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

[Keith Busch <kbusch@kernel.org>]

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;
 }
 
 /**
--