Re: [RFC PATCH v3 8/9] md: Implement ->corrupted_range()

From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: Ruan Shiyang <ruansy.fnst@cn.fujitsu.com>
Cc: linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org,
	linux-nvdimm@lists.01.org, linux-mm@kvack.org,
	linux-fsdevel@vger.kernel.org, linux-raid@vger.kernel.org,
	david@fromorbit.com, hch@lst.de, song@kernel.org,
	rgoldwyn@suse.de, qi.fuli@fujitsu.com, y-goto@fujitsu.com,
	Theodore Ts'o <tytso@mit.edu>
Subject: Re: [RFC PATCH v3 8/9] md: Implement ->corrupted_range()
Date: Mon, 4 Jan 2021 15:34:23 -0800	[thread overview]
Message-ID: <20210104233423.GR6918@magnolia> (raw)
In-Reply-To: <cc48c42d-d8af-c915-5aef-17b7d4853c3c@cn.fujitsu.com>

On Fri, Dec 18, 2020 at 10:11:54AM +0800, Ruan Shiyang wrote:
> 
> 
> On 2020/12/16 上午4:51, Darrick J. Wong wrote:
> > On Tue, Dec 15, 2020 at 08:14:13PM +0800, Shiyang Ruan wrote:
> > > With the support of ->rmap(), it is possible to obtain the superblock on
> > > a mapped device.
> > > 
> > > If a pmem device is used as one target of mapped device, we cannot
> > > obtain its superblock directly.  With the help of SYSFS, the mapped
> > > device can be found on the target devices.  So, we iterate the
> > > bdev->bd_holder_disks to obtain its mapped device.
> > > 
> > > Signed-off-by: Shiyang Ruan <ruansy.fnst@cn.fujitsu.com>
> > > ---
> > >   drivers/md/dm.c       | 66 +++++++++++++++++++++++++++++++++++++++++++
> > >   drivers/nvdimm/pmem.c |  9 ++++--
> > >   fs/block_dev.c        | 21 ++++++++++++++
> > >   include/linux/genhd.h |  7 +++++
> > >   4 files changed, 100 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > > index 4e0cbfe3f14d..9da1f9322735 100644
> > > --- a/drivers/md/dm.c
> > > +++ b/drivers/md/dm.c
> > > @@ -507,6 +507,71 @@ static int dm_blk_report_zones(struct gendisk *disk, sector_t sector,
> > >   #define dm_blk_report_zones		NULL
> > >   #endif /* CONFIG_BLK_DEV_ZONED */
> > > +struct dm_blk_corrupt {
> > > +	struct block_device *bdev;
> > > +	sector_t offset;
> > > +};
> > > +
> > > +static int dm_blk_corrupt_fn(struct dm_target *ti, struct dm_dev *dev,
> > > +				sector_t start, sector_t len, void *data)
> > > +{
> > > +	struct dm_blk_corrupt *bc = data;
> > > +
> > > +	return bc->bdev == (void *)dev->bdev &&
> > > +			(start <= bc->offset && bc->offset < start + len);
> > > +}
> > > +
> > > +static int dm_blk_corrupted_range(struct gendisk *disk,
> > > +				  struct block_device *target_bdev,
> > > +				  loff_t target_offset, size_t len, void *data)
> > > +{
> > > +	struct mapped_device *md = disk->private_data;
> > > +	struct block_device *md_bdev = md->bdev;
> > > +	struct dm_table *map;
> > > +	struct dm_target *ti;
> > > +	struct super_block *sb;
> > > +	int srcu_idx, i, rc = 0;
> > > +	bool found = false;
> > > +	sector_t disk_sec, target_sec = to_sector(target_offset);
> > > +
> > > +	map = dm_get_live_table(md, &srcu_idx);
> > > +	if (!map)
> > > +		return -ENODEV;
> > > +
> > > +	for (i = 0; i < dm_table_get_num_targets(map); i++) {
> > > +		ti = dm_table_get_target(map, i);
> > > +		if (ti->type->iterate_devices && ti->type->rmap) {
> > > +			struct dm_blk_corrupt bc = {target_bdev, target_sec};
> > > +
> > > +			found = ti->type->iterate_devices(ti, dm_blk_corrupt_fn, &bc);
> > > +			if (!found)
> > > +				continue;
> > > +			disk_sec = ti->type->rmap(ti, target_sec);
> > 
> > What happens if the dm device has multiple reverse mappings because the
> > physical storage is being shared at multiple LBAs?  (e.g. a
> > deduplication target)
> 
> I thought that the dm device knows the mapping relationship, and it can be
> done by implementation of ->rmap() in each target.  Did I understand it
> wrong?

