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Message-ID: <9dc4ba32-5be5-26d8-5dd2-9bd48d6b0af4@kernel.org>
Date: Thu, 6 Apr 2023 09:50:01 +0800
From: Chao Yu <chao@...nel.org>
To: Jaegeuk Kim <jaegeuk@...nel.org>,
Matthew Wilcox <willy@...radead.org>
Cc: linux-kernel@...r.kernel.org,
linux-f2fs-devel@...ts.sourceforge.net, stable@...r.kernel.org,
linux-mm@...ck.org, linux-fsdevel@...r.kernel.org
Subject: Re: [f2fs-dev] [PATCH] f2fs: get out of a repeat loop when getting a
locked data page
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@...r.kernel.org>
>>>>> Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.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);
>>>>> }
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