[v4,1/5] btrfs: use the page-cache for super block reading
diff mbox series

Message ID 20200205143831.13959-2-johannes.thumshirn@wdc.com
State New
Headers show
Series
  • btrfs: remove buffer heads form superblock handling
Related show

Commit Message

Johannes Thumshirn Feb. 5, 2020, 2:38 p.m. UTC
Super-block reading in BTRFS is done using buffer_heads. Buffer_heads have
some drawbacks, like not being able to propagate errors from the lower
layers.

Directly use the page cache for reading the super-blocks from disk or
invalidating an on-disk super-block. We have to use the page-cache so to
avoid races between mkfs and udev. See also 6f60cbd3ae44 ("btrfs: access
superblock via pagecache in scan_one_device").

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>

---
Changes to v3:
- Use read_cache_pages() and write_one_page() for IO (hch)
- Changed subject (David)
- Dropped Josef's R-b due to change

Changes to v2:
- open-code kunmap() + put_page() (David)
- fix double kunmap() (David)
- don't use bi_set_op_attrs() (David)

Changes to v1:
- move 'super_page' into for-loop in btrfs_scratch_superblocks() (Nikolay)
- switch to using pagecahce instead of alloc_pages() (Nikolay, David)
---
 fs/btrfs/disk-io.c | 78 +++++++++++++++++++++++++---------------------
 fs/btrfs/disk-io.h |  4 +--
 fs/btrfs/volumes.c | 57 +++++++++++++++++----------------
 fs/btrfs/volumes.h |  2 --
 4 files changed, 76 insertions(+), 65 deletions(-)

Comments

hch@infradead.org Feb. 5, 2020, 4:53 p.m. UTC | #1
On Wed, Feb 05, 2020 at 11:38:27PM +0900, Johannes Thumshirn wrote:
> Super-block reading in BTRFS is done using buffer_heads. Buffer_heads have
> some drawbacks, like not being able to propagate errors from the lower
> layers.
> 
> Directly use the page cache for reading the super-blocks from disk or
> invalidating an on-disk super-block. We have to use the page-cache so to
> avoid races between mkfs and udev. See also 6f60cbd3ae44 ("btrfs: access
> superblock via pagecache in scan_one_device").
> 
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> 
> ---
> Changes to v3:
> - Use read_cache_pages() and write_one_page() for IO (hch)
> - Changed subject (David)
> - Dropped Josef's R-b due to change
> 
> Changes to v2:
> - open-code kunmap() + put_page() (David)
> - fix double kunmap() (David)
> - don't use bi_set_op_attrs() (David)
> 
> Changes to v1:
> - move 'super_page' into for-loop in btrfs_scratch_superblocks() (Nikolay)
> - switch to using pagecahce instead of alloc_pages() (Nikolay, David)
> ---
>  fs/btrfs/disk-io.c | 78 +++++++++++++++++++++++++---------------------
>  fs/btrfs/disk-io.h |  4 +--
>  fs/btrfs/volumes.c | 57 +++++++++++++++++----------------
>  fs/btrfs/volumes.h |  2 --
>  4 files changed, 76 insertions(+), 65 deletions(-)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 28622de9e642..bc14ef1aadda 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -2617,11 +2617,12 @@ int __cold open_ctree(struct super_block *sb,
>  	u64 features;
>  	u16 csum_type;
>  	struct btrfs_key location;
> -	struct buffer_head *bh;
>  	struct btrfs_super_block *disk_super;
>  	struct btrfs_fs_info *fs_info = btrfs_sb(sb);
>  	struct btrfs_root *tree_root;
>  	struct btrfs_root *chunk_root;
> +	struct page *super_page;
> +	u8 *superblock;

I thought you agree to turn this into a struct btrfs_super_block
pointer?

