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

[Chao Yu]

On 2019/9/9 16:38, Jaegeuk Kim wrote:
> On 09/09, Chao Yu wrote:
>> On 2019/9/9 16:21, Jaegeuk Kim wrote:
>>> On 09/09, Chao Yu wrote:
>>>> 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().
>>>
>>> f2fs_ioc_abort_volatile_write() will be triggered.
>>
>> If userspace can do this, we can get rid of this patch, or am I missing sth?
> 
> We don't know when that will come. And, other threads are waiting for GC here.

Yes, however, even atomic_write won't be called sometimes... that's why we add
handling logic in f2fs_gc().

> 
>>
>> - f2fs_ioc_abort_volatile_write
>>  - f2fs_drop_inmem_pages
>>   - clear_inode_flag(inode, FI_ATOMIC_FILE);
>>   - fi->i_gc_failures[GC_FAILURE_ATOMIC] = 0;
>>   - stat_dec_atomic_write(inode);
>>
>>>
>>>>
>>>>>
>>>>>>
>>>>>> 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.

One possible solution is keeping inode in sbi->inode_lock[ATOMIC_FILE] list
until f2fs_do_sync_file() return, then f2fs_drop_inmem_pages_all() can find and
drop the inode.

Thanks,

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