Re: [PATCH v6 4/4] block: enable RWF_DONTCACHE for block devices

[Tal Zussman]

On 5/14/26 5:51 PM, 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 paths, use block_write_begin_iocb() in
> blkdev_write_begin() to thread the kiocb through so that buffer_head
> writeback gets dropbehind support.
> 
> CONFIG_BUFFER_HEAD=n paths are handled by the previously added iomap
> BIO_COMPLETE_IN_TASK support.
> 
> This support is useful for databases that operate on raw block devices,
> among other userspace applications.
> 
> Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>

Responding to Sashiko review inline:

Link: https://sashiko.dev/#/patchset/20260514-blk-dontcache-v6-0-782e2fa7477b%40columbia.edu

Q: "Could this code path be unreachable during block device writes?
Block device buffered writes use blkdev_write_iter(), which unconditionally
delegates to blkdev_buffered_write() and subsequently
iomap_file_buffered_write(). The iomap infrastructure bypasses the legacy
address_space_operations .write_begin method.
During a write, iomap_write_begin() handles buffer head allocation internally
by calling __block_write_begin_int() directly. This naturally inherits the
FGP_DONTCACHE flag passed down from the kiocb via iomap_get_folio().
If the VFS write paths were actually calling .write_begin for block devices, a
CONFIG_BUFFER_HEAD=n kernel would crash with a NULL pointer dereference since
def_blk_aops does not define .write_begin or .write_end in that configuration."

A: So this actually seems legit... doesn't look like anything actually calls 
blkdev_write_begin() or blkdev_write_end(), unless I'm missing something.
block_write_begin_iocb() usage seems necessary for bh-based filesystems, but
block devices seem to use iomap for writes unconditionally.

> ---
>  block/fops.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/block/fops.c b/block/fops.c
> index bb6642b45937..31b073181d87 100644
> --- a/block/fops.c
> +++ b/block/fops.c
> @@ -504,7 +504,8 @@ static int blkdev_write_begin(const struct kiocb *iocb,
>  			      unsigned len, struct folio **foliop,
>  			      void **fsdata)
>  {
> -	return block_write_begin(mapping, pos, len, foliop, blkdev_get_block);
> +	return block_write_begin_iocb(iocb, mapping, pos, len, foliop,
> +				     blkdev_get_block);
>  }
>  
>  static int blkdev_write_end(const struct kiocb *iocb,
> @@ -966,7 +967,7 @@ 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)
>