>  	bytenr = btrfs_sb_offset(copy_num);
>  	if (bytenr + BTRFS_SUPER_INFO_SIZE >= i_size_read(bdev->bd_inode))
>  		return -EINVAL;
>  
> -	bh = __bread(bdev, bytenr / BTRFS_BDEV_BLOCKSIZE, BTRFS_SUPER_INFO_SIZE);
> -	/*
> -	 * If we fail to read from the underlying devices, as of now
> -	 * the best option we have is to mark it EIO.
> -	 */
> -	if (!bh)
> -		return -EIO;
> +	gfp_mask = mapping_gfp_constraint(mapping, ~__GFP_FS) | __GFP_NOFAIL;
> +	page = read_cache_page_gfp(mapping, bytenr >> PAGE_SHIFT, gfp_mask);
> +	if (IS_ERR_OR_NULL(page))
> +		return -ENOMEM;

Why do you need the __GFP_NOFAIL given that failures are handled
properly here?  Also I think instead of using mapping_gfp_constraint you
can use GFP_NOFS directly here.

>  
> -	super = (struct btrfs_super_block *)bh->b_data;
> +	super = kmap(page);
>  	if (btrfs_super_bytenr(super) != bytenr ||
>  		    btrfs_super_magic(super) != BTRFS_MAGIC) {
> -		brelse(bh);
> +		kunmap(page);
> +		put_page(page);
>  		return -EINVAL;
>  	}
> +	kunmap(page);

Also last time I wondered why we can't leave the page mapped for the
caller and also return the virtual address?  That would keep the
callers a little cleaner.  Note that you don't need to pass the
struct page in that case as the unmap helper can use kmap_to_page (and
I think a helper would be really nice for the unmap and put anyway).
Johannes Thumshirn Feb. 6, 2020, 8:17 a.m. UTC | #2
On 05/02/2020 17:53, Christoph Hellwig wrote:
> On Wed, Feb 05, 2020 at 11:38:27PM +0900, Johannes Thumshirn wrote:
>> Super-block reading in BTRFS is done using buffer_heads. Buffer_heads have
>> some drawbacks, like not being able to propagate errors from the lower
>> layers.
>>
>> Directly use the page cache for reading the super-blocks from disk or
>> invalidating an on-disk super-block. We have to use the page-cache so to
>> avoid races between mkfs and udev. See also 6f60cbd3ae44 ("btrfs: access
>> superblock via pagecache in scan_one_device").
>>
>> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
>>
>> ---
>> Changes to v3:
>> - Use read_cache_pages() and write_one_page() for IO (hch)
>> - Changed subject (David)
>> - Dropped Josef's R-b due to change
>>
>> Changes to v2:
>> - open-code kunmap() + put_page() (David)
>> - fix double kunmap() (David)
>> - don't use bi_set_op_attrs() (David)
>>
>> Changes to v1:
>> - move 'super_page' into for-loop in btrfs_scratch_superblocks() (Nikolay)
>> - switch to using pagecahce instead of alloc_pages() (Nikolay, David)
>> ---
>>   fs/btrfs/disk-io.c | 78 +++++++++++++++++++++++++---------------------
>>   fs/btrfs/disk-io.h |  4 +--
>>   fs/btrfs/volumes.c | 57 +++++++++++++++++----------------
>>   fs/btrfs/volumes.h |  2 --
>>   4 files changed, 76 insertions(+), 65 deletions(-)
>>
>> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
>> index 28622de9e642..bc14ef1aadda 100644
>> --- a/fs/btrfs/disk-io.c
>> +++ b/fs/btrfs/disk-io.c
>> @@ -2617,11 +2617,12 @@ int __cold open_ctree(struct super_block *sb,
>>   	u64 features;
>>   	u16 csum_type;
>>   	struct btrfs_key location;
>> -	struct buffer_head *bh;
>>   	struct btrfs_super_block *disk_super;
>>   	struct btrfs_fs_info *fs_info = btrfs_sb(sb);
>>   	struct btrfs_root *tree_root;
>>   	struct btrfs_root *chunk_root;
>> +	struct page *super_page;
>> +	u8 *superblock;
> 
> I thought you agree to turn this into a struct btrfs_super_block
> pointer?

