Re: [PATCH] f2fs: let __get_victim successfully get a segno in corner case

From: Yunlong Song
Date: Thu Jul 20 2017 - 08:30:07 EST

Hi, Jay,
The distribution is like this, unit is segment counts:
cnt_free: 0 (free blocks)
cnt_full: 25182 (segment which has 512 blocks)
cnt_over: 25192 (segment which valid blocks over fggc_threshold)
cnt_below: 870 (segment which valid blocks below fggc_threshold)

The test is fragment test, add and delete small files over and over.

On 2017/7/18 0:56, Jaegeuk Kim wrote:
> On 07/16, sylinux wrote:
>> In factïthis is not "suppose" case yet, we have already met this problem several times in some test suits for corner case, or I cannot notice that this could happen.
> So, have you taken a look at distribution of valid blocks in that case?
> I'm wondering what distribution can cause this.
>
> Thanks,
>
>>
>>
>>
>> åèçæéçåå
>> On 07/16/2017 09:09, Jaegeuk Kim wrote:
>> Hi Yunlong,
>>
>> On 07/14, Yunlong Song wrote:
>>> Suppose that the valid blocks of each section are all over sbi->fggc_threshold,
>> How about adding a kernel message first to detect your *supposed* sceanrio?
>> If that happens, it'll be a sort of bug which we haven't assumed.
>>
>> Thanks,
>>
>>> and even has_not_enough_free_secs is true, f2fs_gc cannot do its job since the
>>> no_fggc_candidate always returns true. As a result, the reserved segments can be
>>> used up, and finally there is no free segment at all, and get_new_segment cannot
>>> get a free segment, filesystem will trap into a wrong status.
>>>
>>> To fix this, we record the segno which has a rough minimum cost and return it to
>>> __get_victim to continue f2fs_gc's job.
>>>
>>> Signed-off-by: Yunlong Song <yunlong.song@xxxxxxxxxx>
>>> ---
>>> fs/f2fs/gc.c | 19 ++++++++++++++-----
>>> fs/f2fs/segment.h | 17 ++++++++++++++---
>>> 2 files changed, 28 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
>>> index fa3d2e2..965e783 100644
>>> --- a/fs/f2fs/gc.c
>>> +++ b/fs/f2fs/gc.c
>>> @@ -178,6 +178,8 @@ static void select_policy(struct f2fs_sb_info *sbi, int gc_type,
>>> p->offset = 0;
>>> else
>>> p->offset = SIT_I(sbi)->last_victim[p->gc_mode];
>>> +
>>> + p->min_cost_r = UINT_MAX;
>>> }
>>>
>>> static unsigned int get_max_cost(struct f2fs_sb_info *sbi,
>>> @@ -194,7 +196,7 @@ static unsigned int get_max_cost(struct f2fs_sb_info *sbi,
>>> return 0;
>>> }
>>>
>>> -static unsigned int check_bg_victims(struct f2fs_sb_info *sbi)
>>> +static unsigned int check_bg_victims(struct f2fs_sb_info *sbi, struct victim_sel_policy *p)
>>> {
>>> struct dirty_seglist_info *dirty_i = DIRTY_I(sbi);
>>> unsigned int secno;
>>> @@ -208,11 +210,12 @@ static unsigned int check_bg_victims(struct f2fs_sb_info *sbi)
>>> if (sec_usage_check(sbi, secno))
>>> continue;
>>>
>>> - if (no_fggc_candidate(sbi, secno))
>>> + p->cur_segno_r = GET_SEG_FROM_SEC(sbi, secno);
>>> + if (no_fggc_candidate(sbi, secno, p))
>>> continue;
>>>
>>> clear_bit(secno, dirty_i->victim_secmap);
>>> - return GET_SEG_FROM_SEC(sbi, secno);
>>> + return p->cur_segno_r;
>>> }
>>> return NULL_SEGNO;
>>> }
>>> @@ -332,7 +335,7 @@ static int get_victim_by_default(struct f2fs_sb_info *sbi,
>>>
>>> last_victim = sm->last_victim[p.gc_mode];
>>> if (p.alloc_mode == LFS && gc_type == FG_GC) {
>>> - p.min_segno = check_bg_victims(sbi);
>>> + p.min_segno = check_bg_victims(sbi, &p);
>>> if (p.min_segno != NULL_SEGNO)
>>> goto got_it;
>>> }
>>> @@ -369,8 +372,9 @@ static int get_victim_by_default(struct f2fs_sb_info *sbi,
>>> goto next;
>>> if (gc_type == BG_GC && test_bit(secno, dirty_i->victim_secmap))
>>> goto next;
>>> + p.cur_segno_r = segno;
>>> if (gc_type == FG_GC && p.alloc_mode == LFS &&
>>> - no_fggc_candidate(sbi, secno))
>>> + no_fggc_candidate(sbi, secno, &p))
>>> goto next;
>>>
>>> cost = get_gc_cost(sbi, segno, &p);
>>> @@ -403,6 +407,11 @@ static int get_victim_by_default(struct f2fs_sb_info *sbi,
>>> trace_f2fs_get_victim(sbi->sb, type, gc_type, &p,
>>> sbi->cur_victim_sec,
>>> prefree_segments(sbi), free_segments(sbi));
>>> + } else if (has_not_enough_free_secs(sbi, 0, 0)) {
>>> + p.min_segno = p.min_segno_r;
>>> + if (p.alloc_mode == LFS && gc_type == FG_GC)
>>> + clear_bit(GET_SEC_FROM_SEG(sbi, p.min_segno), dirty_i->victim_secmap);
>>> + goto got_it;
>>> }
>>> out:
>>> mutex_unlock(&dirty_i->seglist_lock);
>>> diff --git a/fs/f2fs/segment.h b/fs/f2fs/segment.h
>>> index 6b871b4..7d2d0f3 100644
>>> --- a/fs/f2fs/segment.h
>>> +++ b/fs/f2fs/segment.h
>>> @@ -169,6 +169,9 @@ struct victim_sel_policy {
>>> unsigned int ofs_unit; /* bitmap search unit */
>>> unsigned int min_cost; /* minimum cost */
>>> unsigned int min_segno; /* segment # having min. cost */
>>> + unsigned int min_cost_r; /* rough minimum cost */
>>> + unsigned int min_segno_r; /* segment # having rough min. cost */
>>> + unsigned int cur_segno_r; /* segment # rough process is handling */
>>> };
>>>
>>> struct seg_entry {
>>> @@ -743,11 +746,19 @@ static inline block_t sum_blk_addr(struct f2fs_sb_info *sbi, int base, int type)
>>> }
>>>
>>> static inline bool no_fggc_candidate(struct f2fs_sb_info *sbi,
>>> - unsigned int secno)
>>> + unsigned int secno, struct victim_sel_policy *p)
>>> {
>>> - if (get_valid_blocks(sbi, GET_SEG_FROM_SEC(sbi, secno), true) >=
>>> - sbi->fggc_threshold)
>>> + unsigned int cur_cost;
>>> +
>>> + cur_cost = get_valid_blocks(sbi, GET_SEG_FROM_SEC(sbi, secno), true);
>>> + if (cur_cost >= sbi->fggc_threshold) {
>>> + if (p->min_cost_r > cur_cost) {
>>> + p->min_cost_r = cur_cost;
>>> + p->min_segno_r = p->cur_segno_r;
>>> + }
>>> return true;
>>> + }
>>> +
>>> return false;
>>> }
>>>
>>> --
>>> 1.8.5.2
> .
>


--
Thanks,
Yunlong Song