linux-ext4 - Re: [PATCH] ext4: get discard out of jbd2 commit kthread

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Date:   Mon, 17 May 2021 16:52:10 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Wang Jianchao <jianchao.wan9@...il.com>
Cc:     Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext4: get discard out of jbd2 commit kthread

On Mon, May 17, 2021 at 11:57:09AM +0800, Wang Jianchao wrote:
> Right now, discard is issued and waited to be completed in jbd2
> commit kthread context after the logs are committed. When large
> amount of files are deleted and discard is flooding, jbd2 commit
> kthread can be blocked for long time. Then all of the metadata
> operations can be blocked to wait the log space.
> 
> One case is the page fault path with read mm->mmap_sem held, which
> wants to update the file time but has to wait for the log space.
> When other threads in the task wants to do mmap, then write mmap_sem
> is blocked. Finally all of the following read mmap_sem requirements
> are blocked, even the ps command which need to read the /proc/pid/
> -cmdline. Our monitor service which needs to read /proc/pid/cmdline
> used to be blocked for 5 mins.
> 
> This patch moves the discard out of jbd2 kthread context and do it
> in kworker. And drain the kwork when cannot get space in mb buddy.
> This is done out of jbd2 handle and won't block the commit process.
> After that, we could use blk-wbt or other method to throttle the
> discard and needn't to worry it block the jbd2 commit kthread any
> more.

Wouldn't be much simpler to do something like this?

		if (discard_bio) {
-			submit_bio_wait(discard_bio);
-			bio_put(discard_bio);
+			submit_bio(discard_bio);
		}


We're throwing away the return value from submit_bio_wait(), so
there's no real need to wait for I/O to complete so we can fetch the
I/O status.

That way we don't need to move all of this to a kworker context.

     	    	       	       	   - Ted