On Thu, Oct 24, 2019 at 12:19:06PM +0200, Jan Kara wrote:
Correct on both points. Thanks for spotting this! Now I still don't think that calling iput() with running transaction is good. It complicates matters with revoke record reservation but it is also fragile for other reasons - e.g. flush worker could find the allocated inode just before we will call iput() on it, try to write it out, block on starting transaction and we get a deadlock with inode_wait_for_writeback() inside evict(). Now inode *probably* won't be dirty yet by the time we get to ext4_add_nondir() or similar, that's why I say above it's just fragile, not an outright bug.
But we don't ever write the inode itself via inode_wait_for_writeback(), because how ext4 journalling works. (See the comments before ext4_mark_inode_dirty()). And for the special inodes (directories, device nodes, etc.) there's no data dirtyness to worry about. For regular files, we hit this code path when have just created the inode, but were not able to add a link to the parent directory; the fd wasn't been released to userspace yet, so it can't be data dirty either.
So unless I'm missing something, I don't think the deadlock described above is possible?
We can certainly add it to the orphan list if it's necessary, but it's extra overhead and adds a global contention point. So if it's not necessary, I'd rather avoid it if possible, and I think it's safe to do so in this case.
- Ted