On Mon, 03 Mar 2008 18:21:13 +0900
Tejun Heo [off-list ref] wrote:

FUJITA Tomonori wrote:

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I can't see what changing the meaning of rq->data_len (and
investigating all the block drivers) gives us.

No matter which way you go, you change the meaning of rq->data_len and
you MUST inspect rq->data_len usage whichever way you go.

The patch doens't change that rq->data_len means the true data
length. But yeah, it breaks rq->data_len == sum(sg). So it might break
some drivers.

Yeah, that's what I was saying.  You end up breaking one of the two
assumptions.  As sglist is getting modified for any driver if it has DMA
alignment set, whether rq->data_len is adjusted together or not, sglist
and data_len usages have to be audited.

My patch (well, James' original approach) doesn't affect drivers that
don't use drain buffer. rq->data_len still means the true data length
and rq->data_len is equal to sum(sg) for them. So right now we need to
audit only libata.

But your patch changes the meaning of rq->data_len. It affects all the
drivers. So it breaks non libata stuff, like the SMP handler. We need
to audit all the drivers.