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

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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@xxxxxxxxxxxxxxx>
Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
---
   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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