Re: [PATCH v2 2/6] btrfs: remove buffer heads from super block reading

[Johannes Thumshirn <Johannes.Thumshirn@xxxxxxx>]

On 23/01/2020 15:14, Nikolay Borisov wrote:
> 
> 
> On 23.01.20 г. 10:18 ч., Johannes Thumshirn wrote:
> 
> <snip>
> 
>> @@ -3374,40 +3378,60 @@ 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_set_op_attrs(&bio, REQ_OP_READ, 0);
>> +	bio_add_page(&bio, page, BTRFS_SUPER_INFO_SIZE,
>> +		     offset_in_page(bytenr));
>> +
>> +	ret = submit_bio_wait(&bio);
>> +	unlock_page(page);
> 
> So this is based on my code where the page is unlocked as soon as it's
> read. However, as per out off-list discussion, I think this page needs
> to be unlocked once the caller of
> btrfs_read_dev_one_super/btrfs_read_dev_super is finished with it.

As I don't have an answer to that yet, I just gave it a try (see 
attached diff) and it instantly deadlocks on mount. So either I did 
something wrong or holding the lock until the callers have finished 
processing it is a bad idea.

Thanks,
	Johannes

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index ac1755793596..7afac96be17f 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2856,6 +2856,7 @@ int __cold open_ctree(struct super_block *sb,
 	ret = btrfs_init_csum_hash(fs_info, csum_type);
 	if (ret) {
 		err = ret;
+		unlock_page(super_page);
 		btrfs_release_disk_super(super_page);
 		goto fail_alloc;
 	}
@@ -2867,6 +2868,7 @@ int __cold open_ctree(struct super_block *sb,
 	if (btrfs_check_super_csum(fs_info, superblock)) {
 		btrfs_err(fs_info, "superblock checksum mismatch");
 		err = -EINVAL;
+		unlock_page(super_page);
 		btrfs_release_disk_super(super_page);
 		goto fail_csum;
 	}
@@ -2877,6 +2879,7 @@ int __cold open_ctree(struct super_block *sb,
 	 * the whole block of INFO_SIZE
 	 */
 	memcpy(fs_info->super_copy, superblock, sizeof(*fs_info->super_copy));
+	unlock_page(super_page);
 	btrfs_release_disk_super(super_page);
 
 	disk_super = fs_info->super_copy;
@@ -3413,7 +3416,6 @@ int btrfs_read_dev_one_super(struct block_device *bdev, int copy_num,
 		     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.
@@ -3426,6 +3428,7 @@ int btrfs_read_dev_one_super(struct block_device *bdev, int copy_num,
 	super = kmap(page);
 	if (btrfs_super_bytenr(super) != bytenr ||
 		    btrfs_super_magic(super) != BTRFS_MAGIC) {
+		unlock_page(page);
 		btrfs_release_disk_super(page);
 		return -EINVAL;
 	}
@@ -3457,11 +3460,14 @@ int btrfs_read_dev_super(struct block_device *bdev, struct page **page)
 		super = kmap(*page);
 
 		if (!latest || btrfs_super_generation(super) > transid) {
-			if (latest)
+			if (latest) {
+				unlock_page(latest);
 				btrfs_release_disk_super(latest);
+			}
 			latest = *page;
 			transid = btrfs_super_generation(super);
 		} else {
+			unlock_page(*page);
 			btrfs_release_disk_super(*page);
 		}
 
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index f4a6ee518f0c..8bb2f9dead63 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -7344,7 +7344,6 @@ void btrfs_scratch_superblocks(struct block_device *bdev, const char *device_pat
 		bio_add_page(&bio, page, BTRFS_SUPER_INFO_SIZE,
 			     offset_in_page(bytenr));
 
-		lock_page(page);
 		submit_bio_wait(&bio);
 		unlock_page(page);
 		btrfs_release_disk_super(page);