Re: [PATCH] f2fs: skip checkpoint if having a dirty segment but no prefree at BG_GC -> FG_GC

[Jaegeuk Kim <jaegeuk@kernel.org>]

On 02/25, guoweichao wrote:
> Hi Jaegeuk,
> 
> I regard no enough free sections as a precondition when talking about
> BG_GC -> FG_GC. I mean that for both case a) and b) I mentioned has no enough
> free sections implicitly. 
> 
> On 2017/2/25 2:49, Jaegeuk Kim wrote:
> > Hi Weichao,
> > 
> > On 02/25, Weichao Guo wrote:
> >> When turning to FG_GC from BG_GC, we need to write checkpoint in 2 cases:
> >> * a) BG_GC have made some progress, e.g.: some prefree segments.
> >> * b) There is no victim and no prefree segment.
> > 
> > You missed
> >   * c) has_not_enough_free_secs() introduced by
> >       6e17bfbc75a5cb ("f2fs: fix to overcome inline_data floods")
> As we have enabled SSR for warm node(5b6c6be2d8 ("f2fs: use SSR for warm node as well")),
> I think inline data floods should not be a problem in most cases.
> > 
> > And, Yunlong pointed that we can't find a case to avoid write_checkpoint()
> > mostly due to c) condition.
> As inline data floods is an extreme case, and there is little possibility caused panic
> for inline data floods now, there should be lots of chance to skip checkpoint. I mean
> that we can make some accurate inline data floods checking before writing checkpoint.

For now, the safest way is our first option. So, I decided to start with doing
checkpoint due to previous inline_data flooding issue even though it's an
extreme case under SSR.

Anyway, I agree that we need to find a way to detect when to avoid checkpoint.

Thanks,

> > 
> > Thanks,
> > 
> >>
> >> For case a), previously, we also check if there is a dirty segment for
> >> infering blocks moving in last BG_GC. But dirty segments do not always
> >> indicate that, BG_GC may just start and do not move any blocks at all.
> >> Futhermore, skipping checkpoint if there are some dirty segments but no
> >> prefree segments is OK.
> > 
> > 
> >>
> >> Signed-off-by: Weichao Guo <guoweichao@huawei.com>
> >> ---
> >>  fs/f2fs/gc.c | 7 ++++++-
> >>  1 file changed, 6 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
> >> index 6c996e3..30d206a 100644
> >> --- a/fs/f2fs/gc.c
> >> +++ b/fs/f2fs/gc.c
> >> @@ -958,7 +958,12 @@ int f2fs_gc(struct f2fs_sb_info *sbi, bool sync, bool background)
> >>  		 * enough free sections, we should flush dent/node blocks and do
> >>  		 * garbage collections.
> >>  		 */
> >> -		ret = write_checkpoint(sbi, &cpc);
> >> +		if (prefree_segments(sbi))
> >> +			ret = write_checkpoint(sbi, &cpc);
> >> +		else if (!__get_victim(sbi, &segno, gc_type) {
> >> +			segno = NULL_SEGNO;
> >> +			ret = write_checkpoint(sbi, &cpc);
> >> +		}
> >>  		if (ret)
> >>  			goto stop;
> >>  	} else if (gc_type == BG_GC && !background) {
> >> -- 
> >> 2.10.1
> > 
> > .
> > 
> 
> Thanks,
> Weichao

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