Re: [PATCH v2] Btrfs: avoid losing data raid profile when deleting a device

[Liu Bo <bo.li.liu@oracle.com>]

On Mon, Oct 16, 2017 at 12:22:44PM +0800, Anand Jain wrote:
> 
> 
> On 10/14/2017 04:51 AM, Liu Bo wrote:
> >On Wed, Oct 11, 2017 at 10:38:50AM +0300, Nikolay Borisov wrote:
> >>
> >>
> >>On 10.10.2017 20:53, Liu Bo wrote:
> >>>We've avoided data losing raid profile when doing balance, but it
> >>>turns out that deleting a device could also result in the same
> >>>problem
> >>>
> >>>This fixes the problem by creating an empty data chunk before
> >>>relocating the data chunk.
> >>
> >>Why is this needed - copy the metadata of the to-be-relocated chunk into
> >>the newly created empty chunk? I don't entirely understand that code but
> >>doesn't this seem a bit like a hack in order to stash some information?
> >>Perhaps you could elaborate the logic a bit more in the changelog?
> >>
> >>>
> >>>Metadata/System chunk are supposed to have non-zero bytes all the time
> >>>so their raid profile is persistent.
> >>
> >>I think this changelog is a bit scarce on detail as to the culprit of
> >>the problem. Could you perhaps put a sentence or two what the underlying
> >>logic which deletes the raid profile if a chunk is empty ?
> >>
> >
> >Fair enough.
> >
> >The problem is as same as what commit 2c9fe8355258 ("btrfs: Fix
> >lost-data-profile caused by balance bg") had fixed.
> >
> >Similar to doing balance, deleting a device can also move all chunks
> >on this disk to other available disks, after 'move' successfully,
> >it'll remove those chunks.
> >
> >If our last data chunk is empty and part of it happens to be on this
> >disk, then there is no data chunk in this btrfs after deleting the
> >device successfully, any following write will try to create a new data
> >chunk which ends up with a single data chunk because the only
> >available data raid profile is 'single'.
> 
>  So you are referring to a raid1 group profile which contains 3 or more
>  devices otherwise single group file is what it will fit ? Is there
>  reproducer ?

[PATCH] Fstest: btrfs/151: test if device delete ends up with losing raid profile

