linux-kernel - Re: block layer copying user io vectors

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Date:	Mon, 2 Feb 2015 02:00:34 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	"Andy Falanga (afalanga)" <afalanga@...ron.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	Doug Gilbert <dgilbert@...erlog.com>
Subject: Re: block layer copying user io vectors

On Fri, Jan 30, 2015 at 05:43:04PM +0000, Andy Falanga (afalanga) wrote:
> > > This function does something that seems rather strange.  On line 859,
> > > a for loop determines the number of pages needed for the copying of
> > > the user data to kernel space.  Then the memory is allocated (line
> > > 886 bio_kmalloc()).  Then, strangely, on line 895, there is this
> > > conditional:
> > 
> > This is because the function can also be used with preallocated pages,
> > a feature only used by the sg and tape drivers.
> > 
> > Make sure your user memory is 4k aligned, and you should be able to
> > avoid the copy entirely (1).
> 
> Where is this 4k alignment being enforced?  When sg_start_req calls to
> blk_rq_map_user_iov, the only check for alignment is that the data buffers
> are 4-byte aligned (q->dma_alignment == 3).  I have verified that they are.

Indeed, I though we had the more strict direct I/O alignment.

Still doesn't help because the sg driver refuses to directly map user
pages for vectored I/O.
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