The dm device /does/ know the mapping relationship.  I'm asking what
happens if there are *multiple* mappings.  For example, a deduplicating
dm device could observe that the upper level code wrote some data to
sector 200 and now it wants to write the same data to sector 500.
Instead of writing twice, it simply maps sector 500 in its LBA space to
the same space that it mapped sector 200.

Pretend that sector 200 on the dm-dedupe device maps to sector 64 on the
underlying storage (call it /dev/pmem1 and let's say it's the only
target sitting underneath the dm-dedupe device).

If /dev/pmem1 then notices that sector 64 has gone bad, it will start
calling ->corrupted_range handlers until it calls dm_blk_corrupted_range
on the dm-dedupe device.  At least in theory, the dm-dedupe driver's
rmap method ought to return both (64 -> 200) and (64 -> 500) so that
dm_blk_corrupted_range can pass on both corruption notices to whatever's
sitting atop the dedupe device.

At the moment, your ->rmap prototype is only capable of returning one
sector_t mapping per target, and there's only the one target under the
dedupe device, so we cannot report the loss of sectors 200 and 500 to
whatever device is sitting on top of dm-dedupe.

--D

> > 
> > > +			break;
> > > +		}
> > > +	}
> > > +
> > > +	if (!found) {
> > > +		rc = -ENODEV;
> > > +		goto out;
> > > +	}
> > > +
> > > +	sb = get_super(md_bdev);
> > > +	if (!sb) {
> > > +		rc = bd_disk_holder_corrupted_range(md_bdev, to_bytes(disk_sec), len, data);
> > > +		goto out;
> > > +	} else if (sb->s_op->corrupted_range) {
> > > +		loff_t off = to_bytes(disk_sec - get_start_sect(md_bdev));
> > > +
> > > +		rc = sb->s_op->corrupted_range(sb, md_bdev, off, len, data);
> > 
> > This "call bd_disk_holder_corrupted_range or sb->s_op->corrupted_range"
> > logic appears twice; should it be refactored into a common helper?
> > 
> > Or, should the superblock dispatch part move to
> > bd_disk_holder_corrupted_range?
> 
> bd_disk_holder_corrupted_range() requires SYSFS configuration.  I introduce
> it to handle those block devices that can not obtain superblock by
> `get_super()`.
> 
> Usually, if we create filesystem directly on a pmem device, or make some
> partitions at first, we can use `get_super()` to get the superblock.  In
> other case, such as creating a LVM on pmem device, `get_super()` does not
> work.
> 
> So, I think refactoring it into a common helper looks better.
> 
> 
> --
> Thanks,
> Ruan Shiyang.
> 
> > 
> > > +	}
> > > +	drop_super(sb);
> > > +
> > > +out:
> > > +	dm_put_live_table(md, srcu_idx);
> > > +	return rc;
> > > +}
> > > +
> > >   static int dm_prepare_ioctl(struct mapped_device *md, int *srcu_idx,
> > >   			    struct block_device **bdev)
> > >   {
> > > @@ -3084,6 +3149,7 @@ static const struct block_device_operations dm_blk_dops = {
> > >   	.getgeo = dm_blk_getgeo,
> > >   	.report_zones = dm_blk_report_zones,
> > >   	.pr_ops = &dm_pr_ops,
> > > +	.corrupted_range = dm_blk_corrupted_range,
> > >   	.owner = THIS_MODULE
> > >   };
> > > diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> > > index 4688bff19c20..e8cfaf860149 100644
