Re: [PATCH] iomap: enforce DIO alignment check in iomap]

[Ming Lei <tom.leiming@gmail.com>]

On Wed, Jun 10, 2026 at 01:54:48PM -0600, Keith Busch wrote:
> On Wed, Jun 10, 2026 at 08:19:53PM +0200, Carlos Maiolino wrote:
> > On Wed, Jun 10, 2026 at 11:14:30AM -0600, Keith Busch wrote:
> > > 
> > > It does require that someone calls the bio split-to-limits routine,
> > > which I had taken for granted as a given, but I realize that some
> > > drivers don't do that. What block device are you using for your test?
> > 
> > In the PPC machine, it's a virtual scsi vdasd device from one of the
> > virtual nodes
> > 
> > NAME HCTL       TYPE VENDOR   MODEL  REV SERIAL                             TRAN
> > sda  0:0:1:0    disk AIX      VDASD 0001 000a508a00007a0000000175dcba35ac.5
> > 
> > ibmvfc                262144  0
> > ibmvscsi              196608  2
> > 
> > For my x86 machine (remind I reduce the buffer size to 512 on x86), it's
> > a commodity sata samsung SSD:
> 
> Okay, these are under blk-mq so always call __bio_split_to_limits.
> However, I see there's an optimization to skip the checks we're
> depending on if bio_may_need_split doesn't think it needs to be split,
> which is a problem for your observation. I don't think the current
> expecations can allow us to take this optimization anymore when page
> offsets are used.
> 
> This should fix it:
> 
> ---
> diff --git a/block/blk.h b/block/blk.h
> index 1a2d9101bba04..3731f3c5ed140 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -404,7 +404,7 @@ static inline bool bio_may_need_split(struct bio *bio,
>  	bv = __bvec_iter_bvec(bio->bi_io_vec, bio->bi_iter);
>  	if (bio->bi_iter.bi_size > bv->bv_len - bio->bi_iter.bi_bvec_done)
>  		return true;
> -	return bv->bv_len + bv->bv_offset > lim->max_fast_segment_size;
> +	return bv.bv_offset || bv->bv_len > lim->max_fast_segment_size;
>  }

This should work for the un-aligned DMA buffer, but might hurt perf for
any sub-page IO.

Given you have switched to validate dio buffer alignment to bio splitting, it
should be fine to check ->dma_alignment here by putting the three limits
fields into same cache line.


Thanks,
Ming