Re: [PATCH] ide-scsi: DMA alignment bug fixed

[Terry Kyriacopoulos]

On Fri, 5 Nov 2004, Jens Axboe wrote:

> I don't think this is the right way to go. The design of ide-scsi's dma
> setup is based on the old buffer_heads, where you can only have a single
> page at the time so you need to string them manually. We actually have
> kernel helpers for mapping _user_ data to a request, we could add one
> for mapping kernel data to a request as well.
>
> Here's a patch that I did some time ago for mapping a kernel pointer to
> a bio. bio_copy_user() would be good inspiration, too. Mapping an sglist
> to a new bio should be even simpler and it would clean up the ide-scsi
> stuff a lot. It's not exactly pretty.

...

>
> +struct bio *bio_map_data(request_queue_t *q, unsigned char *ptr,
> +			 unsigned int len, int read)
> +{
> +
> +	unsigned long data = (unsigned long) ptr;
> +	unsigned long end = (data + len + PAGE_SIZE - 1) >> PAGE_SHIFT;
> +	unsigned long start = data >> PAGE_SHIFT;
> +	const int nr_pages = end - start;
> +	unsigned int bytes, offset;
> +	struct bio *bio;
> +	struct page *page;
> +	int i;
> +
> +	if (!len || !nr_pages)
> +		return ERR_PTR(-EINVAL);
> +
> +	bio = bio_alloc(GFP_KERNEL, nr_pages);
> +	if (!bio)
> +		return ERR_PTR(-ENOMEM);
> +
> +	for (i = 0; i < nr_pages; i++) {
> +		if (len <= 0)
> +			break;
> +
> +		page = virt_to_page(ptr);
> +		offset = offset_in_page(ptr);
> +
> +		bytes = PAGE_SIZE - offset;
> +		if (bytes > len)
> +			bytes = len;
> +
> +		if (__bio_add_page(q, bio, page, bytes, offset) < bytes)
> +			break;
> +
> +		len -= bytes;
> +		offset = 0;
> +		ptr += bytes;
> +	}
> +
> +	bio->bi_end_io = bio_map_data_endio;
> +	bio->bi_rw |= (!read << BIO_RW);
> +	return bio;
> +}
> +

This code maps a pointer+length into a bio, but ide-scsi is fed an sglist,
just like all low-level SCSI drivers, so this doesn't really help.

Please understand the way I handle misalignment.  Originally I used
bounce buffers for the entire transfer if there was any misalignment,
but this doesn't give optimal performance.  To maintain security, the
bounce buffers must be zeroed before the I/O, otherwise a data scavenger
can read potentially sensitive data if a transfer doesn't fully overwrite
them.

This is where I got the idea to minimize the size of the extra bounce
buffers by using the largest aligned subset of the user area.  If the
base isn't aligned, the data must be shifted into place, hence the need
for the overlapped copying in 'idescsi_bounce_dma'.  If only the length
is misaligned, a small bounce buffer is appended to the bio just for the
partial block.  Notice what it does for performance:

- Extra memory allocation is minimized
- The user area doesn't need to be zeroed
- Copying is limited to the last block if the base is aligned

cdda2wav/cdrecord/cdrdao use buffers much larger than 512 bytes and
typically provide base aligned buffers, so this advanced technique
really pays off.

Yes, some code is ugly, but the bug it fixes is uglier.

Another goal of mine was to confine the patch to the ide-scsi module.
Making changes to high-level SCSI drivers or to IDE services would
definitely not be a good approach.

Given the above, the code you presented can't really simplify ide-scsi.
(If you have something that maps an sglist to a single contiguous block,
I could use it.:-)
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