Re: [PATCH 60/78] zram: remove the claim mechanism

Minchan Kim
2020-11-26T01:11:23Z

On Mon, Nov 16, 2020 at 03:57:51PM +0100, Christoph Hellwig wrote:
> The zram claim mechanism was added to ensure no new opens come in
> during teardown. But the proper way to archive that is to call
> del_gendisk first, which takes care of all that. Once del_gendisk
> is called in the right place, the reset side can also be simplified
> as no I/O can be outstanding on a block device that is not open.

It would be great if it makes the mess simple. Let me have a question
Please see below.

>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> drivers/block/zram/zram_drv.c | 76 ++++++++++-------------------------
> 1 file changed, 21 insertions(+), 55 deletions(-)
>
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 6d15d51cee2b7e..3641434a9b154d 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -1756,64 +1756,33 @@ static ssize_t disksize_store(struct device *dev,
> static ssize_t reset_store(struct device *dev,
> struct device_attribute *attr, const char *buf, size_t len)
> {
> - int ret;
> - unsigned short do_reset;
> - struct zram *zram;
> + struct zram *zram = dev_to_zram(dev);
> struct block_device *bdev;
> + unsigned short do_reset;
> + int ret = 0;
>
> ret = kstrtou16(buf, 10, &do_reset);
> if (ret)
> return ret;
> -
> if (!do_reset)
> return -EINVAL;
>
> - zram = dev_to_zram(dev);
> bdev = bdget_disk(zram->disk, 0);
> if (!bdev)
> return -ENOMEM;
>
> mutex_lock(&bdev->bd_mutex);
> - /* Do not reset an active device or claimed device */
> - if (bdev->bd_openers || zram->claim) {
> - mutex_unlock(&bdev->bd_mutex);
> - bdput(bdev);
> - return -EBUSY;
> - }
> -
> - /* From now on, anyone can't open /dev/zram[0-9] */
> - zram->claim = true;
> + if (bdev->bd_openers)
> + ret = -EBUSY;
> + else
> + zram_reset_device(zram);
> mutex_unlock(&bdev->bd_mutex);
> -
> - /* Make sure all the pending I/O are finished */
> - fsync_bdev(bdev);
> - zram_reset_device(zram);
> bdput(bdev);
>
> - mutex_lock(&bdev->bd_mutex);
> - zram->claim = false;
> - mutex_unlock(&bdev->bd_mutex);
> -
> - return len;
> -}
> -
> -static int zram_open(struct block_device *bdev, fmode_t mode)
> -{
> - int ret = 0;
> - struct zram *zram;
> -
> - WARN_ON(!mutex_is_locked(&bdev->bd_mutex));
> -
> - zram = bdev->bd_disk->private_data;
> - /* zram was claimed to reset so open request fails */
> - if (zram->claim)
> - ret = -EBUSY;
> -
> - return ret;
> + return ret ? ret : len;
> }
>
> static const struct block_device_operations zram_devops = {
> - .open = zram_open,
> .submit_bio = zram_submit_bio,
> .swap_slot_free_notify = zram_slot_free_notify,
> .rw_page = zram_rw_page,
> @@ -1821,7 +1790,6 @@ static const struct block_device_operations zram_devops = {
> };
>
> static const struct block_device_operations zram_wb_devops = {
> - .open = zram_open,
> .submit_bio = zram_submit_bio,
> .swap_slot_free_notify = zram_slot_free_notify,
> .owner = THIS_MODULE
> @@ -1972,34 +1940,32 @@ static int zram_add(void)
> return ret;
> }
>
> -static int zram_remove(struct zram *zram)
> +static bool zram_busy(struct zram *zram)
> {
> struct block_device *bdev;
> + bool busy = false;
>
> bdev = bdget_disk(zram->disk, 0);
> - if (!bdev)
> - return -ENOMEM;
> -
> - mutex_lock(&bdev->bd_mutex);
> - if (bdev->bd_openers || zram->claim) {
> - mutex_unlock(&bdev->bd_mutex);
> + if (bdev) {
> + if (bdev->bd_openers)
> + busy = true;
> bdput(bdev);
> - return -EBUSY;
> }
>
> - zram->claim = true;
> - mutex_unlock(&bdev->bd_mutex);
> + return busy;
> +}
>
> - zram_debugfs_unregister(zram);
> +static int zram_remove(struct zram *zram)
> +{
> + if (zram_busy(zram))
> + return -EBUSY;
>
> - /* Make sure all the pending I/O are finished */
> - fsync_bdev(bdev);
> + del_gendisk(zram->disk);
> + zram_debugfs_unregister(zram);
> zram_reset_device(zram);
> - bdput(bdev);
>
> pr_info("Removed device: %s\n", zram->disk->disk_name);
>
> - del_gendisk(zram->disk);
> blk_cleanup_queue(zram->disk->queue);
> put_disk(zram->disk);
> kfree(zram);
> --
> 2.29.2
>

With this patch, how deal with the race?

CPU 1 CPU 2

hot_remove_store
zram_remove
zram_busy
return -EBUSY
open /dev/zram0
del_gendisk
zram_reset and destroy