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Subject: Re: [PATCH RFC] f2fs: fix to avoid selecting full segment w/ {AT,}SSR
 allocator
To:     <jaegeuk@kernel.org>
CC:     Chao Yu <chao@kernel.org>,
        <linux-f2fs-devel@lists.sourceforge.net>,
        <linux-kernel@vger.kernel.org>
References: <20210220094052.64905-1-yuchao0@huawei.com>
 <ad627dde-23f9-cbbc-ce58-7c799161f290@kernel.org>
From:   Chao Yu <yuchao0@huawei.com>
Message-ID: <2b96b5e5-abc0-615f-0220-5d46b8e2c329@huawei.com>
Date:   Tue, 23 Feb 2021 20:26:49 +0800
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Jaegeuk,

Could you please help to review this patch? since I doubt that this
issue can happen in real world... :(

Thanks,

On 2021/2/22 21:43, Chao Yu wrote:
> Ping,
> 
> On 2021/2/20 17:40, Chao Yu wrote:
>> In cp disabling mode, there could be a condition
>> - target segment has 128 ckpt valid blocks
>> - GC migrates 128 valid blocks to other segment (segment is still in
>> dirty list)
>> - GC migrates 384 blocks to target segment (segment has 128 cp_vblocks
>> and 384 vblocks)
>> - If GC selects target segment via {AT,}SSR allocator, however there is
>> no free space in targe segment.
>>
>> Fixes: 4354994f097d ("f2fs: checkpoint disabling")
>> Fixes: 093749e296e2 ("f2fs: support age threshold based garbage collection")
>> Signed-off-by: Chao Yu <yuchao0@huawei.com>
>> ---
>>    fs/f2fs/f2fs.h    |  1 +
>>    fs/f2fs/gc.c      | 17 +++++++++++++----
>>    fs/f2fs/segment.c | 20 ++++++++++++++++++++
>>    3 files changed, 34 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
>> index ed7807103c8e..9c753eff0814 100644
>> --- a/fs/f2fs/f2fs.h
>> +++ b/fs/f2fs/f2fs.h
>> @@ -3376,6 +3376,7 @@ block_t f2fs_get_unusable_blocks(struct f2fs_sb_info *sbi);
>>    int f2fs_disable_cp_again(struct f2fs_sb_info *sbi, block_t unusable);
>>    void f2fs_release_discard_addrs(struct f2fs_sb_info *sbi);
>>    int f2fs_npages_for_summary_flush(struct f2fs_sb_info *sbi, bool for_ra);
>> +bool segment_has_free_slot(struct f2fs_sb_info *sbi, int segno);
>>    void f2fs_init_inmem_curseg(struct f2fs_sb_info *sbi);
>>    void f2fs_save_inmem_curseg(struct f2fs_sb_info *sbi);
>>    void f2fs_restore_inmem_curseg(struct f2fs_sb_info *sbi);
>> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
>> index 86ba8ed0b8a7..a1d8062cdace 100644
>> --- a/fs/f2fs/gc.c
>> +++ b/fs/f2fs/gc.c
>> @@ -392,10 +392,6 @@ static void add_victim_entry(struct f2fs_sb_info *sbi,
>>    		if (p->gc_mode == GC_AT &&
>>    			get_valid_blocks(sbi, segno, true) == 0)
>>    			return;
>> -
>> -		if (p->alloc_mode == AT_SSR &&
>> -			get_seg_entry(sbi, segno)->ckpt_valid_blocks == 0)
>> -			return;
>>    	}
>>    
>>    	for (i = 0; i < sbi->segs_per_sec; i++)
>> @@ -736,6 +732,19 @@ static int get_victim_by_default(struct f2fs_sb_info *sbi,
>>    		if (gc_type == BG_GC && test_bit(secno, dirty_i->victim_secmap))
>>    			goto next;
>>    
>> +		if (unlikely(is_sbi_flag_set(sbi, SBI_CP_DISABLED))) {
>> +			/*
>> +			 * to avoid selecting candidate which has below valid
>> +			 * block distribution:
>> +			 * partial blocks are valid and all left ones are valid
>> +			 * in previous checkpoint.
>> +			 */
>> +			if (p.alloc_mode == SSR || p.alloc_mode == AT_SSR) {
>> +				if (!segment_has_free_slot(sbi, segno))
>> +					goto next;
>> +			}
>> +		}
>> +
>>    		if (is_atgc) {
>>    			add_victim_entry(sbi, &p, segno);
>>    			goto next;
>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>> index 2d5a82c4ca15..deaf57e13125 100644
>> --- a/fs/f2fs/segment.c
>> +++ b/fs/f2fs/segment.c
>> @@ -2650,6 +2650,26 @@ static void __refresh_next_blkoff(struct f2fs_sb_info *sbi,
>>    		seg->next_blkoff++;
>>    }
>>    
>> +bool segment_has_free_slot(struct f2fs_sb_info *sbi, int segno)
>> +{
>> +	struct sit_info *sit = SIT_I(sbi);
>> +	struct seg_entry *se = get_seg_entry(sbi, segno);
>> +	int entries = SIT_VBLOCK_MAP_SIZE / sizeof(unsigned long);
>> +	unsigned long *target_map = SIT_I(sbi)->tmp_map;
>> +	unsigned long *ckpt_map = (unsigned long *)se->ckpt_valid_map;
>> +	unsigned long *cur_map = (unsigned long *)se->cur_valid_map;
>> +	int i, pos;
>> +
>> +	down_write(&sit->sentry_lock);
>> +	for (i = 0; i < entries; i++)
>> +		target_map[i] = ckpt_map[i] | cur_map[i];
>> +
>> +	pos = __find_rev_next_zero_bit(target_map, sbi->blocks_per_seg, 0);
>> +	up_write(&sit->sentry_lock);
>> +
>> +	return pos < sbi->blocks_per_seg;
>> +}
>> +
>>    /*
>>     * This function always allocates a used segment(from dirty seglist) by SSR
>>     * manner, so it should recover the existing segment information of valid blocks
>>
> .
>