Re: [PATCH V4] f2fs: Avoid double lock for cp_rwsem during checkpoint

[Chao Yu]

On 2020/5/10 3:03, Jaegeuk Kim wrote:
> On 05/09, Chao Yu wrote:
>> On 2020/5/9 0:10, Jaegeuk Kim wrote:
>>> Hi Sayali,
>>>
>>> In order to address the perf regression, how about this?
>>>
>>> >From 48418af635884803ffb35972df7958a2e6649322 Mon Sep 17 00:00:00 2001
>>> From: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
>>> Date: Fri, 8 May 2020 09:08:37 -0700
>>> Subject: [PATCH] f2fs: avoid double lock for cp_rwsem during checkpoint
>>>
>>> There could be a scenario where f2fs_sync_node_pages gets
>>> called during checkpoint, which in turn tries to flush
>>> inline data and calls iput(). This results in deadlock as
>>> iput() tries to hold cp_rwsem, which is already held at the
>>> beginning by checkpoint->block_operations().
>>>
>>> Call stack :
>>>
>>> Thread A		Thread B
>>> f2fs_write_checkpoint()
>>> - block_operations(sbi)
>>>  - f2fs_lock_all(sbi);
>>>   - down_write(&sbi->cp_rwsem);
>>>
>>>                         - open()
>>>                          - igrab()
>>>                         - write() write inline data
>>>                         - unlink()
>>> - f2fs_sync_node_pages()
>>>  - if (is_inline_node(page))
>>>   - flush_inline_data()
>>>    - ilookup()
>>>      page = f2fs_pagecache_get_page()
>>>      if (!page)
>>>       goto iput_out;
>>>      iput_out:
>>> 			-close()
>>> 			-iput()
>>>        iput(inode);
>>>        - f2fs_evict_inode()
>>>         - f2fs_truncate_blocks()
>>>          - f2fs_lock_op()
>>>            - down_read(&sbi->cp_rwsem);
>>>
>>> Fixes: 2049d4fcb057 ("f2fs: avoid multiple node page writes due to inline_data")
>>> Signed-off-by: Sayali Lokhande <sayalil@xxxxxxxxxxxxxx>
>>> Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
>>> ---
>>>  fs/f2fs/node.c | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
>>> index 1db8cabf727ef..626d7daca09de 100644
>>> --- a/fs/f2fs/node.c
>>> +++ b/fs/f2fs/node.c
>>> @@ -1870,8 +1870,8 @@ int f2fs_sync_node_pages(struct f2fs_sb_info *sbi,
>>>  				goto continue_unlock;
>>>  			}
>>>  
>>> -			/* flush inline_data */
>>> -			if (is_inline_node(page)) {
>>> +			/* flush inline_data, if it's not sync path. */
>>> +			if (do_balance && is_inline_node(page)) {
>>
>> IIRC, this flow was designed to avoid running out of free space issue
>> during checkpoint:
>>
>> 2049d4fcb057 ("f2fs: avoid multiple node page writes due to inline_data")
>>
>> The sceanrio is:
>> 1. create fully node blocks
>> 2. flush node blocks
>> 3. write inline_data for all the node blocks again
>> 4. flush node blocks redundantly
>>
>> I guess this may cause failing one case of fstest.
> 
> Yeah, actually I was hitting 204 failure, and thus, revised like this.
> Now, I don't see any regression in fstest.
> 
>>From 8f1882acfb0a5fc43e5a2bbd576a8f3c681a7d2c Mon Sep 17 00:00:00 2001
> From: Sayali Lokhande <sayalil@xxxxxxxxxxxxxx>
> Date: Thu, 30 Apr 2020 16:28:29 +0530
> Subject: [PATCH] f2fs: Avoid double lock for cp_rwsem during checkpoint
> 
> There could be a scenario where f2fs_sync_node_pages gets
> called during checkpoint, which in turn tries to flush
> inline data and calls iput(). This results in deadlock as
> iput() tries to hold cp_rwsem, which is already held at the
> beginning by checkpoint->block_operations().
> 
> Call stack :
> 
> Thread A		Thread B
> f2fs_write_checkpoint()
> - block_operations(sbi)
>  - f2fs_lock_all(sbi);
>   - down_write(&sbi->cp_rwsem);
> 
>                         - open()
>                          - igrab()
>                         - write() write inline data
>                         - unlink()
> - f2fs_sync_node_pages()
>  - if (is_inline_node(page))
>   - flush_inline_data()
>    - ilookup()
>      page = f2fs_pagecache_get_page()
>      if (!page)
>       goto iput_out;
>      iput_out:
> 			-close()
> 			-iput()
>        iput(inode);
>        - f2fs_evict_inode()
>         - f2fs_truncate_blocks()
>          - f2fs_lock_op()
>            - down_read(&sbi->cp_rwsem);
> 
> Fixes: 2049d4fcb057 ("f2fs: avoid multiple node page writes due to inline_data")
> Signed-off-by: Sayali Lokhande <sayalil@xxxxxxxxxxxxxx>
> Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> ---
>  fs/f2fs/checkpoint.c |  9 ++++++++-
>  fs/f2fs/f2fs.h       |  4 ++--
>  fs/f2fs/node.c       | 10 +++++-----
>  3 files changed, 15 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
> index d49f7a01d8a26..928aea4ff663d 100644
> --- a/fs/f2fs/checkpoint.c
> +++ b/fs/f2fs/checkpoint.c
> @@ -1168,6 +1168,12 @@ static int block_operations(struct f2fs_sb_info *sbi)
>  	};
>  	int err = 0, cnt = 0;
>  
> +	/*
> +	 * Let's flush node pages first to flush inline_data.
> +	 * We'll actually guarantee everything below under f2fs_lock_all.
> +	 */
> +	f2fs_sync_node_pages(sbi, &wbc, false, false, FS_CP_NODE_IO);

