[PATCH v3 1/5] btrfs: remove buffer heads from super block reading

Johannes Thumshirn
2020-01-27T15:59:40Z

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.

Change the buffer_heads to BIOs and utilize the page cache for the page
allocation. Compared to buffer_heads using BIOs are more lightweight and
we skip several layers of buffer_head code until we either reach the page
cache or build a BIO and submit it to read the blocks from disk.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>

---
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 | 89 ++++++++++++++++++++++++++++++----------------
fs/btrfs/disk-io.h | 4 +--
fs/btrfs/volumes.c | 66 +++++++++++++++++++++-------------
fs/btrfs/volumes.h | 2 --
4 files changed, 102 insertions(+), 59 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index aea48d6ddc0c..1f30b234ac07 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2635,11 +2635,12 @@ 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;
@@ -2832,28 +2833,33 @@ 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;
}

@@ -2861,10 +2867,11 @@ 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;
}

@@ -2873,8 +2880,9 @@ 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;

@@ -3374,40 +3382,61 @@ 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 bio_vec bio_vec;
+ struct bio bio;
+ struct page *page;
u64 bytenr;
+ struct address_space *mapping = bdev->bd_inode->i_mapping;
+ gfp_t gfp_mask;
+ int ret;

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);
+ gfp_mask = mapping_gfp_constraint(mapping, ~__GFP_FS) | __GFP_NOFAIL;
+ page = find_or_create_page(mapping, bytenr >> PAGE_SHIFT, gfp_mask);
+ if (!page)
+ return -ENOMEM;
+
+ bio_init(&bio, &bio_vec, 1);
+ bio.bi_iter.bi_sector = bytenr >> SECTOR_SHIFT;
+ bio_set_dev(&bio, bdev);
+ bio.bi_opf = REQ_OP_READ;
+ bio_add_page(&bio, page, BTRFS_SUPER_INFO_SIZE,
+ offset_in_page(bytenr));
+
+ ret = submit_bio_wait(&bio);
+ unlock_page(page);
/*
* If we fail to read from the underlying devices, as of now
* the best option we have is to mark it EIO.
*/
- if (!bh)
+ if (ret) {
+ put_page(page);
return -EIO;
+ }

- 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;
@@ -3419,25 +3448,25 @@ 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 8c2d6cf1ce59..e04b233c436a 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_fs_root(struct btrfs_root *tree_root,
struct btrfs_key *location);
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 9cfc668f91f4..fd8b3db14d62 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);
@@ -7319,25 +7320,40 @@ 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 bio_vec bio_vec;
+ struct bio bio;
struct btrfs_super_block *disk_super;
int copy_num;

if (!bdev)
return;

+ bio_init(&bio, &bio_vec, 1);
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);
+
+ bio.bi_iter.bi_sector = bytenr >> SECTOR_SHIFT;
+ bio_set_dev(&bio, bdev);
+ bio.bi_opf = REQ_OP_WRITE;
+ bio_add_page(&bio, page, BTRFS_SUPER_INFO_SIZE,
+ offset_in_page(bytenr));
+
+ lock_page(page);
+ submit_bio_wait(&bio);
+ unlock_page(page);
+ kunmap(page);
+ put_page(page);
+ bio_reset(&bio);
+
}

/* 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;
--
2.24.1