> > > --- a/drivers/nvdimm/pmem.c
> > > +++ b/drivers/nvdimm/pmem.c
> > > @@ -267,11 +267,14 @@ static int pmem_corrupted_range(struct gendisk *disk, struct block_device *bdev,
> > >   	bdev_offset = (disk_sector - get_start_sect(bdev)) << SECTOR_SHIFT;
> > >   	sb = get_super(bdev);
> > > -	if (sb && sb->s_op->corrupted_range) {
> > > +	if (!sb) {
> > > +		rc = bd_disk_holder_corrupted_range(bdev, bdev_offset, len, data);
> > > +		goto out;
> > > +	} else if (sb->s_op->corrupted_range)
> > >   		rc = sb->s_op->corrupted_range(sb, bdev, bdev_offset, len, data);
> > > -		drop_super(sb);
> > 
> > This is out of scope for this patch(set) but do you think that the scsi
> > disk driver should intercept media errors from sense data and call
> > ->corrupted_range too?  ISTR Ted muttering that one of his employers had
> > a patchset to do more with sense data than the upstream kernel currently
> > does...
> > 
> > > -	}
> > > +	drop_super(sb);
> > > +out:
> > >   	bdput(bdev);
> > >   	return rc;
> > >   }
> > > diff --git a/fs/block_dev.c b/fs/block_dev.c
> > > index 9e84b1928b94..d3e6bddb8041 100644
> > > --- a/fs/block_dev.c
> > > +++ b/fs/block_dev.c
> > > @@ -1171,6 +1171,27 @@ struct bd_holder_disk {
> > >   	int			refcnt;
> > >   };
> > > +int bd_disk_holder_corrupted_range(struct block_device *bdev, loff_t off, size_t len, void *data)
> > > +{
> > > +	struct bd_holder_disk *holder;
> > > +	struct gendisk *disk;
> > > +	int rc = 0;
> > > +
> > > +	if (list_empty(&(bdev->bd_holder_disks)))
> > > +		return -ENODEV;
> > > +
> > > +	list_for_each_entry(holder, &bdev->bd_holder_disks, list) {
> > > +		disk = holder->disk;
> > > +		if (disk->fops->corrupted_range) {
> > > +			rc = disk->fops->corrupted_range(disk, bdev, off, len, data);
> > > +			if (rc != -ENODEV)
> > > +				break;
> > > +		}
> > > +	}
> > > +	return rc;
> > > +}
> > > +EXPORT_SYMBOL_GPL(bd_disk_holder_corrupted_range);
> > > +
> > >   static struct bd_holder_disk *bd_find_holder_disk(struct block_device *bdev,
> > >   						  struct gendisk *disk)
> > >   {
> > > diff --git a/include/linux/genhd.h b/include/linux/genhd.h
> > > index ed06209008b8..fba247b852fa 100644
> > > --- a/include/linux/genhd.h
> > > +++ b/include/linux/genhd.h
> > > @@ -382,9 +382,16 @@ int blkdev_ioctl(struct block_device *, fmode_t, unsigned, unsigned long);
> > >   long compat_blkdev_ioctl(struct file *, unsigned, unsigned long);
> > >   #ifdef CONFIG_SYSFS
> > > +int bd_disk_holder_corrupted_range(struct block_device *bdev, loff_t off,
> > > +				   size_t len, void *data);
> > >   int bd_link_disk_holder(struct block_device *bdev, struct gendisk *disk);
> > >   void bd_unlink_disk_holder(struct block_device *bdev, struct gendisk *disk);
> > >   #else
> > > +int bd_disk_holder_corrupted_range(struct block_device *bdev, loff_t off,
> > > +				   size_t len, void *data)
> > > +{
> > > +	return 0;
> > > +}
> > >   static inline int bd_link_disk_holder(struct block_device *bdev,
> > >   				      struct gendisk *disk)
> > >   {
> > > -- 
> > > 2.29.2
> > > 
> > > 
> > > 
> > 
> > 
> 
> 
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