Re: [f2fs-dev] [PATCH] f2fs: get out of a repeat loop when getting a locked data page

[Chao Yu <chao@kernel.org>]

On 2023/3/27 23:30, Jaegeuk Kim wrote:
> On 03/26, Chao Yu wrote:
>> On 2023/3/24 5:39, Jaegeuk Kim wrote:
>>> https://bugzilla.kernel.org/show_bug.cgi?id=216050
>>>
>>> Somehow we're getting a page which has a different mapping.
>>> Let's avoid the infinite loop.
>>>
>>> Cc: <stable@vger.kernel.org>
>>> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
>>> ---
>>>    fs/f2fs/data.c | 8 ++------
>>>    1 file changed, 2 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>>> index bf51e6e4eb64..80702c93e885 100644
>>> --- a/fs/f2fs/data.c
>>> +++ b/fs/f2fs/data.c
>>> @@ -1329,18 +1329,14 @@ struct page *f2fs_get_lock_data_page(struct inode *inode, pgoff_t index,
>>>    {
>>>    	struct address_space *mapping = inode->i_mapping;
>>>    	struct page *page;
>>> -repeat:
>>> +
>>>    	page = f2fs_get_read_data_page(inode, index, 0, for_write, NULL);
>>>    	if (IS_ERR(page))
>>>    		return page;
>>>    	/* wait for read completion */
>>>    	lock_page(page);
>>> -	if (unlikely(page->mapping != mapping)) {
>>
>> How about using such logic only for move_data_page() to limit affect for
>> other paths?
> 
> Why move_data_page() only? If this happens, we'll fall into a loop in anywhere?

Actually, we only suffer dead loop from foreground GC path, right? I suspect the
bug was triggered in this path only.

It looks there are a lot of cases in where we repeat triggering read once
two mappings are mismatched, e.g.
- __get_meta_page
- __get_node_page
- f2fs_write_begin
- f2fs_quota_read

Thanks,

> 
>>
>> Jaegeuk, any thoughts about why mapping is mismatch in between page's one and
>> inode->i_mapping?
> 
>>
>> After several times code review, I didn't get any clue about why f2fs always
>> get the different mapping in a loop.
> 
> I couldn't find the path to happen this. So weird. Please check the history in the
> bug.
> 
>>
>> Maybe we can loop MM guys to check whether below folio_file_page() may return
>> page which has different mapping?
> 
> Matthew may have some idea on this?
> 
>>
>> struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
>> 		int fgp_flags, gfp_t gfp)
>> {
>> 	struct folio *folio;
>>
>> 	folio = __filemap_get_folio(mapping, index, fgp_flags, gfp);
>> 	if (IS_ERR(folio))
>> 		return NULL;
>> 	return folio_file_page(folio, index);
>> }
>>
>> Thanks,
>>
>>> -		f2fs_put_page(page, 1);
>>> -		goto repeat;
>>> -	}
>>> -	if (unlikely(!PageUptodate(page))) {
>>> +	if (unlikely(page->mapping != mapping || !PageUptodate(page))) {
>>>    		f2fs_put_page(page, 1);
>>>    		return ERR_PTR(-EIO);
>>>    	}


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