As stated in the cover letter, I lost track of the TODOs ;-)

>>   	bytenr = btrfs_sb_offset(copy_num);
>>   	if (bytenr + BTRFS_SUPER_INFO_SIZE >= i_size_read(bdev->bd_inode))
>>   		return -EINVAL;
>>   
>> -	bh = __bread(bdev, bytenr / BTRFS_BDEV_BLOCKSIZE, BTRFS_SUPER_INFO_SIZE);
>> -	/*
>> -	 * If we fail to read from the underlying devices, as of now
>> -	 * the best option we have is to mark it EIO.
>> -	 */
>> -	if (!bh)
>> -		return -EIO;
>> +	gfp_mask = mapping_gfp_constraint(mapping, ~__GFP_FS) | __GFP_NOFAIL;
>> +	page = read_cache_page_gfp(mapping, bytenr >> PAGE_SHIFT, gfp_mask);
>> +	if (IS_ERR_OR_NULL(page))
>> +		return -ENOMEM;
> 
> Why do you need the __GFP_NOFAIL given that failures are handled
> properly here?  Also I think instead of using mapping_gfp_constraint you
> can use GFP_NOFS directly here.

OK

>>   
>> -	super = (struct btrfs_super_block *)bh->b_data;
>> +	super = kmap(page);
>>   	if (btrfs_super_bytenr(super) != bytenr ||
>>   		    btrfs_super_magic(super) != BTRFS_MAGIC) {
>> -		brelse(bh);
>> +		kunmap(page);
>> +		put_page(page);
>>   		return -EINVAL;
>>   	}
>> +	kunmap(page);
> 
> Also last time I wondered why we can't leave the page mapped for the
> caller and also return the virtual address?  That would keep the
> callers a little cleaner.  Note that you don't need to pass the
> struct page in that case as the unmap helper can use kmap_to_page (and
> I think a helper would be really nice for the unmap and put anyway).
> 

There's btrfs_release_disk_super() but David didn't like the use of it 
in v2 of this series. But when using a 'struct btrfs_disk_super' instead 
of a 'struct page' I think he could be ok.
Johannes Thumshirn Feb. 6, 2020, 9:29 a.m. UTC | #3
On 05/02/2020 17:53, Christoph Hellwig wrote:
[...]
>> +	struct page *super_page;
>> +	u8 *superblock;
> 
> I thought you agree to turn this into a struct btrfs_super_block
> pointer?
> 

I'll do this in an add-on patch, otherwise the diff will get messy with 
lots of unrelated changes.
hch@infradead.org Feb. 6, 2020, 2:57 p.m. UTC | #4
On Thu, Feb 06, 2020 at 08:17:20AM +0000, Johannes Thumshirn wrote:
> >> +	super = kmap(page);
> >>   	if (btrfs_super_bytenr(super) != bytenr ||
> >>   		    btrfs_super_magic(super) != BTRFS_MAGIC) {
> >> -		brelse(bh);
> >> +		kunmap(page);
> >> +		put_page(page);
> >>   		return -EINVAL;
> >>   	}
> >> +	kunmap(page);
> > 
> > Also last time I wondered why we can't leave the page mapped for the
> > caller and also return the virtual address?  That would keep the
> > callers a little cleaner.  Note that you don't need to pass the
> > struct page in that case as the unmap helper can use kmap_to_page (and
> > I think a helper would be really nice for the unmap and put anyway).
> > 
> 
> There's btrfs_release_disk_super() but David didn't like the use of it 
> in v2 of this series. But when using a 'struct btrfs_disk_super' instead 
> of a 'struct page' I think he could be ok.

