Messages in this thread | | | Subject | Re: [f2fs-dev] [PATCH 1/2] f2fs: do not select same victim right again | From | Chao Yu <> | Date | Thu, 19 Sep 2019 08:53:34 +0800 |
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On 2019/9/19 0:47, Jaegeuk Kim wrote: > On 09/18, Chao Yu wrote: >> On 2019/9/18 11:12, Jaegeuk Kim wrote: >>> 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. >> >> I guess I didn't catch your point, do you mean, if we reset it in the end of >> FGGC, we may encounter the loop during BGGC/SSR? > > FGGC failed in a loop and last victim was remained in cur_victim_sec.
It won't run into a loop because we keep below condition?
+ if (gc_type == FG_GC && seg_freed) + sbi->cur_victim_sec = NULL_SEGNO;
if (sync) goto stop;
I meant add below logic in addition:
+ if (gc_type == FG_GC) + sbi->cur_victim_sec = NULL_SEGNO;
mutex_unlock(&sbi->gc_mutex);
Thanks,
> Next FGGC kicked in and did the same thing again. I don't expect BGGC/SSR > wants to select this victim much, since it will have CB policy. > >> >> I meant: >> >> f2fs_gc() >> ... >> >> + if (gc_type == FG_GC) >> + sbi->cur_victim_sec = NULL_SEGNO; >> >> mutex_unlock(&sbi->gc_mutex); >> >> put_gc_inode(&gc_list); >> ... >> >> Thanks, >> >>> >>>> >>>> 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) >>>>>>>>>>> >>>>>>>>> . >>>>>>>>> >>>>>>> . >>>>>>> >>>>> . >>>>> >>> . >>> > . >
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