Re: [PATCH v2] f2fs: avoid reading already updated pages during GC

[Chao Yu]

On 2026/3/5 09:18, Jianan Huang wrote:
> We found the following issue during fuzz testing:
>
> page: refcount:3 mapcount:0 mapping:00000000b6e89c65 index:0x18b2dc pfn:0x161ba9
> memcg:f8ffff800e269c00
> aops:f2fs_meta_aops ino:2
> flags: 0x52880000000080a9(locked|waiters|uptodate|lru|private|zone=1|kasantag=0x4a)
> raw: 52880000000080a9 fffffffec6e17588 fffffffec0ccc088 a7ffff8067063618
> raw: 000000000018b2dc 0000000000000009 00000003ffffffff f8ffff800e269c00
> page dumped because: VM_BUG_ON_FOLIO(folio_test_uptodate(folio))
> page_owner tracks the page as allocated
>   post_alloc_hook+0x58c/0x5ec
>   prep_new_page+0x34/0x284
>   get_page_from_freelist+0x2dcc/0x2e8c
>   __alloc_pages_noprof+0x280/0x76c
>   __folio_alloc_noprof+0x18/0xac
>   __filemap_get_folio+0x6bc/0xdc4
>   pagecache_get_page+0x3c/0x104
>   do_garbage_collect+0x5c78/0x77a4
>   f2fs_gc+0xd74/0x25f0
>   gc_thread_func+0xb28/0x2930
>   kthread+0x464/0x5d8
>   ret_from_fork+0x10/0x20
> ------------[ cut here ]------------
> kernel BUG at mm/filemap.c:1563!
>   folio_end_read+0x140/0x168
>   f2fs_finish_read_bio+0x5c4/0xb80
>   f2fs_read_end_io+0x64c/0x708
>   bio_endio+0x85c/0x8c0
>   blk_update_request+0x690/0x127c
>   scsi_end_request+0x9c/0xb8c
>   scsi_io_completion+0xf0/0x250
>   scsi_finish_command+0x430/0x45c
>   scsi_complete+0x178/0x6d4
>   blk_mq_complete_request+0xcc/0x104
>   scsi_done_internal+0x214/0x454
>   scsi_done+0x24/0x34
>
> which is similar to the problem reported by syzbot:
> https://syzkaller.appspot.com/bug?extid=3686758660f980b402dc
>
> This case is consistent with the description in commit 9bf1a3f
> ("f2fs: avoid GC causing encrypted file corrupted"):
> Page 1 is moved from blkaddr A to blkaddr B by move_data_block, and after
> being written it is marked as uptodate. Then, Page 1 is moved from blkaddr
> B to blkaddr C, VM_BUG_ON_FOLIO was triggered in the endio initiated by
> ra_data_block.
>
> There is no need to read Page 1 again from blkaddr B, since it has already
> been updated. Therefore, avoid initiating I/O in this case.
>
> Fixes: 6aa58d8ad20a ("f2fs: readahead encrypted block during GC")
> Signed-off-by: Jianan Huang <huangjianan@xxxxxxxxxx>
> Signed-off-by: Sheng Yong <shengyong1@xxxxxxxxxx>

Reviewed-by: Chao Yu <chao@xxxxxxxxxx>

Thanks,