Also I just noticed don't even need the kmap/kunmap at all given that the
block device mapping is never in highmem.
Johannes Thumshirn Feb. 6, 2020, 3:29 p.m. UTC | #5
On 06/02/2020 15:58, Christoph Hellwig wrote:
> Also I just noticed don't even need the kmap/kunmap at all given that the
> block device mapping is never in highmem.
> 

This potentially touches more places, I'll cover that in a dedicated 
patchset.
David Sterba Feb. 7, 2020, 4:13 p.m. UTC | #6
On Thu, Feb 06, 2020 at 03:29:57PM +0000, Johannes Thumshirn wrote:
> On 06/02/2020 15:58, Christoph Hellwig wrote:
> > Also I just noticed don't even need the kmap/kunmap at all given that the
> > block device mapping is never in highmem.
> > 
> 
> This potentially touches more places, I'll cover that in a dedicated 
> patchset.

Are the kmap/kunmaps anywhere on the buffer_head call paths? I can't
find it anywhere, and given that the mapping does not contain highmem
pages we could rather avoid adding it from the beginning.
Johannes Thumshirn Feb. 10, 2020, 7:16 a.m. UTC | #7
On 07/02/2020 17:14, David Sterba wrote:
> On Thu, Feb 06, 2020 at 03:29:57PM +0000, Johannes Thumshirn wrote:
>> On 06/02/2020 15:58, Christoph Hellwig wrote:
>>> Also I just noticed don't even need the kmap/kunmap at all given that the
>>> block device mapping is never in highmem.
>>>
>>
>> This potentially touches more places, I'll cover that in a dedicated
>> patchset.
> 
> Are the kmap/kunmaps anywhere on the buffer_head call paths? I can't
> find it anywhere, and given that the mapping does not contain highmem
> pages we could rather avoid adding it from the beginning.


There's at least one more in btrfs_read_disk_super(), but yes I can 
avoid them in the BH path and remove the ones I find in the next go.

Patch
diff mbox series

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 28622de9e642..bc14ef1aadda 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2617,11 +2617,12 @@  int __cold open_ctree(struct super_block *sb,
 	u64 features;
 	u16 csum_type;
 	struct btrfs_key location;
-	struct buffer_head *bh;
 	struct btrfs_super_block *disk_super;
 	struct btrfs_fs_info *fs_info = btrfs_sb(sb);
 	struct btrfs_root *tree_root;
 	struct btrfs_root *chunk_root;
+	struct page *super_page;
+	u8 *superblock;
 	int ret;
 	int err = -EINVAL;
 	int clear_free_space_tree = 0;
@@ -2815,28 +2816,33 @@  int __cold open_ctree(struct super_block *sb,
 	/*
 	 * Read super block and check the signature bytes only
 	 */
-	bh = btrfs_read_dev_super(fs_devices->latest_bdev);
-	if (IS_ERR(bh)) {
-		err = PTR_ERR(bh);
+	ret = btrfs_read_dev_super(fs_devices->latest_bdev, &super_page);
+	if (ret) {
+		err = ret;
 		goto fail_alloc;
 	}
 
+	superblock = kmap(super_page);
 	/*
 	 * Verify the type first, if that or the the checksum value are
 	 * corrupted, we'll find out
 	 */
-	csum_type = btrfs_super_csum_type((struct btrfs_super_block *)bh->b_data);
+	csum_type = btrfs_super_csum_type((struct btrfs_super_block *)
+					  superblock);
 	if (!btrfs_supported_super_csum(csum_type)) {
 		btrfs_err(fs_info, "unsupported checksum algorithm: %u",
 			  csum_type);
 		err = -EINVAL;
-		brelse(bh);
+		kunmap(super_page);
+		put_page(super_page);
 		goto fail_alloc;
 	}
 
 	ret = btrfs_init_csum_hash(fs_info, csum_type);
 	if (ret) {
 		err = ret;
+		kunmap(super_page);
+		put_page(super_page);
 		goto fail_alloc;
 	}
 
@@ -2844,10 +2850,11 @@  int __cold open_ctree(struct super_block *sb,
 	 * We want to check superblock checksum, the type is stored inside.
 	 * Pass the whole disk block of size BTRFS_SUPER_INFO_SIZE (4k).
 	 */
-	if (btrfs_check_super_csum(fs_info, bh->b_data)) {
+	if (btrfs_check_super_csum(fs_info, superblock)) {
 		btrfs_err(fs_info, "superblock checksum mismatch");
 		err = -EINVAL;
-		brelse(bh);
+		kunmap(super_page);
+		put_page(super_page);
 		goto fail_csum;
 	}
 
