linux-kernel - Re: [f2fs-dev] [PATCH 1/2] f2fs: do not select same victim right again

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Message-ID: <20190918031257.GA82722@jaegeuk-macbookpro.roam.corp.google.com>
Date:   Tue, 17 Sep 2019 20:12:57 -0700
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 1/2] f2fs: do not select same victim right
 again

On 09/18, Chao Yu wrote:
> On 2019/9/18 4:55, Jaegeuk Kim wrote:
> > On 09/17, Chao Yu wrote:
> >> On 2019/9/16 23:37, Jaegeuk Kim wrote:
> >>> On 09/16, Chao Yu wrote:
> >>>> On 2019/9/9 20:04, Jaegeuk Kim wrote:
> >>>>> On 09/09, Chao Yu wrote:
> >>>>>> On 2019/9/9 16:06, Jaegeuk Kim wrote:
> >>>>>>> On 09/09, Chao Yu wrote:
> >>>>>>>> On 2019/9/9 9:25, Jaegeuk Kim wrote:
> >>>>>>>>> GC must avoid select the same victim again.
> >>>>>>>>
> >>>>>>>> Blocks in previous victim will occupy addition free segment, I doubt after this
> >>>>>>>> change, FGGC may encounter out-of-free space issue more frequently.
> >>>>>>>
> >>>>>>> Hmm, actually this change seems wrong by sec_usage_check().
> >>>>>>> We may be able to avoid this only in the suspicious loop?
> >>>>>>>
> >>>>>>> ---
> >>>>>>>  fs/f2fs/gc.c | 2 +-
> >>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>>>>
> >>>>>>> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> >>>>>>> index e88f98ddf396..5877bd729689 100644
> >>>>>>> --- a/fs/f2fs/gc.c
> >>>>>>> +++ b/fs/f2fs/gc.c
> >>>>>>> @@ -1326,7 +1326,7 @@ int f2fs_gc(struct f2fs_sb_info *sbi, bool sync,
> >>>>>>>  		round++;
> >>>>>>>  	}
> >>>>>>>  
> >>>>>>> -	if (gc_type == FG_GC)
> >>>>>>> +	if (gc_type == FG_GC && seg_freed)
> >>>>>>
> >>>>>> That's original solution Sahitya provided to avoid infinite loop of GC, but I
> >>>>>> suggest to find the root cause first, then we added .invalid_segmap for that
> >>>>>> purpose.
> >>>>>
> >>>>> I've checked the Sahitya's patch. So, it seems the problem can happen due to
> >>>>> is_alive or atomic_file.
> >>>>
> >>>> For some conditions, this doesn't help, for example, two sections contain the
> >>>> same fewest valid blocks, it will cause to loop selecting them if it fails to
> >>>> migrate blocks.
> >>>>
> >>>> How about keeping it as it is to find potential bug.
> >>>
> >>> I think it'd be fine to merge this. Could you check the above scenario in more
> >>> detail?
> >>
> >> I haven't saw this in real scenario yet.
> >>
> >> What I mean is if there is a bug (maybe in is_alive()) failing us to GC on one
> >> section, when that bug happens in two candidates, there could be the same
> >> condition that GC will run into loop (select A, fail to migrate; select B, fail
> >> to migrate, select A...).
> >>
> >> But I guess the benefit of this change is, if FGGC fails to migrate block due to
> >> i_gc_rwsem race, selecting another section and later retrying previous one may
> >> avoid lock race, right?
> > 
> > In any case, I think this can avoid potenial GC loop. At least to me, it'd be
> > quite risky, if we remain this just for debugging purpose only.
> 
> Yup,
> 
> One more concern is would this cur_victim_sec remain after FGGC? then BGGC/SSR
> will always skip the section cur_victim_sec points to.

Then, we can get another loop before using it by BGGC/SSR.

> 
> So could we reset cur_victim_sec in the end of FGGC?
> 
> Thanks,
> 
> > 
> >>
> >> Thanks,
> >>
> >>>
> >>> Thanks,
> >>>
> >>>>
> >>>> Thanks,
> >>>>
> >>>>>
> >>>>>>
> >>>>>> Thanks,
> >>>>>>
> >>>>>>>  		sbi->cur_victim_sec = NULL_SEGNO;
> >>>>>>>  
> >>>>>>>  	if (sync)
> >>>>>>>
> >>>>> .
> >>>>>
> >>> .
> >>>
> > .
> >