Message-ID: <dd72cec6-cbc9-4c00-a967-841ec614102f@gmail.com>
Date:   Sun, 10 Dec 2023 20:55:29 +0800
From:   Wu Bo <wubo.oduw@...il.com>
To:     Chao Yu <chao@...nel.org>, Wu Bo <bo.wu@...o.com>,
        Jaegeuk Kim <jaegeuk@...nel.org>
Cc:     linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] f2fs: fix fallocate failed under pinned block
 situation

On 2023/12/9 17:46, Chao Yu wrote:
> On 2023/11/28 20:51, Wu Bo wrote:
>>
>> On 2023/11/28 14:22, Chao Yu wrote:
>>> On 2023/11/17 7:34, Wu Bo wrote:
>>>> On 2023/11/11 12:49, Chao Yu wrote:
>>>>> On 2023/11/8 21:48, Wu Bo wrote:
>>>>>> On 2023/11/7 22:39, Chao Yu wrote:
>>>>>>> On 2023/10/30 17:40, Wu Bo wrote:
>>>>>>>> If GC victim has pinned block, it can't be recycled.
>>>>>>>> And if GC is foreground running, after many failure try, the 
>>>>>>>> pinned file
>>>>>>>> is expected to be clear pin flag. To enable the section be 
>>>>>>>> recycled.
>>>>>>>>
>>>>>>>> But when fallocate trigger FG_GC, GC can never recycle the pinned
>>>>>>>> section. Because GC will go to stop before the failure try meet 
>>>>>>>> the
>>>>>>>> threshold:
>>>>>>>>      if (has_enough_free_secs(sbi, sec_freed, 0)) {
>>>>>>>>          if (!gc_control->no_bg_gc &&
>>>>>>>>              total_sec_freed < gc_control->nr_free_secs)
>>>>>>>>              goto go_gc_more;
>>>>>>>>          goto stop;
>>>>>>>>      }
>>>>>>>>
>>>>>>>> So when fallocate trigger FG_GC, at least recycle one.
>>>>>>>
>>>>>>> Hmm... it may break pinfile's semantics at least on one pinned 
>>>>>>> file?
>>>>>>> In this case, I prefer to fail fallocate() rather than unpinning 
>>>>>>> file,
>>>>>>> in order to avoid leaving invalid LBA references of unpinned 
>>>>>>> file held
>>>>>>> by userspace.
>>>>>>
>>>>>> As f2fs designed now, FG_GC is able to unpin the pinned file.
>>>>>>
>>>>>> fallocate() triggered FG_GC, but can't recycle space. It breaks the
>>>>>> design logic of FG_GC.
>>>>>
>>>>> Yes, contradictoriness exists.
>>>>>
>>>>> IMO, unpin file by GC looks more dangerous, it may cause potential 
>>>>> data
>>>>> corruption w/ below case:
>>>>> 1. app pins file & holds LBAs of data blocks.
>>>>> 2. GC unpins file and migrates its data to new LBAs.
>>>>> 3. other file reuses previous LBAs.
>>>>> 4. app read/write data via previous LBAs.
>>>>>
>>>>> So I suggest to normalize use of pinfile and do not add more unpin 
>>>>> cases
>>>>> in filesystem inner processes.
>>>>>
>>>>>>
>>>>>> This issue is happened in Android OTA scenario. fallocate() always
>>>>>> return failure cause OTA fail.
>>>>>
>>>>> Can you please check why other pinned files were so fragmented 
>>>>> that f2fs_gc()
>>>>> can not recycle one free section?
>>>>>
>>>> Not because pinned files were fragmented, but if the GC victim 
>>>> section has one block is pinned will cause this issue.
>>>>
>>>> If the section don't unpin the block, it can't be recycled. But 
>>>> there is high chance that the pinned section will be chosen next 
>>>> time under f2fs current victim selection strategy.
>>>>
>>>> So if we want to avoid unpin files, I think change victim selection 
>>>> to considering pinned blocks can fix this issue.
>>>
>>> Oh, I get it.
>>>
>>> How about this?
>>>
>>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
>>> index 325dab01a29d..3fb52dec5df8 100644
>>> --- a/fs/f2fs/file.c
>>> +++ b/fs/f2fs/file.c
>>> @@ -1730,7 +1730,10 @@ next_alloc:
>>>              f2fs_down_write(&sbi->gc_lock);
>>>              stat_inc_gc_call_count(sbi, FOREGROUND);
>>>              err = f2fs_gc(sbi, &gc_control);
>>> -            if (err && err != -ENODATA)
>>> +
>>> +            if (err == -EAGAIN)
>>> +                f2fs_balance_fs(sbi, true);
>>> +            else if (err && err != -ENODATA)
>>>                  goto out_err;
>>>          }
>> Do you mean to call f2fs_balance_fs() to recycle one section?
>> But in this situation, f2fs_balance_fs() will return at 
>> enough-free-section check:
>>      if (has_enough_free_secs(sbi, 0, 0))
>>          return;
>
> As you said, there are lots of free segments, so I guess it's fine for
> latter 2m-aligned allocation, and for the case number of free section is
> lower than fggc threshold, we can call f2fs_balance_fs() to reclaim 
> enough
> free sections.