@@ -2856,8 +2863,9 @@  int __cold open_ctree(struct super_block *sb,
 	 * following bytes up to INFO_SIZE, the checksum is calculated from
 	 * the whole block of INFO_SIZE
 	 */
-	memcpy(fs_info->super_copy, bh->b_data, sizeof(*fs_info->super_copy));
-	brelse(bh);
+	memcpy(fs_info->super_copy, superblock, sizeof(*fs_info->super_copy));
+	kunmap(super_page);
+	put_page(super_page);
 
 	disk_super = fs_info->super_copy;
 
@@ -3355,40 +3363,40 @@  static void btrfs_end_buffer_write_sync(struct buffer_head *bh, int uptodate)
 }
 
 int btrfs_read_dev_one_super(struct block_device *bdev, int copy_num,
-			struct buffer_head **bh_ret)
+			struct page **super_page)
 {
-	struct buffer_head *bh;
 	struct btrfs_super_block *super;
+	struct page *page;
 	u64 bytenr;
+	struct address_space *mapping = bdev->bd_inode->i_mapping;
+	gfp_t gfp_mask;
 
 	bytenr = btrfs_sb_offset(copy_num);
 	if (bytenr + BTRFS_SUPER_INFO_SIZE >= i_size_read(bdev->bd_inode))
 		return -EINVAL;
 
-	bh = __bread(bdev, bytenr / BTRFS_BDEV_BLOCKSIZE, BTRFS_SUPER_INFO_SIZE);
-	/*
-	 * If we fail to read from the underlying devices, as of now
-	 * the best option we have is to mark it EIO.
-	 */
-	if (!bh)
-		return -EIO;
+	gfp_mask = mapping_gfp_constraint(mapping, ~__GFP_FS) | __GFP_NOFAIL;
+	page = read_cache_page_gfp(mapping, bytenr >> PAGE_SHIFT, gfp_mask);
+	if (IS_ERR_OR_NULL(page))
+		return -ENOMEM;
 
-	super = (struct btrfs_super_block *)bh->b_data;
+	super = kmap(page);
 	if (btrfs_super_bytenr(super) != bytenr ||
 		    btrfs_super_magic(super) != BTRFS_MAGIC) {
-		brelse(bh);
+		kunmap(page);
+		put_page(page);
 		return -EINVAL;
 	}
+	kunmap(page);
 
-	*bh_ret = bh;
+	*super_page = page;
 	return 0;
 }
 
 
-struct buffer_head *btrfs_read_dev_super(struct block_device *bdev)
+int btrfs_read_dev_super(struct block_device *bdev, struct page **page)
 {
-	struct buffer_head *bh;
-	struct buffer_head *latest = NULL;
+	struct page *latest = NULL;
 	struct btrfs_super_block *super;
 	int i;
 	u64 transid = 0;
@@ -3400,25 +3408,25 @@  struct buffer_head *btrfs_read_dev_super(struct block_device *bdev)
 	 * later supers, using BTRFS_SUPER_MIRROR_MAX instead
 	 */
 	for (i = 0; i < 1; i++) {
-		ret = btrfs_read_dev_one_super(bdev, i, &bh);
+		ret = btrfs_read_dev_one_super(bdev, i, page);
 		if (ret)
 			continue;
 
-		super = (struct btrfs_super_block *)bh->b_data;
+		super = kmap(*page);
 
 		if (!latest || btrfs_super_generation(super) > transid) {
-			brelse(latest);
-			latest = bh;
+			if (latest) {
+				kunmap(latest);
+				put_page(latest);
+			}
+			latest = *page;
 			transid = btrfs_super_generation(super);
-		} else {
-			brelse(bh);
 		}
-	}
 
