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

[Chao Yu]

On 2019/9/9 16:01, Jaegeuk Kim wrote:
> 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@xxxxxxxxxx>
>>>>> ---
>>>>>  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.

What I mean is, for any error returned from atomic_commit() interface, should
userspace application handle it with consistent way, like trigger
f2fs_drop_inmem_pages(), so we don't need to handle it inside atomic_commit().

> 
>>
>> 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);
>>>>>  	}
>>>>>
>>> .
>>>
> .
>