It is possible that user write a large number of inline data in between
f2fs_sync_node_pages() and f2fs_lock_all(), it will cause the no-space issue in
race condition.

Also, if there is huge number of F2FS_DIRTY_IMETA, after this change, we will
flush inode page twice which is unneeded.

f2fs_sync_node_pages() --- flush dirty inode page
f2fs_lock_all()
...
f2fs_sync_inode_meta() --- update dirty inode page
f2fs_sync_node_pages() --- flush dirty inode page again.

Thanks,

> +
>  retry_flush_quotas:
>  	f2fs_lock_all(sbi);
>  	if (__need_flush_quota(sbi)) {
> @@ -1222,7 +1228,8 @@ static int block_operations(struct f2fs_sb_info *sbi)
>  	if (get_pages(sbi, F2FS_DIRTY_NODES)) {
>  		up_write(&sbi->node_write);
>  		atomic_inc(&sbi->wb_sync_req[NODE]);
> -		err = f2fs_sync_node_pages(sbi, &wbc, false, FS_CP_NODE_IO);
> +		err = f2fs_sync_node_pages(sbi, &wbc, false,
> +					true, FS_CP_NODE_IO);
>  		atomic_dec(&sbi->wb_sync_req[NODE]);
>  		if (err) {
>  			up_write(&sbi->node_change);
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index d916540f12813..ac6ae42b9dd4e 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -3286,8 +3286,8 @@ int f2fs_fsync_node_pages(struct f2fs_sb_info *sbi, struct inode *inode,
>  			struct writeback_control *wbc, bool atomic,
>  			unsigned int *seq_id);
>  int f2fs_sync_node_pages(struct f2fs_sb_info *sbi,
> -			struct writeback_control *wbc,
> -			bool do_balance, enum iostat_type io_type);
> +		struct writeback_control *wbc,
> +		bool do_balance, bool sync, enum iostat_type io_type);
>  int f2fs_build_free_nids(struct f2fs_sb_info *sbi, bool sync, bool mount);
>  bool f2fs_alloc_nid(struct f2fs_sb_info *sbi, nid_t *nid);
>  void f2fs_alloc_nid_done(struct f2fs_sb_info *sbi, nid_t nid);
> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
> index 1db8cabf727ef..fd00a8c119088 100644
> --- a/fs/f2fs/node.c
> +++ b/fs/f2fs/node.c
> @@ -1808,8 +1808,8 @@ static bool flush_dirty_inode(struct page *page)
>  }
>  
>  int f2fs_sync_node_pages(struct f2fs_sb_info *sbi,
> -				struct writeback_control *wbc,
> -				bool do_balance, enum iostat_type io_type)
> +			struct writeback_control *wbc,
> +			bool do_balance, bool sync, enum iostat_type io_type)
>  {
>  	pgoff_t index;
>  	struct pagevec pvec;
> @@ -1870,8 +1870,8 @@ int f2fs_sync_node_pages(struct f2fs_sb_info *sbi,
>  				goto continue_unlock;
>  			}
>  
> -			/* flush inline_data */
> -			if (is_inline_node(page)) {
> +			/* flush inline_data, if it's async context. */
> +			if (!sync && is_inline_node(page)) {
>  				clear_inline_node(page);
>  				unlock_page(page);
>  				flush_inline_data(sbi, ino_of_node(page));
> @@ -1999,7 +1999,7 @@ static int f2fs_write_node_pages(struct address_space *mapping,
>  
>  	diff = nr_pages_to_write(sbi, NODE, wbc);
>  	blk_start_plug(&plug);
> -	f2fs_sync_node_pages(sbi, wbc, true, FS_NODE_IO);
> +	f2fs_sync_node_pages(sbi, wbc, true, false, FS_NODE_IO);
>  	blk_finish_plug(&plug);
>  	wbc->nr_to_write = max((long)0, wbc->nr_to_write - diff);
>  
>