Re: [f2fs-dev] [PATCH] f2fs: modify the procedure of scan free nid

[Chao Yu]

On 2017/11/2 10:38, Fan Li wrote:
> 
> 
>> -----Original Message-----
>> From: Chao Yu [mailto:chao@xxxxxxxxxx]
>> Sent: Wednesday, November 01, 2017 8:47 PM
>> To: Fan Li; 'Jaegeuk Kim'
>> Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
>> Subject: Re: [f2fs-dev] [PATCH] f2fs: modify the procedure of scan free nid
>>
>> On 2017/11/1 18:03, Fan Li wrote:
>>>
>>>
>>>> -----Original Message-----
>>>> From: Chao Yu [mailto:chao@xxxxxxxxxx]
>>>> Sent: Tuesday, October 31, 2017 10:32 PM
>>>> To: Fan Li; 'Jaegeuk Kim'
>>>> Cc: linux-kernel@xxxxxxxxxxxxxxx;
>>>> linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
>>>> Subject: Re: [f2fs-dev] [PATCH] f2fs: modify the procedure of scan
>>>> free nid
>>>>
>>>> On 2017/10/31 21:37, Fan Li wrote:
>>>>> In current version, we preserve 8 pages of nat blocks as free nids,
>>>>> build bitmaps for it and use them to allocate nids until its number
>>>>> drops below NAT_ENTRY_PER_BLOCK.
>>>>>
>>>>> After that, we have a problem, scan_free_nid_bits will scan the same
>>>>> 8 pages trying to find more free nids, but in most cases the free
>>>>> nids in these bitmaps are already in free list, scan them won't get
>>>>> us any new nids.
>>>>> Further more, after scan_free_nid_bits, the search is over if
>>>>> nid_cnt[FREE_NID] != 0.
>>>>> It causes that we scan the same pages over and over again, yet no
>>>>> new free nids are found until nid_cnt[FREE_NID]==0.
>>>>>
>>>>> This patch mark the range where new free nids could exist and keep
>>>>> scan for free nids until nid_cnt[FREE_NID] >= NAT_ENTRY_PER_BLOCK.
>>>>> The new vairable first_scan_block marks the start of the range, it's
>>>>> initialized with NEW_ADDR, which means all free nids before
>>>>> next_scan_nid are already in free list; and use next_scan_nid as the
>>>>> end of the range since all free nids which are scanned must be
>>>>> smaller next_scan_nid.
>>>>>
>>>>>
>>>>> Signed-off-by: Fan li <fanofcode.li@xxxxxxxxxxx>
>>>>> ---
>>>>>  fs/f2fs/f2fs.h |  1 +
>>>>>  fs/f2fs/node.c | 30 ++++++++++++++++++++++++++----
>>>>>  2 files changed, 27 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h index e0ef31c..ae1cf91
>>>>> 100644
>>>>> --- a/fs/f2fs/f2fs.h
>>>>> +++ b/fs/f2fs/f2fs.h
>>>>> @@ -705,6 +705,7 @@ struct f2fs_nm_info {
>>>>>  	nid_t max_nid;			/* maximum possible node ids */
>>>>>  	nid_t available_nids;		/* # of available node ids */
>>>>>  	nid_t next_scan_nid;		/* the next nid to be scanned */
>>>>> +	block_t first_scan_block;       /* the first NAT block to be scanned */
>>>>
>>>> As we are traveling bitmap, so how about using smaller granularity for tracking last-scanned-position. like:
>>>>
>>>> unsigned next_bitmap_pos; ?
>>>>
>>> Yes, I think it's a good idea, but original code scans nids by blocks,
>>> if I change that, I need to change some other details too, and before that, I want to make sure this idea of patch is right.
>>> I also have some ideas about it, if that's OK, I tend to submit other patches to implement them.
>>>
>>>>>  	unsigned int ram_thresh;	/* control the memory footprint */
>>>>>  	unsigned int ra_nid_pages;	/* # of nid pages to be readaheaded */
