Re: [syzbot] [mm?] [fs?] KCSAN: data-race in __filemap_add_folio / invalidate_bdev (8)

[Matthew Wilcox]

On Mon, Mar 10, 2025 at 02:40:26AM -0700, syzbot wrote:
> Reported-by: syzbot+f2aaf773187f5cae54f3@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> EXT4-fs (loop0): unmounting filesystem 00000000-0000-0000-0000-000000000000.
> ==================================================================
> BUG: KCSAN: data-race in __filemap_add_folio / invalidate_bdev
> 
> read-write to 0xffff888100630570 of 8 bytes by task 3291 on cpu 0:
>  __filemap_add_folio+0x430/0x6f0 mm/filemap.c:929

This is a write to mapping->nrpages with the i_pages lock held, as it
should be.

>  filemap_add_folio+0x9c/0x1b0 mm/filemap.c:981
>  page_cache_ra_unbounded+0x1c1/0x350 mm/readahead.c:276
>  do_page_cache_ra mm/readahead.c:328 [inline]
>  force_page_cache_ra mm/readahead.c:357 [inline]
>  page_cache_sync_ra+0x252/0x680 mm/readahead.c:585
>  filemap_get_pages+0x2ca/0x11a0 mm/filemap.c:2580
>  filemap_read+0x230/0x8c0 mm/filemap.c:2691
>  blkdev_read_iter+0x228/0x2d0 block/fops.c:796
>  new_sync_read fs/read_write.c:484 [inline]
>  vfs_read+0x5cc/0x6f0 fs/read_write.c:565
>  ksys_read+0xe8/0x1b0 fs/read_write.c:708
> 
> read to 0xffff888100630570 of 8 bytes by task 3306 on cpu 1:
>  invalidate_bdev+0x25/0x70 block/bdev.c:99

This is a read of mapping->nrpages with no lock held.  So we could
silence this warning by making this a READ_ONCE or data_race().

The problem is that I'm not sure this is the right answer.  Obviously
here we only care about zero-vs-non-zero, but what if we race with
0 being incremented to 1?  Should there be some locking higher up
that prevents this?  Or is this "yes, root can do this and screw
themselves"?

>  ext4_put_super+0x571/0x810 fs/ext4/super.c:1356
>  generic_shutdown_super+0xe5/0x220 fs/super.c:642
>  kill_block_super+0x2a/0x70 fs/super.c:1710
>  ext4_kill_sb+0x44/0x80 fs/ext4/super.c:7368
>  deactivate_locked_super+0x7d/0x1c0 fs/super.c:473
>  deactivate_super+0x9f/0xb0 fs/super.c:506
>  cleanup_mnt+0x268/0x2e0 fs/namespace.c:1413
>  __cleanup_mnt+0x19/0x20 fs/namespace.c:1420
>  task_work_run+0x13a/0x1a0 kernel/task_work.c:227