Re: [PATCH v3 1/6] block: disable block device DAX by default

Jan Kara
2016-02-17T21:55:15Z

On Tue 16-02-16 20:34:14, Ross Zwisler wrote:
> From: Dan Williams <dan.j.williams@intel.com>
>
> The recent *sync enabling discovered that we are inserting into the
> block_device pagecache counter to the expectations of the dirty data
> tracking for dax mappings. This can lead to data corruption.
>
> We want to support DAX for block devices eventually, but it requires
> wider changes to properly manage the pagecache.
>
> [<ffffffff81576d93>] dump_stack+0x85/0xc2
> [<ffffffff812b9ee0>] dax_writeback_mapping_range+0x60/0xe0
> [<ffffffff812a1d4f>] blkdev_writepages+0x3f/0x50
> [<ffffffff811db011>] do_writepages+0x21/0x30
> [<ffffffff811cb6a6>] __filemap_fdatawrite_range+0xc6/0x100
> [<ffffffff811cb75a>] filemap_write_and_wait+0x4a/0xa0
> [<ffffffff812a15e0>] set_blocksize+0x70/0xd0
> [<ffffffff812a273d>] sb_set_blocksize+0x1d/0x50
> [<ffffffff8132ac9b>] ext4_fill_super+0x75b/0x3360
> [<ffffffff81583381>] ? vsnprintf+0x201/0x4c0
> [<ffffffff815836d9>] ? snprintf+0x49/0x60
> [<ffffffff81263010>] mount_bdev+0x180/0x1b0
> [<ffffffff8132a540>] ? ext4_calculate_overhead+0x370/0x370
> [<ffffffff8131ad95>] ext4_mount+0x15/0x20
> [<ffffffff81263908>] mount_fs+0x38/0x170
>
> Mark the support broken so its disabled by default, but otherwise still
> available for testing.
>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Jens Axboe <axboe@fb.com>
> Cc: Matthew Wilcox <matthew.r.wilcox@intel.com>
> Cc: Al Viro <viro@ftp.linux.org.uk>
> Reported-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> Suggested-by: Dave Chinner <david@fromorbit.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>

Makes sense. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

> ---
> block/Kconfig | 13 +++++++++++++
> fs/block_dev.c | 6 +++++-
> 2 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/block/Kconfig b/block/Kconfig
> index 161491d..0363cd7 100644
> --- a/block/Kconfig
> +++ b/block/Kconfig
> @@ -88,6 +88,19 @@ config BLK_DEV_INTEGRITY
> T10/SCSI Data Integrity Field or the T13/ATA External Path
> Protection. If in doubt, say N.
>
> +config BLK_DEV_DAX
> + bool "Block device DAX support"
> + depends on FS_DAX
> + depends on BROKEN
> + help
> + When DAX support is available (CONFIG_FS_DAX) raw block
> + devices can also support direct userspace access to the
> + storage capacity via MMAP(2) similar to a file on a
> + DAX-enabled filesystem. However, the DAX I/O-path disables
> + some standard I/O-statistics, and the MMAP(2) path has some
> + operational differences due to bypassing the page
> + cache. If in doubt, say N.
> +
> config BLK_DEV_THROTTLING
> bool "Block layer bio throttling support"
> depends on BLK_CGROUP=y
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 39b3a17..31c6d10 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1201,7 +1201,11 @@ static int __blkdev_get(struct block_device *bdev, fmode_t mode, int for_part)
> bdev->bd_disk = disk;
> bdev->bd_queue = disk->queue;
> bdev->bd_contains = bdev;
> - bdev->bd_inode->i_flags = disk->fops->direct_access ? S_DAX : 0;
> + if (IS_ENABLED(CONFIG_BLK_DEV_DAX) && disk->fops->direct_access)
> + bdev->bd_inode->i_flags = S_DAX;
> + else
> + bdev->bd_inode->i_flags = 0;
> +
> if (!partno) {
> ret = -ENXIO;
> bdev->bd_part = disk_get_part(disk, partno);
> --
> 2.5.0
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR