[PATCH 6/7] f2fs: fix to serialize truncation of COW blocks w/ GC

[Chao Yu <chao@kernel.org>]

From: Chao Yu <chao@kernel.org>

GC migrates blocks of COW inode w/ the COW inode and its i_gc_rwsem,
however f2fs_ioc_start_atomic_write() truncates blocks of the reused
COW inode w/o holding COW inode's i_gc_rwsem, it only holds the
original inode's locks, so the truncation can race w/ GC.

Case #1: GC caches stale data of a freed block in meta cache

- GC (phase 3, cow inode C)		- ioctl(F2FS_IOC_START_ATOMIC_WRITE)
- f2fs_down_write_trylock(C's i_gc_rwsem[WRITE])
- ra_data_block(C, j)
 - f2fs_grab_cache_folio(orig mapping, j)
 - f2fs_get_dnode_of_data(C, j)
 : get P
 - f2fs_put_dnode()
 - f2fs_is_valid_blkaddr(P)
 - f2fs_folio_wait_writeback()
 - f2fs_wait_on_block_writeback()
					- f2fs_ioc_start_atomic_write()
					 - f2fs_down_write(orig's i_gc_rwsem)
					 - f2fs_do_truncate_blocks(C, 0, true)
					  - f2fs_truncate_data_blocks_range()
					   - f2fs_set_data_blkaddr(NULL_ADDR)
					   - f2fs_invalidate_blocks(P)
					    - f2fs_invalidate_internal_cache(P)
					    : nothing is cached
 - f2fs_grab_cache(META, P)
 : cache P in META[P]
 - f2fs_submit_cache_read(P)
 : SIT of P is still valid
					    - update_sit_entry(P, -1)
					    : P was freed
- end_io: f2fs_cache_set_uptodate(META[P])
: META[P] is stale for a freed block

On superblock w/o encrypt, verity and compression features, checkpoint
does not truncate meta cache, so META[P] survives after checkpoint, then:

- checkpoint, P can be reused
- writeback of victim's atomic write
 - do_write_block()
 : allocate P, write victim's data to P, META[P] is not invalidated
- GC phase 3: ra_data_block(victim's cow inode, k)
: META[P] is uptodate, skip reading
- GC phase 4: move_data_block(victim's cow inode, k)
: META[P] is uptodate, skip reading
 - memcpy(tentry, sentry)
 - f2fs_update_data_blkaddr(newaddr)
: victim's block was overwritten w/ stale data

Case #2: GC reports a false inconsistency error

- GC (phase 3, cow inode C)		- ioctl(F2FS_IOC_START_ATOMIC_WRITE)
- ra_data_block(C, j)
 - f2fs_get_dnode_of_data(C, j)
 : get P
 - f2fs_put_dnode()
					- f2fs_do_truncate_blocks(C, 0, true)
					 - f2fs_invalidate_blocks(P)
					  - update_sit_entry(P, -1)
 - f2fs_is_valid_blkaddr(P, DATA_GENERIC_ENHANCE)
 : or in f2fs_submit_cache_read()
  - __is_bitmap_valid()
   - f2fs_err("Inconsistent error blkaddr:...")
   - set_sbi_flag(SBI_NEED_FSCK)
   - dump_stack()
   - f2fs_handle_error(ERROR_INVALID_BLKADDR)

All other truncation paths hold i_gc_rwsem[WRITE] of the truncated
inode, let's hold COW inode's i_gc_rwsem[WRITE] during truncation of
COW blocks as well, then GC will skip COW blocks due to failure of
f2fs_down_write_trylock().

Cc: stable@kernel.org
Fixes: f18d00769336 ("f2fs: use meta inode for GC of COW file")
Signed-off-by: Chao Yu <chao@kernel.org>
---
 fs/f2fs/file.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 363039da3ab2..3a7093a7c12e 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -2445,11 +2445,20 @@ static int f2fs_ioc_start_atomic_write(struct file *filp, bool truncate)
 		F2FS_I(fi->cow_inode)->atomic_inode = inode;
 	} else {
 		/* Reuse the already created COW inode */
+		struct f2fs_inode_info *cfi = F2FS_I(fi->cow_inode);
+
 		f2fs_bug_on(sbi, get_dirty_pages(fi->cow_inode));
 
 		invalidate_mapping_pages(fi->cow_inode->i_mapping, 0, -1);
 
+		/*
+		 * GC migrates COW blocks w/ cow inode and its i_gc_rwsem, so
+		 * hold it to serialize truncation of COW blocks w/ GC.
+		 */
+		f2fs_down_write_nested(&cfi->i_gc_rwsem[WRITE],
+						SINGLE_DEPTH_NESTING);
 		ret = f2fs_do_truncate_blocks(fi->cow_inode, 0, true);
+		f2fs_up_write(&cfi->i_gc_rwsem[WRITE]);
 		if (ret)
 			goto out_unlock;
 	}
-- 
2.49.0