On Thu, 2018-10-18 at 07:03 -0700, Matthew Wilcox wrote:

On Thu, Oct 18, 2018 at 09:18:12PM +0800, Ming Lei wrote:

quoted

Filesystems may allocate io buffer from slab, and use this buffer to
submit bio. This way may break storage drivers if they have special
requirement on DMA alignment.

Before we go down this road, could we have a discussion about what
hardware actually requires this?  Storage has this weird assumption that
I/Os must be at least 512 byte aligned in memory, and I don't know where
this idea comes from.  Network devices can do arbitrary byte alignment.
Even USB controllers can do arbitrary byte alignment.  Sure, performance
is going to suck and there are definite risks on some architectures
with doing IOs that are sub-cacheline aligned, but why is storage such a
special snowflake that we assume that host controllers are only capable
of doing 512-byte aligned DMAs?

I just dragged out the NCR53c810 data sheet from 1993, and it's capable of
doing arbitrary alignment of DMA.  NVMe is capable of 4-byte aligned DMA.
What hardware needs this 512 byte alignment?

How about starting with modifying the queue_dma_alignment() function? The
current implementation of that function is as follows:

static inline int queue_dma_alignment(struct request_queue *q)
{
	return q ? q->dma_alignment : 511;
}

In other words, for block drivers that do not set the DMA alignment
explicitly it is assumed that these drivers need 512 byte alignment. I think
the "512 byte alignment as default" was introduced in 2002. From Thomas
Gleixner's history tree, commit ad519c6902fb:

+static inline int queue_dma_alignment(request_queue_t *q)
+{
+       int retval = 511;
+
+       if (q && q->dma_alignment)
+               retval = q->dma_alignment;
+
+       return retval;
+}
+
+static inline int bdev_dma_aligment(struct block_device *bdev)
+{
+       return queue_dma_alignment(bdev_get_queue(bdev));
+}
+

Bart.