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Subject: Re: [PATCH v2] f2fs: fix to keep isolation of atomic write
To:     Jaegeuk Kim <jaegeuk@kernel.org>
CC:     <linux-f2fs-devel@lists.sourceforge.net>,
        <linux-kernel@vger.kernel.org>, <chao@kernel.org>
References: <20201230075557.108818-1-yuchao0@huawei.com>
 <YAC9a6quO2VOirLi@google.com>
 <3923906d-f208-f6c2-f121-5e77e8fb6b28@huawei.com>
 <YActuF2es8IGJfGj@google.com>
From:   Chao Yu <yuchao0@huawei.com>
Message-ID: <98ddd61e-1429-5152-10b1-ac267ec4493d@huawei.com>
Date:   Wed, 20 Jan 2021 09:18:28 +0800
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On 2021/1/20 3:06, Jaegeuk Kim wrote:
> On 01/15, Chao Yu wrote:
>> On 2021/1/15 5:53, Jaegeuk Kim wrote:
>>> On 12/30, Chao Yu wrote:
>>>> ThreadA					ThreadB
>>>> - f2fs_ioc_start_atomic_write
>>>> - write
>>>> - f2fs_ioc_commit_atomic_write
>>>>    - f2fs_commit_inmem_pages
>>>>    - f2fs_drop_inmem_pages
>>>>    - f2fs_drop_inmem_pages
>>>>     - __revoke_inmem_pages
>>>> 					- f2fs_vm_page_mkwrite
>>>> 					 - set_page_dirty
>>>> 					  - tag ATOMIC_WRITTEN_PAGE and add page
>>>> 					    to inmem_pages list
>>>>     - clear_inode_flag(FI_ATOMIC_FILE)
>>>> 					- f2fs_vm_page_mkwrite
>>>> 					  - set_page_dirty
>>>> 					   - f2fs_update_dirty_page
>>>> 					    - f2fs_trace_pid
>>>> 					     - tag inmem page private to pid
>>>
>>> Hmm, how about removing fs/f2fs/trace.c to make private more complicated
>>> like this? I think we can get IO traces from tracepoints.
>>
>> Hmm, actually, there is are issues, one is the trace IO, the other is the
>> race issue (atomic_start,commit,drop vs mkwrite) which can make isolation
>> semantics of transaction be broken.
>>
>> Or can we avoid atomic file racing with file mmap?

Otherwise I think we should add i_mmap_sem to avoid the race.

> 
> No, we can't. We may need to find other way to check the race. :)

Well, any thoughts about this issue?

Thanks,

> 
>>
>> - atomic_start			- file_mmap
>> 				 - inode_lock
>> 				 - if (FI_ATOMIC_FILE) return
>>   - inode_lock
>>   - if (FI_MMAP_FILE) return
>>
>> Thanks,
>>
>>>
>>>> 					- truncate
>>>> 					 - f2fs_invalidate_page
>>>> 					 - set page->mapping to NULL
>>>> 					  then it will cause panic once we
>>>> 					  access page->mapping
>>>>
>>>> The root cause is we missed to keep isolation of atomic write in the case
>>>> of commit_atomic_write vs mkwrite, let commit_atomic_write helds i_mmap_sem
>>>> lock to avoid this issue.
>>>>
>>>> Signed-off-by: Chao Yu <yuchao0@huawei.com>
>>>> ---
>>>> v2:
>>>> - use i_mmap_sem to avoid mkwrite racing with below flows:
>>>>    * f2fs_ioc_start_atomic_write
>>>>    * f2fs_drop_inmem_pages
>>>>    * f2fs_commit_inmem_pages
>>>>
>>>>    fs/f2fs/file.c    | 3 +++
>>>>    fs/f2fs/segment.c | 7 +++++++
>>>>    2 files changed, 10 insertions(+)
>>>>
>>>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
>>>> index 4e6d4b9120a8..a48ec650d691 100644
>>>> --- a/fs/f2fs/file.c
>>>> +++ b/fs/f2fs/file.c
>>>> @@ -2050,6 +2050,7 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
>>>>    		goto out;
>>>>    	down_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
>>>> +	down_write(&F2FS_I(inode)->i_mmap_sem);
>>>>    	/*
>>>>    	 * Should wait end_io to count F2FS_WB_CP_DATA correctly by
>>>> @@ -2060,6 +2061,7 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
>>>>    			  inode->i_ino, get_dirty_pages(inode));
>>>>    	ret = filemap_write_and_wait_range(inode->i_mapping, 0, LLONG_MAX);
>>>>    	if (ret) {
>>>> +		up_write(&F2FS_I(inode)->i_mmap_sem);
>>>>    		up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
>>>>    		goto out;
>>>>    	}
>>>> @@ -2073,6 +2075,7 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
>>>>    	/* add inode in inmem_list first and set atomic_file */
>>>>    	set_inode_flag(inode, FI_ATOMIC_FILE);
>>>>    	clear_inode_flag(inode, FI_ATOMIC_REVOKE_REQUEST);
>>>> +	up_write(&F2FS_I(inode)->i_mmap_sem);
>>>>    	up_write(&F2FS_I(inode)->i_gc_rwsem[WRITE]);
>>>>    	f2fs_update_time(F2FS_I_SB(inode), REQ_TIME);
>>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>>>> index d8570b0359f5..dab870d9faf6 100644
>>>> --- a/fs/f2fs/segment.c
>>>> +++ b/fs/f2fs/segment.c
>>>> @@ -327,6 +327,8 @@ void f2fs_drop_inmem_pages(struct inode *inode)
>>>>    	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
>>>>    	struct f2fs_inode_info *fi = F2FS_I(inode);
>>>> +	down_write(&F2FS_I(inode)->i_mmap_sem);
>>>> +
>>>>    	while (!list_empty(&fi->inmem_pages)) {
>>>>    		mutex_lock(&fi->inmem_lock);
>>>>    		__revoke_inmem_pages(inode, &fi->inmem_pages,
>>>> @@ -344,6 +346,8 @@ void f2fs_drop_inmem_pages(struct inode *inode)
>>>>    		sbi->atomic_files--;
>>>>    	}
>>>>    	spin_unlock(&sbi->inode_lock[ATOMIC_FILE]);
>>>> +
>>>> +	up_write(&F2FS_I(inode)->i_mmap_sem);
>>>>    }
>>>>    void f2fs_drop_inmem_page(struct inode *inode, struct page *page)
>>>> @@ -467,6 +471,7 @@ int f2fs_commit_inmem_pages(struct inode *inode)
>>>>    	f2fs_balance_fs(sbi, true);
>>>>    	down_write(&fi->i_gc_rwsem[WRITE]);
>>>> +	down_write(&F2FS_I(inode)->i_mmap_sem);
>>>>    	f2fs_lock_op(sbi);
>>>>    	set_inode_flag(inode, FI_ATOMIC_COMMIT);
>>>> @@ -478,6 +483,8 @@ int f2fs_commit_inmem_pages(struct inode *inode)
>>>>    	clear_inode_flag(inode, FI_ATOMIC_COMMIT);
>>>>    	f2fs_unlock_op(sbi);
>>>> +
>>>> +	up_write(&F2FS_I(inode)->i_mmap_sem);
>>>>    	up_write(&fi->i_gc_rwsem[WRITE]);
>>>>    	return err;
>>>> -- 
>>>> 2.29.2
>>> .
>>>
> .
>