>
> Thanks,
Yes, this make sense. I didn't see allocation will continue after 
f2fs_balance_fs() return.
>
>>>
>>> However, the code won't fix contradictoriness issue, because the 
>>> root cause
>>> is we left fragmented pinned data in filesystem, which should be 
>>> avoided in
>>> GC-reliance LFS filesyetem as much as possible.
>>>
>>> Thanks,
>>>
>>>>
>>>>> Thanks,
>>>>>
>>>>>>
>>>>>>    And this commit changed previous behavior of fallocate():
>>>>>>
>>>>>> Commit 2e42b7f817ac ("f2fs: stop allocating pinned sections if 
>>>>>> EAGAIN
>>>>>> happens")
>>>>>>
>>>>>> Before this commit, if fallocate() meet this situation, it will 
>>>>>> trigger
>>>>>> FG_GC to recycle pinned space finally.
>>>>>>
>>>>>> FG_GC is expected to recycle pinned space when there is no more free
>>>>>> space.  And this is the right time to do it when fallocate() need 
>>>>>> free
>>>>>> space.
>>>>>>
>>>>>> It is weird when f2fs shows enough spare space but can't 
>>>>>> fallocate(). So
>>>>>> I think it should be fixed.
>>>>>>
>>>>>>>
>>>>>>> Thoughts?
>>>>>>>
>>>>>>> Thanks,
>>>>>>>
>>>>>>>>
>>>>>>>> This issue can be reproduced by filling f2fs space as following 
>>>>>>>> layout.
>>>>>>>> Every segment has one block is pinned:
>>>>>>>> +-+-+-+-+-+-+-----+-+
>>>>>>>> | | |p| | | | ... | | seg_n
>>>>>>>> +-+-+-+-+-+-+-----+-+
>>>>>>>> +-+-+-+-+-+-+-----+-+
>>>>>>>> | | |p| | | | ... | | seg_n+1
>>>>>>>> +-+-+-+-+-+-+-----+-+
>>>>>>>> ...
>>>>>>>> +-+-+-+-+-+-+-----+-+
>>>>>>>> | | |p| | | | ... | | seg_n+k
>>>>>>>> +-+-+-+-+-+-+-----+-+
>>>>>>>>
>>>>>>>> And following are steps to reproduce this issue:
>>>>>>>> dd if=/dev/zero of=./f2fs_pin.img bs=2M count=1024
>>>>>>>> mkfs.f2fs f2fs_pin.img
>>>>>>>> mkdir f2fs
>>>>>>>> mount f2fs_pin.img ./f2fs
>>>>>>>> cd f2fs
>>>>>>>> dd if=/dev/zero of=./large_padding bs=1M count=1760
>>>>>>>> ./pin_filling.sh
>>>>>>>> rm padding*
>>>>>>>> sync
>>>>>>>> touch fallocate_40m
>>>>>>>> f2fs_io pinfile set fallocate_40m
>>>>>>>> fallocate -l 41943040 fallocate_40m
>>>>>>>>
>>>>>>>> fallocate always fail with EAGAIN even there has enough free 
>>>>>>>> space.
>>>>>>>>
>>>>>>>> 'pin_filling.sh' is:
>>>>>>>> count=1
>>>>>>>> while :
>>>>>>>> do
>>>>>>>>       # filling the seg space
>>>>>>>>       for i in {1..511}:
>>>>>>>>       do
>>>>>>>>           name=padding_$count-$i
>>>>>>>>           echo write $name
>>>>>>>>           dd if=/dev/zero of=./$name bs=4K count=1 > /dev/null 
>>>>>>>> 2>&1
>>>>>>>>           if [ $? -ne 0 ]; then
>>>>>>>>                   exit 0
>>>>>>>>           fi
>>>>>>>>       done
>>>>>>>>       sync
>>>>>>>>
>>>>>>>>       # pin one block in a segment
>>>>>>>>       name=pin_file$count
>>>>>>>>       dd if=/dev/zero of=./$name bs=4K count=1 > /dev/null 2>&1
>>>>>>>>       sync
>>>>>>>>       f2fs_io pinfile set $name
>>>>>>>>       count=$(($count + 1))
>>>>>>>> done
>>>>>>>>
>>>>>>>> Signed-off-by: Wu Bo <bo.wu@...o.com>
>>>>>>>> ---
>>>>>>>>    fs/f2fs/file.c | 2 +-
>>>>>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>>>
>>>>>>>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
>>>>>>>> index ca5904129b16..e8a13616543f 100644
>>>>>>>> --- a/fs/f2fs/file.c
>>>>>>>> +++ b/fs/f2fs/file.c
>>>>>>>> @@ -1690,7 +1690,7 @@ static int f2fs_expand_inode_data(struct 
>>>>>>>> inode
>>>>>>>> *inode, loff_t offset,
>>>>>>>>                .init_gc_type = FG_GC,
>>>>>>>>                .should_migrate_blocks = false,
>>>>>>>>                .err_gc_skipped = true,
>>>>>>>> -            .nr_free_secs = 0 };
>>>>>>>> +            .nr_free_secs = 1 };
>>>>>>>>        pgoff_t pg_start, pg_end;
>>>>>>>>        loff_t new_size;
>>>>>>>>        loff_t off_end;

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