-	if (!latest)
-		return ERR_PTR(ret);
+		kunmap(*page);
+	}
 
-	return latest;
+	return ret;
 }
 
 /*
diff --git a/fs/btrfs/disk-io.h b/fs/btrfs/disk-io.h
index 8add2e14aab1..a89283ce8ca2 100644
--- a/fs/btrfs/disk-io.h
+++ b/fs/btrfs/disk-io.h
@@ -54,9 +54,9 @@  int __cold open_ctree(struct super_block *sb,
 	       char *options);
 void __cold close_ctree(struct btrfs_fs_info *fs_info);
 int write_all_supers(struct btrfs_fs_info *fs_info, int max_mirrors);
-struct buffer_head *btrfs_read_dev_super(struct block_device *bdev);
+int btrfs_read_dev_super(struct block_device *bdev, struct page **super_page);
 int btrfs_read_dev_one_super(struct block_device *bdev, int copy_num,
-			struct buffer_head **bh_ret);
+			     struct page **super_page);
 int btrfs_commit_super(struct btrfs_fs_info *fs_info);
 struct btrfs_root *btrfs_read_tree_root(struct btrfs_root *tree_root,
 					struct btrfs_key *key);
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 0b3167bd9f35..d381225aaff0 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -6,7 +6,6 @@ 
 #include <linux/sched.h>
 #include <linux/bio.h>
 #include <linux/slab.h>
-#include <linux/buffer_head.h>
 #include <linux/blkdev.h>
 #include <linux/ratelimit.h>
 #include <linux/kthread.h>
@@ -500,7 +499,7 @@  static struct btrfs_fs_devices *find_fsid_with_metadata_uuid(
 static int
 btrfs_get_bdev_and_sb(const char *device_path, fmode_t flags, void *holder,
 		      int flush, struct block_device **bdev,
-		      struct buffer_head **bh)
+		      struct page **super_page)
 {
 	int ret;
 
@@ -519,9 +518,8 @@  btrfs_get_bdev_and_sb(const char *device_path, fmode_t flags, void *holder,
 		goto error;
 	}
 	invalidate_bdev(*bdev);
-	*bh = btrfs_read_dev_super(*bdev);
-	if (IS_ERR(*bh)) {
-		ret = PTR_ERR(*bh);
+	ret = btrfs_read_dev_super(*bdev, super_page);
+	if (ret) {
 		blkdev_put(*bdev, flags);
 		goto error;
 	}
@@ -530,7 +528,6 @@  btrfs_get_bdev_and_sb(const char *device_path, fmode_t flags, void *holder,
 
 error:
 	*bdev = NULL;
-	*bh = NULL;
 	return ret;
 }
 
@@ -611,7 +608,7 @@  static int btrfs_open_one_device(struct btrfs_fs_devices *fs_devices,
 {
 	struct request_queue *q;
 	struct block_device *bdev;
-	struct buffer_head *bh;
+	struct page *super_page;
 	struct btrfs_super_block *disk_super;
 	u64 devid;
 	int ret;
@@ -622,17 +619,17 @@  static int btrfs_open_one_device(struct btrfs_fs_devices *fs_devices,
 		return -EINVAL;
 
 	ret = btrfs_get_bdev_and_sb(device->name->str, flags, holder, 1,
-				    &bdev, &bh);
+				    &bdev, &super_page);
 	if (ret)
 		return ret;
 
-	disk_super = (struct btrfs_super_block *)bh->b_data;
+	disk_super = kmap(super_page);
 	devid = btrfs_stack_device_id(&disk_super->dev_item);
 	if (devid != device->devid)
-		goto error_brelse;
+		goto error_free_page;
 
 	if (memcmp(device->uuid, disk_super->dev_item.uuid, BTRFS_UUID_SIZE))
-		goto error_brelse;
+		goto error_free_page;
 
 	device->generation = btrfs_super_generation(disk_super);
 
