On 8.01.21 г. 7:36 ч., Qu Wenruo wrote:

There are several qgroup flush related bugs fixed recently, all of them
are caused by the fact that we can trigger qgroup metadata space
reservation holding a transaction handle.

Thankfully the only situation to trigger above reservation is
btrfs_dirty_inode().

Currently btrfs_dirty_inode() will try join transactio first, then
update the inode.
If btrfs_update_inode() fails with -ENOSPC, then it retry to start
transaction to reserve metadata space.

This not only forces us to reserve metadata space with a transaction
handle hold, but can't handle other errors like -EDQUOT.

This patch will make btrfs_dirty_inode() to call
btrfs_start_transaction() directly without first try joining then
starting, so that in try_flush_qgroup() we won't hold a trans handle.

This will slow down btrfs_dirty_inode() but my fstests doesn't show too
much different for most test cases, thus it may be worthy to skip such
performance "optimization".

btrfs_dirty_inode is called everytime file_update_time is called or
atime is updated. Given the default mount option is to use lazy a_time
I'd say we needn't worry about it. file_update_time, however, is called
within the write path so this change potentially makes file write more
expensive. So instead of testing with fstests it needs proper
benchmarking with fio.

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