On Wed, Dec 14, 2022 at 06:01:25PM +0100, Jan Kara wrote:
>
> Besides some naming nits (see below) I think this should work. But I have
> to say I'm a bit uneasy about this because we will now be changing block
> mapping from unwritten to written only with shared i_rwsem. OTOH that
> happens during writeback as well so we should be fine and the gain is very
> nice.
Hmm.... when I was looking potential impacts of the change what
ext4_overwrite_io() would do, I looked at the current user of that
function in ext4_dio_write_checks().
/*
* Determine whether the IO operation will overwrite allocated
* and initialized blocks.
* We need exclusive i_rwsem for changing security info
* in file_modified().
*/
if (*ilock_shared && (!IS_NOSEC(inode) || *extend ||
!ext4_overwrite_io(inode, offset, count))) {
if (iocb->ki_flags & IOCB_NOWAIT) {
ret = -EAGAIN;
goto out;
}
inode_unlock_shared(inode);
*ilock_shared = false;
inode_lock(inode);
goto restart;
}
ret = file_modified(file);
if (ret < 0)
goto out;
What is confusing me is the comment, "We need exclusive i_rwsem for
changing security info in file_modified().". But then we end up
calling file_modified() unconditionally, regardless of whether we've
transitioned from a shared lock to an exclusive lock.
So file_modified() can get called either with or without the inode
locked r/w. I realize that this patch doesn't change this
inconsistency, but it appears either the comment is wrong, or the code
is wrong.
What am I missing?
- Ted
