Date:   Tue, 15 Nov 2016 14:40:45 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
CC:     <linux-f2fs-devel@...ts.sourceforge.net>,
        <linux-kernel@...r.kernel.org>, <chao@...nel.org>
Subject: Re: [PATCH v2] f2fs: don't wait writeback for datas during checkpoint

Hi Jaegeuk,

On 2016/11/15 7:32, Jaegeuk Kim wrote:
> Hi Chao,
> 
> On Mon, Nov 14, 2016 at 07:04:12PM +0800, Chao Yu wrote:
>> Normally, while committing checkpoint, we will wait on all pages to be
>> writebacked no matter the page is data or metadata, so in scenario where
>> there are lots of data IO being submitted with metadata, we may suffer
>> long latency for waiting writeback during checkpoint.
>>
>> Indeed, we only care about persistence for pages with metadata, but not
>> pages with data, as file system consistent are only related to metadate,
>> so in order to avoid encountering long latency in above scenario, let's
>> recognize and reference metadata in submitted IOs, wait writeback only
>> for metadatas.
> 
> Hmm, another concern comes, which is related to GCed data like below scenario.
> 
> 1. Write data X
> 2. Sync
> 3. Move data X by GC
> 4. Checkpoint
> 5. Power-cut
> 
> In this case, we should guarantee data X which was migrated by GC during #3.
> If we don't care about end_io in #4 Checkpoint, we can lose the data after
> #5 Power-cut.
> 
> Any idea?

Yes, good catch. :)

What about tagging these GCed page as cold data through set_cold_data, and clear
the tag in end_io, then we can keep reference count and wait on writeback for them?

