f2fs: fix to detect gcing page in f2fs_is_cp_guaranteed()

Jan Prusakowski reported a f2fs bug as below:

f2fs/007 will hang kernel during testing w/ below configs:

kernel 6.12.18 (from pixel-kernel/android16-6.12)
export MKFS_OPTIONS="-O encrypt -O extra_attr -O project_quota -O quota"
export F2FS_MOUNT_OPTIONS="test_dummy_encryption,discard,fsync_mode=nobarrier,reserve_root=32768,checkpoint_merge,atgc"

cat /proc/<umount_proc_id>/stack
f2fs_wait_on_all_pages+0xa3/0x130
do_checkpoint+0x40c/0x5d0
f2fs_write_checkpoint+0x258/0x550
kill_f2fs_super+0x14f/0x190
deactivate_locked_super+0x30/0xb0
cleanup_mnt+0xba/0x150
task_work_run+0x59/0xa0
syscall_exit_to_user_mode+0x12d/0x130
do_syscall_64+0x57/0x110
entry_SYSCALL_64_after_hwframe+0x76/0x7e

cat /sys/kernel/debug/f2fs/status

  - IO_W (CP: -256, Data:  256, Flush: (   0    0    1), Discard: (   0    0)) cmd:    0 undiscard:   0

CP IOs reference count becomes negative.

The root cause is:

After 4961acd ("f2fs: fix to tag gcing flag on page during block
migration"), we will tag page w/ gcing flag for raw page of cluster
during its migration.

However, if the inode is both encrypted and compressed, during
ioc_decompress(), it will tag page w/ gcing flag, and it increase
F2FS_WB_DATA reference count:
- f2fs_write_multi_page
 - f2fs_write_raw_page
  - f2fs_write_single_page
   - do_write_page
    - f2fs_submit_page_write
     - WB_DATA_TYPE(bio_page, fio->compressed_page)
     : bio_page is encrypted, so mapping is NULL, and fio->compressed_page
       is NULL, it returns F2FS_WB_DATA
     - inc_page_count(.., F2FS_WB_DATA)

Then, during end_io(), it decrease F2FS_WB_CP_DATA reference count:
- f2fs_write_end_io
 - f2fs_compress_write_end_io
  - fscrypt_pagecache_folio
  : get raw page from encrypted page
  - WB_DATA_TYPE(&folio->page, false)
  : raw page has gcing flag, it returns F2FS_WB_CP_DATA
  - dec_page_count(.., F2FS_WB_CP_DATA)

In order to fix this issue, we need to detect gcing flag in raw page
in f2fs_is_cp_guaranteed().

Fixes: 4961acd ("f2fs: fix to tag gcing flag on page during block migration")
Reported-by: Jan Prusakowski <jprusakowski@google.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: chaseyu

fs/f2fs/data.c

@@ -54,7 +54,7 @@ bool f2fs_is_cp_guaranteed(struct page *page)
 	struct f2fs_sb_info *sbi;
 
 	if (fscrypt_is_bounce_page(page))
-		return false;
+		return page_private_gcing(fscrypt_pagecache_page(page));
 
 	inode = mapping->host;
 	sbi = F2FS_I_SB(inode);