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

[Chao Yu]

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; ?

>  	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?

> +		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.
> +	 */
> +	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.

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?

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)
>