Re: [PATCH] f2fs: avoid race between zero_range and background GC

[Chao Yu <chao@kernel.org>]

On 2018/7/27 18:29, Jaegeuk Kim wrote:
> On 07/26, Chao Yu wrote:
>> Thread A				Background GC
>> - f2fs_zero_range
>>  - truncate_pagecache_range
>> 					- gc_data_segment
>> 					 - get_read_data_page
>> 					  - move_data_page
>> 					   - set_page_dirty
>> 					   - set_cold_data
>>  - f2fs_do_zero_range
>>   - dn->data_blkaddr = NEW_ADDR;
>>   - f2fs_set_data_blkaddr
>>
>> Actually, we don't need to set dirty & checked flag on the page, since
>> all valid data in the page should be zeroed by zero_range().
> 
> But, it doesn't matter too much, right?

No, if the dirtied page is writebacked after f2fs_do_zero_range(), result of
zero_range() should be wrong, as zeroed page contains valid user data.

> 
>> Use i_gc_rwsem[WRITE] to avoid such race condition.
> 
> Hope to avoid abusing i_gc_rwsem[] tho.

Agreed, let's try avoiding until we have to use it.

Thanks,

> 
>>
>> Signed-off-by: Chao Yu <yuchao0@huawei.com>
>> ---
>>  fs/f2fs/file.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
>> index 267ec3794e1e..7bd2412a8c37 100644
>> --- a/fs/f2fs/file.c
>> +++ b/fs/f2fs/file.c
>> @@ -1309,6 +1309,7 @@ static int f2fs_zero_range(struct inode *inode, loff_t offset, loff_t len,
>>  	if (ret)
>>  		return ret;
>>  
>> +	down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
>>  	down_write(&F2FS_I(inode)->i_mmap_sem);
>>  	ret = filemap_write_and_wait_range(mapping, offset, offset + len - 1);
>>  	if (ret)
>> @@ -1389,6 +1390,7 @@ static int f2fs_zero_range(struct inode *inode, loff_t offset, loff_t len,
>>  	}
>>  out_sem:
>>  	up_write(&F2FS_I(inode)->i_mmap_sem);
>> +	up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
>>  
>>  	return ret;
>>  }
>> -- 
>> 2.18.0.rc1