed, 17 insertions(+), 27 deletions(-)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 102d79575895..7c7d2d9c47d0 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -1212,6 +1212,7 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
>  	kuid_t uid = current_uid();
>  	struct block_device *bdev;
>  	bool partscan = false;
> +	bool drop_caches = false;
>  
>  	err = mutex_lock_killable(&loop_ctl_mutex);
>  	if (err)
> @@ -1232,10 +1233,8 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
>  	}
>  
>  	if (lo->lo_offset != info->lo_offset ||
> -	    lo->lo_sizelimit != info->lo_sizelimit) {
> -		sync_blockdev(lo->lo_device);
> -		kill_bdev(lo->lo_device);
> -	}
> +	    lo->lo_sizelimit != info->lo_sizelimit)
> +		drop_caches = true;

I don't think this solution of moving buffer cache invalidation after loop
device is updated is really correct.

If there's any dirty data in the buffer cache, god knows where it ends
up being written after the loop device is reconfigured. Think e.g. of a
file offset being changed. It may not be even possible to write it if say
block size increased and we have now improperly sized buffers in the buffer
cache...

Frankly, I have rather limited sympathy to someone trying to reconfigure a
loop device while it is in use. Is there any sane usecase? I'd be inclined
to just use a similar trick as we did with LOOP_SET_FD and allow these
changes only if the caller has the loop device open exclusively or we are
able to upgrade to exclusive open. As otherwise say mounted filesystem on
top of loop device being reconfigured is very likely to be in serious
trouble (e.g. it's impossible to fully invalidate buffer cache in that
case).

But that's probably somewhat tangential to the problem you have. For your
case I don't really see a race-free way to invalidate buffer cache and
update loop configuration - the best I can see is to flush & invalidate
the cache, freeze the bdev so that new data cannot be read into the
buffer cache, check the cache is still empty - if yes, go ahead. If not,
unfreeze and try again...

								Honza

>  	/* I/O need to be drained during transfer transition */
>  	blk_mq_freeze_queue(lo->lo_queue);
> @@ -1265,14 +1264,6 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
>  
>  	if (lo->lo_offset != info->lo_offset ||
>  	    lo->lo_sizelimit != info->lo_sizelimit) {
> -		/* kill_bdev should have truncated all the pages */
> -		if (lo->lo_device->bd_inode->i_mapping->nrpages) {
> -			err = -EAGAIN;
> -			pr_warn("%s: loop%d (%s) has still dirty pages (nrpages=%lu)\n",
> -				__func__, lo->lo_number, lo->lo_file_name,
> -				lo->lo_device->bd_inode->i_mapping->nrpages);
> -			goto out_unfreeze;
> -		}
>  		if (figure_loop_size(lo, info->lo_offset, info->lo_sizelimit)) {
>  			err = -EFBIG;
>  			goto out_unfreeze;
> @@ -1317,6 +1308,12 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
>  		bdev = lo->lo_device;
>  		partscan = true;
>  	}
> +
> +	/* truncate stale pages cached by previous operations */
> +	if (!err && drop_caches) {
> +		sync_blockdev(lo->lo_device);
> +		kill_bdev(lo->lo_device);
> +	}
>  out_unlock:
>  	mutex_unlock(&loop_ctl_mutex);
>  	if (partscan)
> @@ -1498,6 +1495,7 @@ static int loop_set_dio(struct loop_device *lo, unsigned long arg)
>  
>  static int loop_set_block_size(struct loop_device *lo, unsigned long arg)
>  {
> +	bool drop_caches = false;
>  	int err = 0;
>  
>  	if (lo->lo_state != Lo_bound)
> @@ -1506,23 +1504,10 @@ static int loop_set_block_size(struct loop_device *lo, unsigned long arg)
>  	if (arg < 512 || arg > PAGE_SIZE || !is_power_of_2(arg))
>  		return -EINVAL;
>  
> -	if (lo->lo_queue->limits.logical_block_size != arg) {
> -		sync_blockdev(lo->lo_device);
> -		kill_bdev(lo->lo_device);
> -	}
> +	if (lo->lo_queue->limits.logical_block_size != arg)
> +		drop_caches = true;
>  
>  	blk_mq_freeze_queue(lo->lo_queue);
> -
> -	/* kill_bdev should have truncated all the pages */
> -	if (lo->lo_queue->limits.logical_block_size != arg &&
> -			lo->lo_device->bd_inode->i_mapping->nrpages) {
> -		err = -EAGAIN;
> -		pr_warn("%s: loop%d (%s) has still dirty pages (nrpages=%lu)\n",
> -			__func__, lo->lo_number, lo->lo_file_name,
> -			lo->lo_device->bd_inode->i_mapping->nrpages);
> -		goto out_unfreeze;
> -	}
> -
>  	blk_queue_logical_block_size(lo->lo_queue, arg);
>  	blk_queue_physical_block_size(lo->lo_queue, arg);
>  	blk_queue_io_min(lo->lo_queue, arg);
> @@ -1530,6 +1515,11 @@ static int loop_set_block_size(struct loop_device *lo, unsigned long arg)
>  out_unfreeze:
>  	blk_mq_unfreeze_queue(lo->lo_queue);
>  
> +	/* truncate stale pages cached by previous operations */
> +	if (drop_caches) {
> +		sync_blockdev(lo->lo_device);
> +		kill_bdev(lo->lo_device);
> +	}
>  	return err;
>  }
>  
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR