RE: [PATCH] exfat: check disk status during buffer write

["Sungjong Seo" <sj1557.seo@samsung.com>]

> We found that when writing a large file through buffer write,
> if the disk is inaccessible, exFAT does not return an error
> normally, which leads to the writing process not stopping properly.
> 
> To easily reproduce this issue, you can follow the steps below:
> 
> 1. format a device to exFAT and then mount (with a full disk erase)
> 2. dd if=/dev/zero of=/exfat_mount/test.img bs=1M count=8192
> 3. eject the device
> 
> You may find that the dd process does not stop immediately and may
> continue for a long time.
> 
> We compared it with the FAT, where FAT would prompt an EIO error and
> immediately stop the dd operation.
> 
> The root cause of this issue is that when the exfat_inode contains the
> ALLOC_NO_FAT_CHAIN flag, exFAT does not need to access the disk to
> look up directory entries or the FAT table (whereas FAT would do)
> every time data is written. Instead, exFAT simply marks the buffer as
> dirty and returns, delegating the writeback operation to the writeback
> process.
> 
> If the disk cannot be accessed at this time, the error will only be
> returned to the writeback process, and the original process will not
> receive the error, so it cannot be returned to the user side.
> 
> Therefore, we think that when writing files with ALLOC_NO_FAT_CHAIN,
> it is necessary to continuously check the status of the disk.
> 
> When the disk cannot be accessed normally, an error should be returned
> to stop the writing process.
> 
> Signed-off-by: Dongliang Cui <dongliang.cui@unisoc.com>
> Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
> ---
>  fs/exfat/exfat_fs.h | 5 +++++
>  fs/exfat/inode.c    | 5 +++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/fs/exfat/exfat_fs.h b/fs/exfat/exfat_fs.h
> index ecc5db952deb..c5f5a7a8b672 100644
> --- a/fs/exfat/exfat_fs.h
> +++ b/fs/exfat/exfat_fs.h
> @@ -411,6 +411,11 @@ static inline unsigned int
> exfat_sector_to_cluster(struct exfat_sb_info *sbi,
>  		EXFAT_RESERVED_CLUSTERS;
>  }
> 
> +static inline bool exfat_check_disk_error(struct block_device *bdev)
> +{
> +	return blk_queue_dying(bdev_get_queue(bdev));
Why don't you check it like ext4?

static int block_device_ejected(struct super_block *sb)
{
       struct inode *bd_inode = sb->s_bdev->bd_inode;
       struct backing_dev_info *bdi = inode_to_bdi(bd_inode);

       return bdi->dev == NULL;
}
> +}
> +
>  static inline bool is_valid_cluster(struct exfat_sb_info *sbi,
>  		unsigned int clus)
>  {
> diff --git a/fs/exfat/inode.c b/fs/exfat/inode.c
> index dd894e558c91..efd02c1c83a6 100644
> --- a/fs/exfat/inode.c
> +++ b/fs/exfat/inode.c
> @@ -147,6 +147,11 @@ static int exfat_map_cluster(struct inode *inode,
> unsigned int clu_offset,
>  	*clu = last_clu = ei->start_clu;
> 
>  	if (ei->flags == ALLOC_NO_FAT_CHAIN) {
> +		if (exfat_check_disk_error(sb->s_bdev)) {
> +			exfat_fs_error(sb, "device inaccessiable!\n");
> +			return -EIO;
This patch looks useful when using removable storage devices.
BTW, in case of "ei->flags != ALLOC_NO_FAT_CHAIN", There could be
the same problem if it can be found from lru_cache. So, it would be nice
to check disk_error regardless ei->flags. Also, Calling exfat_fs_error()
seems unnecessary. Instead, let's return -ENODEV instead of -EIO.
I believe that these errors will be handled on exfat_get_block()

Thanks.
> +		}
> +
>  		if (clu_offset > 0 && *clu != EXFAT_EOF_CLUSTER) {
>  			last_clu += clu_offset - 1;
> 
> --
> 2.25.1