>>>>>  	unsigned int dirty_nats_ratio;	/* control dirty nats ratio threshold */
>>>>> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c index 3d0d1be..7834097
>>>>> 100644
>>>>> --- a/fs/f2fs/node.c
>>>>> +++ b/fs/f2fs/node.c
>>>>> @@ -1950,10 +1950,23 @@ static void scan_free_nid_bits(struct f2fs_sb_info *sbi)
>>>>>  	struct curseg_info *curseg = CURSEG_I(sbi, CURSEG_HOT_DATA);
>>>>>  	struct f2fs_journal *journal = curseg->journal;
>>>>>  	unsigned int i, idx;
>>>>> +	unsigned int max_blocks = NAT_BLOCK_OFFSET(nm_i->next_scan_nid);
>>>>>
>>>>> -	down_read(&nm_i->nat_tree_lock);
>>>>> +	/* every free nid in blocks scanned previously is in the free list */
>>>>> +	if (nm_i->first_scan_block == NEW_ADDR)
>>>>
>>>> How about using nm_i->max_nid as no more free nids in bitmap?
>>>>
>>> For now, I use the block as the unit of variable first_scan_block, for
>>> the same reason above, I tend to change it in another patch.
>>>
>>>>> +		return;
>>>>>
>>>>> -	for (i = 0; i < nm_i->nat_blocks; i++) {
>>>>> +	/*
>>>>> +	 * TODO: "next_scan_nid == 0" means after searching every nat block,
>>>>> +	 *       we still can't find enough free nids, there may not be any
>>>>> +	 *       more nid left to be found, we should stop at somewhere
>>>>> +	 *       instead of going through these all over again.
>>>>> +	 */
>>
>> How about trying avoid todo thing in our patch, if our new feature is not so complicate or big.
>>
> Sure, I will delete this.
> 
>>>>> +	if (max_blocks == 0)
>>>>> +		max_blocks = nm_i->nat_blocks;
>>>>> +
>>>>> +	down_read(&nm_i->nat_tree_lock);
>>>>> +	for (i = nm_i->first_scan_block; i < max_blocks; i++) {
>>>>
>>>> Free nids could be set free after nodes were truncated & checkpoint,
>>>> if we start from first_scan_block, we will miss some free
>>> nids.
>>>>
>>> This is the part I'm not sure. To my understanding, after nodes were
>>> truncated, the nats will be cached as dirty nats, the IS_CHECKPOINTED
>>> flag will be removed from them, as a result, in original code these nats will not be added to free list in scan, so I also
> didn't add these nats
>> in this patch, but I don't know why it's designed this way in the first place.
>>> Please tell me what's wrong about my understanding or why it's like this.
>>> And what do you mean by the free nid which could be set free after checkpoint?
>>
>> You can check the code in __flush_nat_entries:
>>
>> 		if (nat_get_blkaddr(ne) == NULL_ADDR) {
>> 			add_free_nid(sbi, nid, false);
>> 			spin_lock(&NM_I(sbi)->nid_list_lock);
>> 			NM_I(sbi)->available_nids++;
>> 			update_free_nid_bitmap(sbi, nid, true, false);
>> 			spin_unlock(&NM_I(sbi)->nid_list_lock);
>> 		}
>>
>> I mean that we will try to:
>> 1. add_free_nid
>> 2. update_free_nid_bitmap
>>
>> But, you know, there is no guarantee that add_free_nid will success, so nid is been set free just in bitmap, if we do not update
>> first_scan_block here, we may lose chance to scan that bitmap, right?
>>
>> Thanks,
>>
> Now I see it, thanks.
> To be clear, those dirty NULL nats without IS_CHECKPOINTED flag weren't added to the free list in the old codes 
> and still don't need to be added in this patch, right?

