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Message-ID: <ZC46Ccm8xTT4OlE3@google.com>
Date: Wed, 5 Apr 2023 20:18:33 -0700
From: Jaegeuk Kim <jaegeuk@...nel.org>
To: Chao Yu <chao@...nel.org>
Cc: Matthew Wilcox <willy@...radead.org>, 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 04/06, Chao Yu wrote:
> 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?
Ah, I missed folio_test_private() tho, can we really expect get_page(),
SetPagePrivate(), and set_page_private() is in pair with folio_detach_private()?
I feel attach/detach_page_private would look better?
>
> 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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