linux-kernel - Re: [f2fs-dev] [PATCH 3/3] f2fs: flush stale issued discard candidates

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Message-ID: <20181218224351.GC31062@jaegeuk-macbookpro.roam.corp.google.com>
Date:   Tue, 18 Dec 2018 14:43:51 -0800
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <yuchao0@...wei.com>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH 3/3] f2fs: flush stale issued discard
 candidates

On 12/18, Chao Yu wrote:
> On 2018/12/14 13:01, Jaegeuk Kim wrote:
> > Sometimes, I could observe # of issuing_discard to be 1 which blocks background
> > jobs due to is_idle()=false.
> > The only way to get out of it was to trigger gc_urgent. This patch avoids that
> > by checking any candidates as done in the list.
> 
> Well, as below code, once we issued discard commands, we will wait all
> queued discard end their IO, so do you know what flow can cause such
> condition...?

It's very subtle, and I suspect somehow race condition not from here.

> 
> 		issued = __issue_discard_cmd(sbi, &dpolicy);
> 		if (issued > 0) {
> 			__wait_all_discard_cmd(sbi, &dpolicy);
> 
> Or, I doubt that 'issued' statistical info could be wrong.

No, it simply came back to 0 with this patch. So, something is pending
in the queue even if it is done.

Thanks,

> 
> Thanks,
> 
> > 
> > Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
> > ---
> >  fs/f2fs/segment.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> > index 49ea9009ab5a..acbbc924e518 100644
> > --- a/fs/f2fs/segment.c
> > +++ b/fs/f2fs/segment.c
> > @@ -1651,6 +1651,10 @@ static int issue_discard_thread(void *data)
> >  		if (dcc->discard_wake)
> >  			dcc->discard_wake = 0;
> >  
> > +		/* clean up pending candidates before going to sleep */
> > +		if (atomic_read(&dcc->queued_discard))
> > +			__wait_all_discard_cmd(sbi, NULL);
> > +
> >  		if (try_to_freeze())
> >  			continue;
> >  		if (f2fs_readonly(sbi->sb))
> >