Thanks,

> 
> Thanks,
> 
>>
>> Signed-off-by: Chao Yu <yuchao0@...wei.com>
>> ---
>>  fs/f2fs/checkpoint.c |  2 +-
>>  fs/f2fs/data.c       | 36 ++++++++++++++++++++++++++++++++----
>>  fs/f2fs/debug.c      |  7 ++++---
>>  fs/f2fs/f2fs.h       |  8 +++++---
>>  4 files changed, 42 insertions(+), 11 deletions(-)
>>
>> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
>> index 7bece59..bdf8a50 100644
>> --- a/fs/f2fs/checkpoint.c
>> +++ b/fs/f2fs/checkpoint.c
>> @@ -1003,7 +1003,7 @@ static void wait_on_all_pages_writeback(struct f2fs_sb_info *sbi)
>>  	for (;;) {
>>  		prepare_to_wait(&sbi->cp_wait, &wait, TASK_UNINTERRUPTIBLE);
>>  
>> -		if (!atomic_read(&sbi->nr_wb_bios))
>> +		if (!get_pages(sbi, F2FS_WB_META))
>>  			break;
>>  
>>  		io_schedule_timeout(5*HZ);
>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>> index 66d2aee..f52cec3 100644
>> --- a/fs/f2fs/data.c
>> +++ b/fs/f2fs/data.c
>> @@ -29,6 +29,26 @@
>>  #include "trace.h"
>>  #include <trace/events/f2fs.h>
>>  
>> +static bool f2fs_is_meta_data(struct page *page)
>> +{
>> +	struct address_space *mapping = page->mapping;
>> +	struct f2fs_sb_info *sbi;
>> +	struct inode *inode;
>> +
>> +	/* it is bounce page of encrypted regular inode */
>> +	if (!mapping)
>> +		return false;
>> +
>> +	inode = mapping->host;
>> +	sbi = F2FS_I_SB(inode);
>> +
>> +	if ((inode->i_ino == F2FS_META_INO(sbi) &&
>> +			page->index < MAIN_BLKADDR(sbi)) ||
>> +			inode->i_ino ==  F2FS_NODE_INO(sbi) ||
>> +			S_ISDIR(inode->i_mode))
>> +		return true;
>> +	return false;
>> +}
>>  static void f2fs_read_end_io(struct bio *bio)
>>  {
>>  	struct bio_vec *bvec;
>> @@ -73,6 +93,7 @@ static void f2fs_write_end_io(struct bio *bio)
>>  
>>  	bio_for_each_segment_all(bvec, bio, i) {
>>  		struct page *page = bvec->bv_page;
>> +		bool is_meta = f2fs_is_meta_data(page);
>>  
>>  		fscrypt_pullback_bio_page(&page, true);
>>  
>> @@ -80,9 +101,10 @@ static void f2fs_write_end_io(struct bio *bio)
>>  			mapping_set_error(page->mapping, -EIO);
>>  			f2fs_stop_checkpoint(sbi, true);
>>  		}
>> +		dec_page_count(sbi, is_meta ? F2FS_WB_META : F2FS_WB_DATA);
>>  		end_page_writeback(page);
>>  	}
>> -	if (atomic_dec_and_test(&sbi->nr_wb_bios) &&
>> +	if (!get_pages(sbi, F2FS_WB_META) &&
>>  				wq_has_sleeper(&sbi->cp_wait))
>>  		wake_up(&sbi->cp_wait);
>>  
>> @@ -111,7 +133,6 @@ static inline void __submit_bio(struct f2fs_sb_info *sbi,
>>  				struct bio *bio, enum page_type type)
>>  {
>>  	if (!is_read_io(bio_op(bio))) {
>> -		atomic_inc(&sbi->nr_wb_bios);
>>  		if (f2fs_sb_mounted_blkzoned(sbi->sb) &&
>>  			current->plug && (type == DATA || type == NODE))
>>  			blk_finish_plug(current->plug);
>> @@ -272,6 +293,15 @@ void f2fs_submit_page_mbio(struct f2fs_io_info *fio)
>>  		verify_block_addr(sbi, fio->old_blkaddr);
>>  	verify_block_addr(sbi, fio->new_blkaddr);
>>  
>> +	bio_page = fio->encrypted_page ? fio->encrypted_page : fio->page;
>> +
>> +	if (!is_read) {
>> +		bool is_meta;
>> +
>> +		is_meta = f2fs_is_meta_data(bio_page);
>> +		inc_page_count(sbi, is_meta ? F2FS_WB_META : F2FS_WB_DATA);
>> +	}
>> +
>>  	down_write(&io->io_rwsem);
>>  
>>  	if (io->bio && (io->last_block_in_bio != fio->new_blkaddr - 1 ||
>> @@ -284,8 +314,6 @@ void f2fs_submit_page_mbio(struct f2fs_io_info *fio)
>>  		io->fio = *fio;
>>  	}
>>  
>> -	bio_page = fio->encrypted_page ? fio->encrypted_page : fio->page;
>> -
>>  	if (bio_add_page(io->bio, bio_page, PAGE_SIZE, 0) <
>>  							PAGE_SIZE) {
>>  		__submit_merged_bio(io);
>> diff --git a/fs/f2fs/debug.c b/fs/f2fs/debug.c
>> index 2fdf233..f2d87de 100644
>> --- a/fs/f2fs/debug.c
>> +++ b/fs/f2fs/debug.c
>> @@ -50,7 +50,8 @@ static void update_general_status(struct f2fs_sb_info *sbi)
>>  	si->ndirty_files = sbi->ndirty_inode[FILE_INODE];
>>  	si->ndirty_all = sbi->ndirty_inode[DIRTY_META];
>>  	si->inmem_pages = get_pages(sbi, F2FS_INMEM_PAGES);
>> -	si->wb_bios = atomic_read(&sbi->nr_wb_bios);
>> +	si->nr_wb_meta = get_pages(sbi, F2FS_WB_META);
>> +	si->nr_wb_data = get_pages(sbi, F2FS_WB_DATA);
>>  	si->total_count = (int)sbi->user_block_count / sbi->blocks_per_seg;
>>  	si->rsvd_segs = reserved_segments(sbi);
>>  	si->overp_segs = overprovision_segments(sbi);
>> @@ -313,8 +314,8 @@ static int stat_show(struct seq_file *s, void *v)
>>  		seq_printf(s, "  - Inner Struct Count: tree: %d(%d), node: %d\n",
>>  				si->ext_tree, si->zombie_tree, si->ext_node);
>>  		seq_puts(s, "\nBalancing F2FS Async:\n");
>> -		seq_printf(s, "  - inmem: %4d, wb_bios: %4d\n",
>> -			   si->inmem_pages, si->wb_bios);
>> +		seq_printf(s, "  - inmem: %4d, wb_meta: %4d, wb_data: %4d\n",
>> +			   si->inmem_pages, si->nr_wb_meta, si->nr_wb_data);
>>  		seq_printf(s, "  - nodes: %4d in %4d\n",
>>  			   si->ndirty_node, si->node_pages);
>>  		seq_printf(s, "  - dents: %4d in dirs:%4d (%4d)\n",
>> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
>> index 23a937f..4df301c 100644
>> --- a/fs/f2fs/f2fs.h
>> +++ b/fs/f2fs/f2fs.h
>> @@ -682,6 +682,8 @@ enum count_type {
>>  	F2FS_DIRTY_META,
>>  	F2FS_INMEM_PAGES,
>>  	F2FS_DIRTY_IMETA,
>> +	F2FS_WB_META,
>> +	F2FS_WB_DATA,
>>  	NR_COUNT_TYPE,
>>  };
>>  
>> @@ -849,7 +851,6 @@ struct f2fs_sb_info {
>>  	block_t discard_blks;			/* discard command candidats */
>>  	block_t last_valid_block_count;		/* for recovery */
>>  	u32 s_next_generation;			/* for NFS support */
>> -	atomic_t nr_wb_bios;			/* # of writeback bios */
>>  
>>  	/* # of pages, see count_type */
>>  	atomic_t nr_pages[NR_COUNT_TYPE];
>> @@ -1263,7 +1264,8 @@ static inline void inc_page_count(struct f2fs_sb_info *sbi, int count_type)
>>  {
>>  	atomic_inc(&sbi->nr_pages[count_type]);
>>  
>> -	if (count_type == F2FS_DIRTY_DATA || count_type == F2FS_INMEM_PAGES)
>> +	if (count_type == F2FS_DIRTY_DATA || count_type == F2FS_INMEM_PAGES ||
>> +		count_type == F2FS_WB_META || count_type == F2FS_WB_DATA)
>>  		return;
>>  
>>  	set_sbi_flag(sbi, SBI_IS_DIRTY);
>> @@ -2219,7 +2221,7 @@ struct f2fs_stat_info {
>>  	unsigned int ndirty_dirs, ndirty_files, ndirty_all;
>>  	int nats, dirty_nats, sits, dirty_sits, free_nids, alloc_nids;
>>  	int total_count, utilization;
>> -	int bg_gc, wb_bios;
>> +	int bg_gc, nr_wb_meta, nr_wb_data;
>>  	int inline_xattr, inline_inode, inline_dir, orphans;
>>  	unsigned int valid_count, valid_node_count, valid_inode_count, discard_blks;
>>  	unsigned int bimodal, avg_vblocks;
>> -- 
>> 2.8.2.311.gee88674
> 
> .
> 

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