Thanks,

-liubo
> 
> Thanks, Anand
> 
> 
> >thanks,
> >-liubo
> >
> >>>
> >>>Reported-by: James Alandt <James.Alandt@wdc.com>
> >>>Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
> >>>---
> >>>
> >>>v2: - return the correct error.
> >>>     - move helper ahead of __btrfs_balance().
> >>>
> >>>  fs/btrfs/volumes.c | 84 ++++++++++++++++++++++++++++++++++++++++++------------
> >>>  1 file changed, 65 insertions(+), 19 deletions(-)
> >>>
> >>>diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> >>>index 4a72c45..a74396d 100644
> >>>--- a/fs/btrfs/volumes.c
> >>>+++ b/fs/btrfs/volumes.c
> >>>@@ -3018,6 +3018,48 @@ static int btrfs_relocate_sys_chunks(struct btrfs_fs_info *fs_info)
> >>>  	return ret;
> >>>  }
> >>>+/*
> >>>+ * return 1 : allocate a data chunk successfully,
> >>>+ * return <0: errors during allocating a data chunk,
> >>>+ * return 0 : no need to allocate a data chunk.
> >>>+ */
> >>>+static int btrfs_may_alloc_data_chunk(struct btrfs_fs_info *fs_info,
> >>>+				      u64 chunk_offset)
> >>>+{
> >>>+	struct btrfs_block_group_cache *cache;
> >>>+	u64 bytes_used;
> >>>+	u64 chunk_type;
> >>>+
> >>>+	cache = btrfs_lookup_block_group(fs_info, chunk_offset);
> >>>+	ASSERT(cache);
> >>>+	chunk_type = cache->flags;
> >>>+	btrfs_put_block_group(cache);
> >>>+
> >>>+	if (chunk_type & BTRFS_BLOCK_GROUP_DATA) {
> >>>+		spin_lock(&fs_info->data_sinfo->lock);
> >>>+		bytes_used = fs_info->data_sinfo->bytes_used;
> >>>+		spin_unlock(&fs_info->data_sinfo->lock);
> >>>+
> >>>+		if (!bytes_used) {
> >>>+			struct btrfs_trans_handle *trans;
> >>>+			int ret;
> >>>+
> >>>+			trans =	btrfs_join_transaction(fs_info->tree_root);
> >>>+			if (IS_ERR(trans))
> >>>+				return PTR_ERR(trans);
> >>>+
> >>>+			ret = btrfs_force_chunk_alloc(trans, fs_info,
> >>>+						      BTRFS_BLOCK_GROUP_DATA);
> >>>+			btrfs_end_transaction(trans);
> >>>+			if (ret < 0)
> >>>+				return ret;
> >>>+
> >>>+			return 1;
> >>>+		}
> >>>+	}
> >>>+	return 0;
> >>>+}
> >>>+
> >>>  static int insert_balance_item(struct btrfs_fs_info *fs_info,
> >>>  			       struct btrfs_balance_control *bctl)
> >>>  {
> >>>@@ -3476,7 +3518,6 @@ static int __btrfs_balance(struct btrfs_fs_info *fs_info)
> >>>  	u32 count_meta = 0;
> >>>  	u32 count_sys = 0;
> >>>  	int chunk_reserved = 0;
> >>>-	u64 bytes_used = 0;
> >>>  	/* step one make some room on all the devices */
> >>>  	devices = &fs_info->fs_devices->devices;
> >>>@@ -3635,28 +3676,21 @@ static int __btrfs_balance(struct btrfs_fs_info *fs_info)
> >>>  			goto loop;
> >>>  		}
> >>>-		ASSERT(fs_info->data_sinfo);
> >>>-		spin_lock(&fs_info->data_sinfo->lock);
> >>>-		bytes_used = fs_info->data_sinfo->bytes_used;
> >>>-		spin_unlock(&fs_info->data_sinfo->lock);
> >>>-
> >>>-		if ((chunk_type & BTRFS_BLOCK_GROUP_DATA) &&
> >>>-		    !chunk_reserved && !bytes_used) {
> >>>-			trans = btrfs_start_transaction(chunk_root, 0);
> >>>-			if (IS_ERR(trans)) {
> >>>-				mutex_unlock(&fs_info->delete_unused_bgs_mutex);
> >>>-				ret = PTR_ERR(trans);
> >>>-				goto error;
> >>>-			}
> >>>-
> >>>-			ret = btrfs_force_chunk_alloc(trans, fs_info,
> >>>-						      BTRFS_BLOCK_GROUP_DATA);
> >>>-			btrfs_end_transaction(trans);
> >>>+		if (!chunk_reserved) {
> >>>+			/*
> >>>+			 * We may be relocating the only data chunk we have,
> >>>+			 * which could potentially end up with losing data's
> >>>+			 * raid profile, so lets allocate an empty one in
> >>>+			 * advance.
> >>>+			 */
> >>>+			ret = btrfs_may_alloc_data_chunk(fs_info,
> >>>+							 found_key.offset);
> >>>  			if (ret < 0) {
> >>>  				mutex_unlock(&fs_info->delete_unused_bgs_mutex);
> >>>  				goto error;
> >>>+			} else if (ret == 1) {
> >>>+				chunk_reserved = 1;
> >>>  			}
> >>>-			chunk_reserved = 1;
> >>>  		}
> >>>  		ret = btrfs_relocate_chunk(fs_info, found_key.offset);
> >>>@@ -4419,6 +4453,18 @@ int btrfs_shrink_device(struct btrfs_device *device, u64 new_size)
> >>>  		chunk_offset = btrfs_dev_extent_chunk_offset(l, dev_extent);
> >>>  		btrfs_release_path(path);
> >>>+		/*
> >>>+		 * We may be relocating the only data chunk we have,
> >>>+		 * which could potentially end up with losing data's
> >>>+		 * raid profile, so lets allocate an empty one in
> >>>+		 * advance.
> >>>+		 */
> >>>+		ret = btrfs_may_alloc_data_chunk(fs_info, chunk_offset);
> >>>+		if (ret < 0) {
> >>>+			mutex_unlock(&fs_info->delete_unused_bgs_mutex);
> >>>+			goto done;
> >>>+		}
> >>>+
> >>>  		ret = btrfs_relocate_chunk(fs_info, chunk_offset);
> >>>  		mutex_unlock(&fs_info->delete_unused_bgs_mutex);
> >>>  		if (ret && ret != -ENOSPC)
> >>>
> >--
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