Re: [f2fs-dev] [PATCH] f2fs: fix retry logic in f2fs_write_cache_pages()

[Chao Yu]

On 2020/6/1 17:20, Sahitya Tummala wrote:
> Hi Chao,
> 
> Can you please help review the diff given by Jaegeuk below?
> If it looks good, I can post a v2.

Oops, I missed to review that patch...

> 
> Thanks,
> 
> On Thu, May 28, 2020 at 12:18:39PM -0700, Jaegeuk Kim wrote:
>> On 05/28, Chao Yu wrote:
>>> On 2020/5/28 10:45, Chao Yu wrote:
>>>> On 2020/5/27 10:20, Sahitya Tummala wrote:
>>>>> In case a compressed file is getting overwritten, the current retry
>>>>> logic doesn't include the current page to be retried now as it sets
>>>>> the new start index as 0 and new end index as writeback_index - 1.
>>>>> This causes the corresponding cluster to be uncompressed and written
>>>>> as normal pages without compression. Fix this by allowing writeback to
>>>>> be retried for the current page as well (in case of compressed page
>>>>> getting retried due to index mismatch with cluster index). So that
>>>>> this cluster can be written compressed in case of overwrite.
>>>>>
>>>>> Signed-off-by: Sahitya Tummala <stummala@xxxxxxxxxxxxxx>
>>>>> ---
>>>>>  fs/f2fs/data.c | 2 +-
>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>>>>> index 4af5fcd..bfd1df4 100644
>>>>> --- a/fs/f2fs/data.c
>>>>> +++ b/fs/f2fs/data.c
>>>>> @@ -3024,7 +3024,7 @@ static int f2fs_write_cache_pages(struct address_space *mapping,
>>>>>  	if ((!cycled && !done) || retry) {
>>>>
>>>> IMO, we add retry logic in wrong place, you can see that cycled value is
>>>> zero only if wbc->range_cyclic is true, in that case writeback_index is valid.
>>>>
>>>> However if retry is true and wbc->range_cyclic is false, then writeback_index
>>>> would be uninitialized variable.
>>>>
>>>> Thoughts?
>>>>
>>>> Thanks,
>>>>
>>>>>  		cycled = 1;
>>>>>  		index = 0;
>>>>> -		end = writeback_index - 1;
>>>
>>> BTW, I notice that range_cyclic writeback flow was refactored in below commit,
>>> and skeleton of f2fs.writepages was copied from mm/page-writeback.c::write_cache_pages(),
>>> I guess we need follow that change.
>>>
>>> 64081362e8ff ("mm/page-writeback.c: fix range_cyclic writeback vs writepages deadlock")
>>
>> Is that something like this?
>>
>> ---
>>  fs/f2fs/data.c | 11 ++---------
>>  1 file changed, 2 insertions(+), 9 deletions(-)
>>
>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>> index 48a622b95b76e..28fcdf0d4dcb9 100644
>> --- a/fs/f2fs/data.c
>> +++ b/fs/f2fs/data.c
>> @@ -2861,7 +2861,6 @@ static int f2fs_write_cache_pages(struct address_space *mapping,
>>  	pgoff_t index;
>>  	pgoff_t end;		/* Inclusive */
>>  	pgoff_t done_index;
>> -	int cycled;
>>  	int range_whole = 0;
>>  	xa_mark_t tag;
>>  	int nwritten = 0;
>> @@ -2879,17 +2878,12 @@ static int f2fs_write_cache_pages(struct address_space *mapping,
>>  	if (wbc->range_cyclic) {
>>  		writeback_index = mapping->writeback_index; /* prev offset */
>>  		index = writeback_index;
>> -		if (index == 0)
>> -			cycled = 1;
>> -		else
>> -			cycled = 0;
>>  		end = -1;
>>  	} else {
>>  		index = wbc->range_start >> PAGE_SHIFT;
>>  		end = wbc->range_end >> PAGE_SHIFT;
>>  		if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
>>  			range_whole = 1;
>> -		cycled = 1; /* ignore range_cyclic tests */
>>  	}
>>  	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
>>  		tag = PAGECACHE_TAG_TOWRITE;
>> @@ -3054,10 +3048,9 @@ static int f2fs_write_cache_pages(struct address_space *mapping,
>>  		}
>>  	}
>>  #endif
>> -	if ((!cycled && !done) || retry) {
>> -		cycled = 1;
>> +	if (retry) {
>>  		index = 0;
>> -		end = writeback_index - 1;
>> +		end = -1;
>>  		goto retry;
>>  	}

+	if (wbc->range_cyclic && !done)
+		done_index = 0;

Thanks,

>>  	if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
>> -- 
>> 2.27.0.rc0.183.gde8f92d652-goog
>>
>