Re: [f2fs-dev] [PATCH] f2fs: get out of a repeat loop when getting a locked data page
From: Chao Yu
Date: Wed Apr 05 2023 - 21:50:10 EST
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);
}