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Subject[PATCH v5 14/14] ubifs: ubifs_releasepage: Remove ubifs_assert(0) to valid this process
Date
There are two states for ubifs writing pages:
1. Dirty, Private
2. Not Dirty, Not Private

The normal process cannot go to ubifs_releasepage() which means there
exists pages being private but not dirty. Reproducer[1] shows that it
could occur (which maybe related to [2]) with following process:

PA PB PC
lock(page)[PA]
ubifs_write_end
attach_page_private // set Private
__set_page_dirty_nobuffers // set Dirty
unlock(page)

write_cache_pages[PA]
lock(page)
clear_page_dirty_for_io(page) // clear Dirty
ubifs_writepage

do_truncation[PB]
truncate_setsize
i_size_write(inode, newsize) // newsize = 0

i_size = i_size_read(inode) // i_size = 0
end_index = i_size >> PAGE_SHIFT
if (page->index > end_index)
goto out // jump
out:
unlock(page) // Private, Not Dirty

generic_fadvise[PC]
lock(page)
invalidate_inode_page
try_to_release_page
ubifs_releasepage
ubifs_assert(c, 0)
// bad assertion!
unlock(page)

Then we may get following assertion failed:
UBIFS error (ubi0:0 pid 1683): ubifs_assert_failed [ubifs]:
UBIFS assert failed: 0, in fs/ubifs/file.c:1513
UBIFS warning (ubi0:0 pid 1683): ubifs_ro_mode [ubifs]:
switched to read-only mode, error -22
CPU: 2 PID: 1683 Comm: aa Not tainted 5.16.0-rc5-00184-g0bca5994cacc-dirty #308
Call Trace:
dump_stack+0x13/0x1b
ubifs_ro_mode+0x54/0x60 [ubifs]
ubifs_assert_failed+0x4b/0x80 [ubifs]
ubifs_releasepage+0x67/0x1d0 [ubifs]
try_to_release_page+0x57/0xe0
invalidate_inode_page+0xfb/0x130
__invalidate_mapping_pages+0xb9/0x280
invalidate_mapping_pagevec+0x12/0x20
generic_fadvise+0x303/0x3c0
ksys_fadvise64_64+0x4c/0xb0

[1] https://bugzilla.kernel.org/show_bug.cgi?id=215373
[2] https://linux-mtd.infradead.narkive.com/NQoBeT1u/patch-rfc-ubifs-fix-assert-failed-in-ubifs-set-page-dirty

Fixes: 1e51764a3c2ac0 ("UBIFS: add new flash file system")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
---
fs/ubifs/file.c | 19 ++++++++++++++-----
1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/fs/ubifs/file.c b/fs/ubifs/file.c
index 7cc2abcb70ae..4bafcb80d29c 100644
--- a/fs/ubifs/file.c
+++ b/fs/ubifs/file.c
@@ -1494,14 +1494,23 @@ static int ubifs_releasepage(struct page *page, gfp_t unused_gfp_flags)
struct inode *inode = page->mapping->host;
struct ubifs_info *c = inode->i_sb->s_fs_info;

- /*
- * An attempt to release a dirty page without budgeting for it - should
- * not happen.
- */
if (PageWriteback(page))
return 0;
+
+ /*
+ * Page is private but not dirty, weird? There is one condition
+ * making it happened. ubifs_writepage skipped the page because
+ * page index beyonds isize (for example. truncated by other
+ * process named A), then the page is invalidated by fadvise64
+ * syscall before being truncated by process A.
+ */
ubifs_assert(c, PagePrivate(page));
- ubifs_assert(c, 0);
+ if (PageChecked(page))
+ release_new_page_budget(c);
+ else
+ release_existing_page_budget(c);
+
+ atomic_long_dec(&c->dirty_pg_cnt);
detach_page_private(page);
ClearPageChecked(page);
return 1;
--
2.31.1
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