@@ -641,7 +638,7 @@  static int btrfs_open_one_device(struct btrfs_fs_devices *fs_devices,
 		    BTRFS_FEATURE_INCOMPAT_METADATA_UUID) {
 			pr_err(
 		"BTRFS: Invalid seeding and uuid-changed device detected\n");
-			goto error_brelse;
+			goto error_free_page;
 		}
 
 		clear_bit(BTRFS_DEV_STATE_WRITEABLE, &device->dev_state);
@@ -667,12 +664,14 @@  static int btrfs_open_one_device(struct btrfs_fs_devices *fs_devices,
 		fs_devices->rw_devices++;
 		list_add_tail(&device->dev_alloc_list, &fs_devices->alloc_list);
 	}
-	brelse(bh);
+	kunmap(super_page);
+	put_page(super_page);
 
 	return 0;
 
-error_brelse:
-	brelse(bh);
+error_free_page:
+	kunmap(super_page);
+	put_page(super_page);
 	blkdev_put(bdev, flags);
 
 	return -EINVAL;
@@ -2209,14 +2208,15 @@  static struct btrfs_device *btrfs_find_device_by_path(
 	u64 devid;
 	u8 *dev_uuid;
 	struct block_device *bdev;
-	struct buffer_head *bh;
+	struct page *super_page;
 	struct btrfs_device *device;
 
 	ret = btrfs_get_bdev_and_sb(device_path, FMODE_READ,
-				    fs_info->bdev_holder, 0, &bdev, &bh);
+				    fs_info->bdev_holder, 0, &bdev,
+				    &super_page);
 	if (ret)
 		return ERR_PTR(ret);
-	disk_super = (struct btrfs_super_block *)bh->b_data;
+	disk_super = kmap(super_page);
 	devid = btrfs_stack_device_id(&disk_super->dev_item);
 	dev_uuid = disk_super->dev_item.uuid;
 	if (btrfs_fs_incompat(fs_info, METADATA_UUID))
@@ -2226,7 +2226,8 @@  static struct btrfs_device *btrfs_find_device_by_path(
 		device = btrfs_find_device(fs_info->fs_devices, devid, dev_uuid,
 					   disk_super->fsid, true);
 
-	brelse(bh);
+	kunmap(super_page);
+	put_page(super_page);
 	if (!device)
 		device = ERR_PTR(-ENOENT);
 	blkdev_put(bdev, FMODE_READ);
@@ -7319,7 +7320,6 @@  int btrfs_get_dev_stats(struct btrfs_fs_info *fs_info,
 
 void btrfs_scratch_superblocks(struct block_device *bdev, const char *device_path)
 {
-	struct buffer_head *bh;
 	struct btrfs_super_block *disk_super;
 	int copy_num;
 
@@ -7328,16 +7328,21 @@  void btrfs_scratch_superblocks(struct block_device *bdev, const char *device_pat
 
 	for (copy_num = 0; copy_num < BTRFS_SUPER_MIRROR_MAX;
 		copy_num++) {
+		u64 bytenr = btrfs_sb_offset(copy_num);
+		struct page *page;
 
-		if (btrfs_read_dev_one_super(bdev, copy_num, &bh))
+		if (btrfs_read_dev_one_super(bdev, copy_num, &page))
 			continue;
 
-		disk_super = (struct btrfs_super_block *)bh->b_data;
-
+		disk_super = kmap(page) + offset_in_page(bytenr);
 		memset(&disk_super->magic, 0, sizeof(disk_super->magic));
-		set_buffer_dirty(bh);
-		sync_dirty_buffer(bh);
-		brelse(bh);
+		kunmap(page);
+
+		set_page_dirty(page);
+		lock_page(page); /* write_on_page() unlocks the page */
+		write_one_page(page);
+		put_page(page);
+
 	}
 
 	/* Notify udev that device has changed */
diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index 690d4f5a0653..3b8eb2a14960 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -17,8 +17,6 @@  extern struct mutex uuid_mutex;
 
 #define BTRFS_STRIPE_LEN	SZ_64K
 
-struct buffer_head;
-
 struct btrfs_io_geometry {
 	/* remaining bytes before crossing a stripe */
 	u64 len;