Hi,
On Thu, 03 Apr 2008 12:34:13 +0900
Tejun Heo <htejun@xxxxxxxxx> wrote:
> Hello,
>
> FUJITA Tomonori wrote:
> > - /*
> > - * __blk_rq_map_user() copies the buffers if starting address
> > - * or length isn't aligned to dma_pad_mask. As the copied
> > - * buffer is always page aligned, we know that there's enough
> > - * room for padding. Extend the last bio and update
> > - * rq->data_len accordingly.
> > - *
> > - * On unmap, bio_uncopy_user() will use unmodified
> > - * bio_map_data pointed to by bio->bi_private.
> > - */
> > - if (len & q->dma_pad_mask) {
> > - unsigned int pad_len = (q->dma_pad_mask & ~len) + 1;
> > - struct bio *tail = rq->biotail;
> > -
> > - tail->bi_io_vec[tail->bi_vcnt - 1].bv_len += pad_len;
> > - tail->bi_size += pad_len;
> > -
> > - rq->extra_len += pad_len;
> > - }
> > -
> > rq->buffer = rq->data = NULL;
> > return 0;
> > unmap_rq:
> > diff --git a/block/blk-merge.c b/block/blk-merge.c
> > index 0f58616..2a81c87 100644
> > --- a/block/blk-merge.c
> > +++ b/block/blk-merge.c
> > @@ -220,6 +220,13 @@ new_segment:
> > bvprv = bvec;
> > } /* segments in rq */
> >
> > + if (sg && (q->dma_pad_mask & rq->data_len)) {
> > + unsigned int pad_len = (q->dma_pad_mask & ~rq->data_len) + 1;
> > +
> > + sg->length += pad_len;
> > + rq->extra_len += pad_len;
> > + }
> > +
>
> We're not sure it will always have the extra room. For example,
> map_user_iov doesn't do it and there can be in-kernel issuers.
Yeah, I see.
> Maybe we need yet another flag indicating padding space
> availability?
How about doing the exact same thing that the drain buffer does? We
can put pre-allocated buffer to a queue and save one sg entry for it.
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