Re: ext4 out of order when use cfq scheduler

[Theodore Ts'o <tytso@mit.edu>]

On Mon, Mar 14, 2016 at 08:39:28AM +0100, Jan Kara wrote:
> No, that won't be enough. blkdev_issue_flush() is not guaranteed to do
> anything to IOs which have not reported completion before
> blkdev_issue_flush() was called. Specifically, CFQ will queue submitted bio
> in its internal RB tree, following flush request completely bypasses this
> tree and goes directly to the disk where it flushes caches. And only later
> CFQ decides to schedule async writeback from the flusher thread which is
> queued in the RB tree...

Oh, right.  I am forgetting about the flushing mahchinery rewrite.
Thanks for pointing that out.

But what we *could* do is to swap those two calls and then in the case
where delalloc is enabled, could maintain a list of inodes where we
only need to call filemap_fdatawait(), and not initiate writeback for
any dirty pages which had been caused by non-allocating writes.

						- Ted