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Message-ID: <9dc4ba32-5be5-26d8-5dd2-9bd48d6b0af4@kernel.org>
Date:   Thu, 6 Apr 2023 09:50:01 +0800
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Subject: Re: [f2fs-dev] [PATCH] f2fs: get out of a repeat loop when getting a
 locked data page
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To:     Jaegeuk Kim <jaegeuk@kernel.org>,
        Matthew Wilcox <willy@infradead.org>
Cc:     linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, stable@vger.kernel.org,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org
References: <20230323213919.1876157-1-jaegeuk@kernel.org>
 <8aea02b0-86f9-539a-02e9-27b381e68b66@kernel.org>
 <ZCG2mfviZfY1dqb4@google.com> <ZCHCykI/BLcfDzt7@casper.infradead.org>
 <ZC2kSfNUXKK4PfpM@google.com>
From:   Chao Yu <chao@kernel.org>
In-Reply-To: <ZC2kSfNUXKK4PfpM@google.com>
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On 2023/4/6 0:39, Jaegeuk Kim wrote:
> On 03/27, Matthew Wilcox wrote:
>> On Mon, Mar 27, 2023 at 08:30:33AM -0700, 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?
>>>
>>>>
>>>> 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?
>>
>> There's a lot of comments in the bug ... hard to come into this one
>> cold.
>>
>> I did notice this one (#119):
>> : Interestingly, ref count is 514, which looks suspiciously as a binary
>> : flag 1000000010. Is it possible that during 5.17/5.18 implementation
>> : of a "pin", somehow binary flag was written to ref count, or something
>> : like '1 << ...' happens?
>>
>> That indicates to me that somehow you've got hold of a THP that is in
>> the page cache.  Probably shmem/tmpfs.  That indicate to me a refcount
>> problem that looks something like this:
>>
>> f2fs allocates a page
>> f2fs adds the page to the page cache
>> f2fs puts the reference to the page without removing it from the
>> page cache (how?)
> 
> Is it somewhat related to setting a bit in private field?

IIUC, it looks the page reference is added/removed as pair.

> 
> When we migrate the blocks, we do:
> 1) get_lock_page()

- f2fs_grab_cache_page
  - pagecache_get_page
   - __filemap_get_folio
    - no_page  -> filemap_alloc_folio  page_ref = 1 (referenced by caller)
     - filemap_add_folio page_ref = 2 (referenced by radix tree)

> 2) submit read
> 3) lock_page()
> 3) set_page_dirty()
> 4) set_page_private_gcing(page)

page_ref = 3 (reference by private data)

> 
> --- in fs/f2fs/f2fs.h
> 1409 #define PAGE_PRIVATE_SET_FUNC(name, flagname) \
> 1410 static inline void set_page_private_##name(struct page *page) \
> 1411 { \
> 1412         if (!PagePrivate(page)) { \
> 1413                 get_page(page); \
> 1414                 SetPagePrivate(page); \
> 1415                 set_page_private(page, 0); \
> 1416         } \
> 1417         set_bit(PAGE_PRIVATE_NOT_POINTER, &page_private(page)); \
> 1418         set_bit(PAGE_PRIVATE_##flagname, &page_private(page)); \
> 1419 }
> 
> 
> 5) set_page_writebac()
> 6) submit write
> 7) unlock_page()
> 8) put_page(page)

page_ref = 2 (ref by caller was removed)

> 
> Later, f2fs_invalidate_folio will do put_page again by:
> clear_page_private_gcing(&folio->page);

page_ref = 1 (ref by private was removed, and the last left ref is hold by radix tree)

> 
> --- in fs/f2fs/f2fs.h
> 1421 #define PAGE_PRIVATE_CLEAR_FUNC(name, flagname) \
> 1422 static inline void clear_page_private_##name(struct page *page) \
> 1423 { \
> 1424         clear_bit(PAGE_PRIVATE_##flagname, &page_private(page)); \
> 1425         if (page_private(page) == BIT(PAGE_PRIVATE_NOT_POINTER)) { \
> 1426                 set_page_private(page, 0); \
> 1427                 if (PagePrivate(page)) { \
> 1428                         ClearPagePrivate(page); \

Since PagePrivate was cleared, so folio_detach_private in
f2fs_invalidate_folio()/f2fs_release_folio will just skip drop reference.

static inline void *folio_detach_private(struct folio *folio)
{
	void *data = folio_get_private(folio);

	if (!folio_test_private(folio))
		return NULL;
	folio_clear_private(folio);
	folio->private = NULL;
	folio_put(folio);

	return data;
}

Or am I missing something?

Thanks,

> 1429                         put_page(page); \
> 1430                 }\
> 1431         } \
> 1432 }
> 
>> page is now free, gets reallocated into a THP
>> lookup from the f2fs file finds the new THP
>> things explode messily
>>
>> Checking page->mapping is going to avoid the messy explosion, but
>> you'll still have a page in the page cache which doesn't actually
>> belong to you, and that's going to lead to subtle data corruption.
>>
>> This should be caught by page_expected_state(), called from
>> free_page_is_bad(), called from free_pages_prepare().  Do your testers
>> have CONFIG_DEBUG_VM enabled?  That might give you a fighting chance at
>> finding the last place which called put_page().  It won't necessarily be
>> the _wrong_ place to call put_page() (that may have happened earlier),
>> but it may give you a clue.
>>
>>>>
>>>> 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);
>>>>>    	}