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Subject: Re: [PATCH RFC] f2fs: fix to avoid selecting full segment w/ {AT,}SSR
 allocator
To:     Jaegeuk Kim <jaegeuk@kernel.org>
CC:     <linux-f2fs-devel@lists.sourceforge.net>,
        <linux-kernel@vger.kernel.org>, <chao@kernel.org>
References: <20210220094052.64905-1-yuchao0@huawei.com>
 <YFOLNGo+/8sKQ7si@google.com>
 <ec5cda53-d3f4-450c-7567-7bfc68e224f9@huawei.com>
 <YFpy26JZRpZASB7R@google.com>
From:   Chao Yu <yuchao0@huawei.com>
Message-ID: <0a2a17af-8719-6865-554a-f339f367485e@huawei.com>
Date:   Wed, 24 Mar 2021 11:28:09 +0800
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On 2021/3/24 6:59, Jaegeuk Kim wrote:
> On 03/19, Chao Yu wrote:
>> On 2021/3/19 1:17, Jaegeuk Kim wrote:
>>> On 02/20, 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;
>>>
>>> Do we need to change this to check free_slot instead of get_ckpt_valid_blocks()?
>>
>> Jaegeuk,
>>
>> LFS was assigned only for GC case, in this case we are trying to select source
>> section, rather than target segment for SSR/AT_SSR case, so we don't need to
>> check free_slot.
>>
>> - f2fs_gc
>>   - __get_victim
>>    - get_victim(sbi, victim, gc_type, NO_CHECK_TYPE, LFS, 0);
>>
>>>
>>>    732                 if (unlikely(is_sbi_flag_set(sbi, SBI_CP_DISABLED) &&
>>>    733                                         get_ckpt_valid_blocks(sbi, segno) &&
>>>    734                                         p.alloc_mode == LFS))
>>
>> BTW, in LFS mode, GC wants to find source section rather than segment, so we
>> should change to check valid ckpt blocks in every segment of targe section here?
> 
> Alright. I refactored a bit on this patch with new one. Could you please take a look?
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git/commit/?h=dev&id=00152bd7cabd69b4615ebead823ff23887b0e0f7

I see, newly added comment looks good to me.

One more concern is commit title and commit message is out-of-update,
I've revised it in v2:

https://lore.kernel.org/linux-f2fs-devel/20210324031828.67133-1-yuchao0@huawei.com/T/#u

Thanks,

> 
> Thanks,
> 
>>
>> Thanks,
>>
>>>
>>>
>>>> +			}
>>>> +		}
>>>> +
>>>>    		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
>>>> -- 
>>>> 2.29.2
>>> .
>>>
> .
>