Currently, we will call nat_reset_flag in __flush_nat_entry_set to tag
each nat entry with IS_CHECKPOINTED flag, and only try to pick nat entry
with NULL blkaddr into free list. You can check all flew in
__flush_nat_entry_set for details.

> I only need to add those nats which couldn't be added due to system failure, like out of memory or 
> errors of the insertion to radix tree?

I think in this patch we can be aware of that kind of failure, and try to
update first_scan_block if nid is failed to add into free list, like:

if (nat_get_blkaddr(ne) == NULL_ADDR) {
	if (!add_free_nid(sbi, nid, false))
		update_position = true;
	spin_lock(&NM_I(sbi)->nid_list_lock);
	NM_I(sbi)->first_scan_block = NAT_BLOCK_OFFSET(nid);
	NM_I(sbi)->available_nids++;
	update_free_nid_bitmap(sbi, nid, true, false);
	spin_unlock(&NM_I(sbi)->nid_list_lock);
}

Thanks,

> I'm away for quite a while, there are some new development in f2fs I'm still catching up, if there's anything
> else in this patch that doesn't fit in, please let me know, thanks.
> 
>>>
>>>> Thanks,
>>>>
>>>>>  		if (!test_bit_le(i, nm_i->nat_block_bitmap))
>>>>>  			continue;
>>>>>  		if (!nm_i->free_nid_count[i])
>>>>> @@ -1967,10 +1980,13 @@ static void scan_free_nid_bits(struct f2fs_sb_info *sbi)
>>>>>  			nid = i * NAT_ENTRY_PER_BLOCK + idx;
>>>>>  			add_free_nid(sbi, nid, true);
>>>>>
>>>>> -			if (nm_i->nid_cnt[FREE_NID] >= MAX_FREE_NIDS)
>>>>> +			if (nm_i->nid_cnt[FREE_NID] >= MAX_FREE_NIDS) {
>>>>> +				nm_i->first_scan_block = i;
>>>>>  				goto out;
>>>>> +			}
>>>>>  		}
>>>>>  	}
>>>>> +	nm_i->first_scan_block = NEW_ADDR;
>>>>>  out:
>>>>>  	down_read(&curseg->journal_rwsem);
>>>>>  	for (i = 0; i < nats_in_cursum(journal); i++) { @@ -2010,7 +2026,7
>>>>> @@ static void __build_free_nids(struct f2fs_sb_info *sbi, bool sync, bool mount)
>>>>>  		/* try to find free nids in free_nid_bitmap */
>>>>>  		scan_free_nid_bits(sbi);
>>>>>
>>>>> -		if (nm_i->nid_cnt[FREE_NID])
>>>>> +		if (nm_i->nid_cnt[FREE_NID] >= NAT_ENTRY_PER_BLOCK)
>>>>>  			return;
>>>>>  	}
>>>>>
>>>>> @@ -2163,6 +2179,7 @@ int try_to_free_nids(struct f2fs_sb_info *sbi, int nr_shrink)
>>>>>  	struct f2fs_nm_info *nm_i = NM_I(sbi);
>>>>>  	struct free_nid *i, *next;
>>>>>  	int nr = nr_shrink;
>>>>> +	nid_t min_nid = nm_i->max_nid;
>>>>>
>>>>>  	if (nm_i->nid_cnt[FREE_NID] <= MAX_FREE_NIDS)
>>>>>  		return 0;
>>>>> @@ -2176,11 +2193,15 @@ int try_to_free_nids(struct f2fs_sb_info *sbi, int nr_shrink)
>>>>>  				nm_i->nid_cnt[FREE_NID] <= MAX_FREE_NIDS)
>>>>>  			break;
>>>>>
>>>>> +		if (i->nid < min_nid)
>>>>> +			min_nid = i->nid;
>>>>>  		__remove_free_nid(sbi, i, FREE_NID);
>>>>>  		kmem_cache_free(free_nid_slab, i);
>>>>>  		nr_shrink--;
>>>>>  	}
>>>>>  	spin_unlock(&nm_i->nid_list_lock);
>>>>> +	if (min_nid != nm_i->max_nid)
>>>>> +		nm_i->first_scan_block = NAT_BLOCK_OFFSET(min_nid);
>>>>
>>>> Need to update nm_i->first_scan_block during __flush_nat_entry_set?
>>>>
>>> The doubt I have is described in above question.
>>>
>>>> Thanks,
>>>>
>>>>>  	mutex_unlock(&nm_i->build_lock);
>>>>>
>>>>>  	return nr - nr_shrink;
>>>>> @@ -2674,6 +2695,7 @@ static int init_node_manager(struct f2fs_sb_info *sbi)
>>>>>  	init_rwsem(&nm_i->nat_tree_lock);
>>>>>
>>>>>  	nm_i->next_scan_nid = le32_to_cpu(sbi->ckpt->next_free_nid);
>>>>> +	nm_i->first_scan_block = NEW_ADDR;
>>>>>  	nm_i->bitmap_size = __bitmap_size(sbi, NAT_BITMAP);
>>>>>  	version_bitmap = __bitmap_ptr(sbi, NAT_BITMAP);
>>>>>  	if (!version_bitmap)
>>>>>
>>>
>>>
> 
>