Re: [PATCH v2 1/7] block: use iomap_dirty_folio for block devices

[Hannes Reinecke]

On 8/28/26 3:49 PM, Tal Zussman wrote:
> With CONFIG_BUFFER_HEAD=n, block devices are written back through iomap,
> but def_blk_aops uses filemap_dirty_folio, which only sets PG_dirty. It
> does not set the per-block dirty bits in the folio's iomap_folio_state,
> so iomap_writeback_folio() finds no dirty range, submits no I/O and
> clears PG_dirty, resulting in data loss.
>
> Other iomap users set .dirty_folio to iomap_dirty_folio, which marks the
> folio's blocks dirty before calling filemap_dirty_folio().
>
> This is only observable with block size < folio size. With a single
> block there is no iomap_folio_state to get out of sync and
> iomap_writeback_folio() marks the whole folio dirty itself. For a
> page-aligned device, this may require using the BLKBSZSET ioctl to set
> the block size, which requires CAP_SYS_ADMIN. A device whose size is not
> page aligned already gets a sub-page block size from
> set_init_blocksize(), so no ioctl and no privilege is needed.
>
> To reproduce, on a device with a sub-page block size, write a known
> pattern with O_DIRECT, mmap the same range, store to it, msync() and
> fsync(), then read it back with O_DIRECT. A reproducer is available at
> [1].
>
> [1] https://gist.github.com/tzussman/18ab05cba4b3fdc79cce0a69d1fd05b4
>
> Fixes: 925c86a19bac ("fs: add CONFIG_BUFFER_HEAD")
> Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> Link: https://sashiko.dev/#/patchset/20260730-blk-dontcache-v7-0-3e8e6850068d%40columbia.edu?part=5
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>
> ---
>   block/fops.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
Reviewed-by: Hannes Reinecke <hare@xxxxxxxxxx>

Cheers,

Hannes
--
Dr. Hannes Reinecke                  Kernel Storage Architect
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