Re: [PATCH v7 5/5] block: enable RWF_DONTCACHE for block devices

On 7/30/26 2:57 AM, Tal Zussman wrote:
> Block device buffered reads and writes already pass through
> filemap_read() and iomap_file_buffered_write() respectively, both of
> which handle IOCB_DONTCACHE. Enable RWF_DONTCACHE for block device files
> by setting FOP_DONTCACHE in def_blk_fops.
> 
> For CONFIG_BUFFER_HEAD=y, writeback goes through buffer_head's
> __bh_submit() which sets BIO_COMPLETE_IN_TASK on dropbehind folios. For
> CONFIG_BUFFER_HEAD=n, writeback goes through iomap which handles it via
> BIO_COMPLETE_IN_TASK on the ioend bio.
> 
> This support is useful for databases that operate on raw block devices,
> among other userspace applications.
> 
> Reviewed-by: Christoph Hellwig <[email protected]>
> Signed-off-by: Tal Zussman <[email protected]>
> ---
>  block/fops.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block/fops.c b/block/fops.c
> index 15783a6180de..8b39c53b6393 100644
> --- a/block/fops.c
> +++ b/block/fops.c
> @@ -941,7 +941,7 @@ const struct file_operations def_blk_fops = {
>  	.splice_write	= iter_file_splice_write,
>  	.fallocate	= blkdev_fallocate,
>  	.uring_cmd	= blkdev_uring_cmd,
> -	.fop_flags	= FOP_BUFFER_RASYNC,
> +	.fop_flags	= FOP_BUFFER_RASYNC | FOP_DONTCACHE,
>  };
>  
>  static __init int blkdev_init(void)
> 

It looks like Sashiko found a couple pre-existing issues in this file [1].
I've reproduced the first and looking into the second. Will send patches
once confirmed.

(FWIW the other issues Sashiko reported seem overblown.)

[1]: https://sashiko.dev/#/patchset/20260730-blk-dontcache-v7-0-3e8e6850068d%40columbia.edu?part=5