linux-kernel - Re: [f2fs-dev] [PATCH 2/2] f2fs: avoid infinite GC loop due to stale atomic files

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Date:   Mon, 9 Sep 2019 09:01:08 +0100
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <yuchao0@...wei.com>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH 2/2] f2fs: avoid infinite GC loop due to stale
 atomic files

On 09/09, Chao Yu wrote:
> On 2019/9/9 15:30, Jaegeuk Kim wrote:
> > On 09/09, Chao Yu wrote:
> >> On 2019/9/9 9:25, Jaegeuk Kim wrote:
> >>> If committing atomic pages is failed when doing f2fs_do_sync_file(), we can
> >>> get commited pages but atomic_file being still set like:
> >>>
> >>> - inmem:    0, atomic IO:    4 (Max.   10), volatile IO:    0 (Max.    0)
> >>>
> >>> If GC selects this block, we can get an infinite loop like this:
> >>>
> >>> f2fs_submit_page_bio: dev = (253,7), ino = 2, page_index = 0x2359a8, oldaddr = 0x2359a8, newaddr = 0x2359a8, rw = READ(), type = COLD_DATA
> >>> f2fs_submit_read_bio: dev = (253,7)/(253,7), rw = READ(), DATA, sector = 18533696, size = 4096
> >>> f2fs_get_victim: dev = (253,7), type = No TYPE, policy = (Foreground GC, LFS-mode, Greedy), victim = 4355, cost = 1, ofs_unit = 1, pre_victim_secno = 4355, prefree = 0, free = 234
> >>> f2fs_iget: dev = (253,7), ino = 6247, pino = 5845, i_mode = 0x81b0, i_size = 319488, i_nlink = 1, i_blocks = 624, i_advise = 0x2c
> >>> f2fs_submit_page_bio: dev = (253,7), ino = 2, page_index = 0x2359a8, oldaddr = 0x2359a8, newaddr = 0x2359a8, rw = READ(), type = COLD_DATA
> >>> f2fs_submit_read_bio: dev = (253,7)/(253,7), rw = READ(), DATA, sector = 18533696, size = 4096
> >>> f2fs_get_victim: dev = (253,7), type = No TYPE, policy = (Foreground GC, LFS-mode, Greedy), victim = 4355, cost = 1, ofs_unit = 1, pre_victim_secno = 4355, prefree = 0, free = 234
> >>> f2fs_iget: dev = (253,7), ino = 6247, pino = 5845, i_mode = 0x81b0, i_size = 319488, i_nlink = 1, i_blocks = 624, i_advise = 0x2c
> >>>
> >>> In that moment, we can observe:
> >>>
> >>> [Before]
> >>> Try to move 5084219 blocks (BG: 384508)
> >>>   - data blocks : 4962373 (274483)
> >>>   - node blocks : 121846 (110025)
> >>> Skipped : atomic write 4534686 (10)
> >>>
> >>> [After]
> >>> Try to move 5088973 blocks (BG: 384508)
> >>>   - data blocks : 4967127 (274483)
> >>>   - node blocks : 121846 (110025)
> >>> Skipped : atomic write 4539440 (10)
> >>>
> >>> Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
> >>> ---
> >>>  fs/f2fs/file.c | 10 +++++-----
> >>>  1 file changed, 5 insertions(+), 5 deletions(-)
> >>>
> >>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> >>> index 7ae2f3bd8c2f..68b6da734e5f 100644
> >>> --- a/fs/f2fs/file.c
> >>> +++ b/fs/f2fs/file.c
> >>> @@ -1997,11 +1997,11 @@ static int f2fs_ioc_commit_atomic_write(struct file *filp)
> >>>  			goto err_out;
> >>>  
> >>>  		ret = f2fs_do_sync_file(filp, 0, LLONG_MAX, 0, true);
> >>> -		if (!ret) {
> >>> -			clear_inode_flag(inode, FI_ATOMIC_FILE);
> >>> -			F2FS_I(inode)->i_gc_failures[GC_FAILURE_ATOMIC] = 0;
> >>> -			stat_dec_atomic_write(inode);
> >>> -		}
> >>> +
> >>> +		/* doesn't need to check error */
> >>> +		clear_inode_flag(inode, FI_ATOMIC_FILE);
> >>> +		F2FS_I(inode)->i_gc_failures[GC_FAILURE_ATOMIC] = 0;
> >>> +		stat_dec_atomic_write(inode);
> >>
> >> If there are still valid atomic write pages linked in .inmem_pages, it may cause
> >> memory leak when we just clear FI_ATOMIC_FILE flag.
> > 
> > f2fs_commit_inmem_pages() should have flushed them.
> 
> Oh, we failed to flush its nodes.
> 
> However we won't clear such info if we failed to flush inmen pages, it looks
> inconsistent.
> 
> Any interface needed to drop inmem pages or clear ATOMIC_FILE flag in that two
> error path? I'm not very clear how sqlite handle such error.

f2fs_drop_inmem_pages() did that, but not in this case.

> 
> Thanks,
> 
> > 
> >>
> >> So my question is why below logic didn't handle such condition well?
> >>
> >> f2fs_gc()
> >>
> >> 	if (has_not_enough_free_secs(sbi, sec_freed, 0)) {
> >> 		if (skipped_round <= MAX_SKIP_GC_COUNT ||
> >> 					skipped_round * 2 < round) {
> >> 			segno = NULL_SEGNO;
> >> 			goto gc_more;
> >> 		}
> >>
> >> 		if (first_skipped < last_skipped &&
> >> 				(last_skipped - first_skipped) >
> >> 						sbi->skipped_gc_rwsem) {
> >> 			f2fs_drop_inmem_pages_all(sbi, true);
> > 
> > This is doing nothing, since f2fs_commit_inmem_pages() removed the inode
> > from inmem list.
> > 
> >> 			segno = NULL_SEGNO;
> >> 			goto gc_more;
> >> 		}
> >> 		if (gc_type == FG_GC && !is_sbi_flag_set(sbi, SBI_CP_DISABLED))
> >> 			ret = f2fs_write_checkpoint(sbi, &cpc);
> >> 	}
> >>
> >>>  	} else {
> >>>  		ret = f2fs_do_sync_file(filp, 0, LLONG_MAX, 1, false);
> >>>  	}
> >>>
> > .
> >