On Thu, 11 Aug 2011, Lukas Czerner wrote:
> This commit adds discard support for loop devices. Discard is usually
> supported by SSD and thinly provisioned devices as a method for
> reclaiming unused space. This is no different than trying to reclaim
> back space which is not used by the file system on the image, but it
> still occupies space on the host file system.
>
> We can do the reclamation on file system which does support hole
> punching. So when discard request gets to the loop driver we can
> translate that to punch a hole to the underlying file, hence reclaim
> the free space.
>
> This is very useful for trimming down the size of the image to only what
> is really used by the file system on that image. Fstrim may be used for
> that purpose.
>
> It has been tested on ext4, xfs and btrfs with the image file systems
> ext4, ext3, xfs and btrfs. ext4, or ext6 image on ext4 file system has
> some problems but it seems that ext4 punch hole implementation is
> somewhat flawed and it is unrelated to this commit.
>
> Also this is a very good method of validating file systems punch hole
> implementation.
>
> Note that when encryption is used, discard support is disabled, because
> using it might leak some information useful for possible attacker.
Adding linux-fsdevel@xxxxxxxxxxxxxxx cc.
>
> Signed-off-by: Lukas Czerner <lczerner@xxxxxxxxxx>
> CC: Jens Axboe <jaxboe@xxxxxxxxxxxx>
> ---
> drivers/block/loop.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 files changed, 54 insertions(+), 0 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 76c8da7..a6d6873 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -75,6 +75,7 @@
> #include <linux/kthread.h>
> #include <linux/splice.h>
> #include <linux/sysfs.h>
> +#include <linux/falloc.h>
>
> #include <asm/uaccess.h>
>
> @@ -484,6 +485,29 @@ static int do_bio_filebacked(struct loop_device *lo, struct bio *bio)
> }
> }
>
> + /*
> + * We use punch hole to reclaim the free space used by the
> + * image a.k.a. discard. However we do support discard if
> + * encryption is enabled, because it may give an attacker
> + * useful information.
> + */
> + if (bio->bi_rw & REQ_DISCARD) {
> + struct file *file = lo->lo_backing_file;
> + int mode = FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE;
> +
> + if ((!file->f_op->fallocate) ||
> + lo->lo_encrypt_key_size) {
> + ret = -EOPNOTSUPP;
> + goto out;
> + }
> + ret = file->f_op->fallocate(file, mode, pos,
> + bio->bi_size);
> + if (unlikely(ret && ret != -EINVAL &&
> + ret != -EOPNOTSUPP))
> + ret = -EIO;
> + goto out;
> + }
> +
> ret = lo_send(lo, bio, pos);
>
> if ((bio->bi_rw & REQ_FUA) && !ret) {
> @@ -814,6 +838,35 @@ static void loop_sysfs_exit(struct loop_device *lo)
> &loop_attribute_group);
> }
>
> +static void loop_config_discard(struct loop_device *lo)
> +{
> + struct file *file = lo->lo_backing_file;
> + struct inode *inode = file->f_mapping->host;
> + struct request_queue *q = lo->lo_queue;
> +
> + /*
> + * We use punch hole to reclaim the free space used by the
> + * image a.k.a. discard. However we do support discard if
> + * encryption is enabled, because it may give an attacker
> + * useful information.
> + */
> + if ((!file->f_op->fallocate) ||
> + lo->lo_encrypt_key_size) {
> + q->limits.discard_granularity = 0;
> + q->limits.discard_alignment = 0;
> + q->limits.max_discard_sectors = 0;
> + q->limits.discard_zeroes_data = 0;
> + queue_flag_clear_unlocked(QUEUE_FLAG_DISCARD, q);
> + return;
> + }
> +
> + q->limits.discard_granularity = inode->i_sb->s_blocksize;
> + q->limits.discard_alignment = inode->i_sb->s_blocksize;
> + q->limits.max_discard_sectors = UINT_MAX >> 9;
> + q->limits.discard_zeroes_data = 1;
> + queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, q);
> +}
> +
> static int loop_set_fd(struct loop_device *lo, fmode_t mode,
> struct block_device *bdev, unsigned int arg)
> {
> @@ -1113,6 +1166,7 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
> info->lo_encrypt_key_size);
> lo->lo_key_owner = uid;
> }
> + loop_config_discard(lo);
>
> return 0;
> }
>
--
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
