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block: add a dma mapping iterator

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blk_rq_map_sg is maze of nested loops.  Untangle it by creating an
iterator that returns [paddr,len] tuples for DMA mapping, and then
implement the DMA logic on top of this.  This not only removes code
at the source level, but also generates nicer binary code:

$ size block/blk-merge.o.*
   text	   data	    bss	    dec	    hex	filename
  10001	    432	      0	  10433	   28c1	block/blk-merge.o.new
  10317	    468	      0	  10785	   2a21	block/blk-merge.o.old

Last but not least it will be used as a building block for a new
DMA mapping helper that doesn't rely on struct scatterlist.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250106081609.798289-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Christoph Hellwig 2025-01-06 09:15:29 +01:00 • committed by Jens Axboe
parent 2caca8fc7a
commit b7175e24d6
1 changed files with 70 additions and 107 deletions
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177
block/blk-merge.c
View file

@ -473,6 +473,63 @@ unsigned int blk_recalc_rq_segments(struct request *rq)
return nr_phys_segs;
}
struct phys_vec {
phys_addr_t paddr;
u32 len;
};
static bool blk_map_iter_next(struct request *req,
struct req_iterator *iter, struct phys_vec *vec)
{
unsigned int max_size;
struct bio_vec bv;
if (req->rq_flags & RQF_SPECIAL_PAYLOAD) {
if (!iter->bio)
return false;
vec->paddr = bvec_phys(&req->special_vec);
vec->len = req->special_vec.bv_len;
iter->bio = NULL;
return true;
}
if (!iter->iter.bi_size)
return false;
bv = mp_bvec_iter_bvec(iter->bio->bi_io_vec, iter->iter);
vec->paddr = bvec_phys(&bv);
max_size = get_max_segment_size(&req->q->limits, vec->paddr, UINT_MAX);
bv.bv_len = min(bv.bv_len, max_size);
bio_advance_iter_single(iter->bio, &iter->iter, bv.bv_len);
/*
* If we are entirely done with this bi_io_vec entry, check if the next
* one could be merged into it. This typically happens when moving to
* the next bio, but some callers also don't pack bvecs tight.
*/
while (!iter->iter.bi_size || !iter->iter.bi_bvec_done) {
struct bio_vec next;
if (!iter->iter.bi_size) {
if (!iter->bio->bi_next)
break;
iter->bio = iter->bio->bi_next;
iter->iter = iter->bio->bi_iter;
}
next = mp_bvec_iter_bvec(iter->bio->bi_io_vec, iter->iter);
if (bv.bv_len + next.bv_len > max_size ||
!biovec_phys_mergeable(req->q, &bv, &next))
break;
bv.bv_len += next.bv_len;
bio_advance_iter_single(iter->bio, &iter->iter, next.bv_len);
}
vec->len = bv.bv_len;
return true;
}
static inline struct scatterlist *blk_next_sg(struct scatterlist **sg,
struct scatterlist *sglist)
{
@ -490,120 +547,26 @@ static inline struct scatterlist *blk_next_sg(struct scatterlist **sg,
return sg_next(*sg);
}
static unsigned blk_bvec_map_sg(struct request_queue *q,
struct bio_vec *bvec, struct scatterlist *sglist,
struct scatterlist **sg)
{
unsigned nbytes = bvec->bv_len;
unsigned nsegs = 0, total = 0;
while (nbytes > 0) {
unsigned offset = bvec->bv_offset + total;
unsigned len = get_max_segment_size(&q->limits,
bvec_phys(bvec) + total, nbytes);
struct page *page = bvec->bv_page;
/*
* Unfortunately a fair number of drivers barf on scatterlists
* that have an offset larger than PAGE_SIZE, despite other
* subsystems dealing with that invariant just fine. For now
* stick to the legacy format where we never present those from
* the block layer, but the code below should be removed once
* these offenders (mostly MMC/SD drivers) are fixed.
*/
page += (offset >> PAGE_SHIFT);
offset &= ~PAGE_MASK;
*sg = blk_next_sg(sg, sglist);
sg_set_page(*sg, page, len, offset);
total += len;
nbytes -= len;
nsegs++;
}
return nsegs;
}
static inline int __blk_bvec_map_sg(struct bio_vec bv,
struct scatterlist *sglist, struct scatterlist **sg)
{
*sg = blk_next_sg(sg, sglist);
sg_set_page(*sg, bv.bv_page, bv.bv_len, bv.bv_offset);
return 1;
}
/* only try to merge bvecs into one sg if they are from two bios */
static inline bool
__blk_segment_map_sg_merge(struct request_queue *q, struct bio_vec *bvec,
struct bio_vec *bvprv, struct scatterlist **sg)
{
int nbytes = bvec->bv_len;
if (!*sg)
return false;
if ((*sg)->length + nbytes > queue_max_segment_size(q))
return false;
if (!biovec_phys_mergeable(q, bvprv, bvec))
return false;
(*sg)->length += nbytes;
return true;
}
static int __blk_bios_map_sg(struct request_queue *q, struct bio *bio,
struct scatterlist *sglist,
struct scatterlist **sg)
{
struct bio_vec bvec, bvprv = { NULL };
struct bvec_iter iter;
int nsegs = 0;
bool new_bio = false;
for_each_bio(bio) {
bio_for_each_bvec(bvec, bio, iter) {
/*
* Only try to merge bvecs from two bios given we
* have done bio internal merge when adding pages
* to bio
*/
if (new_bio &&
__blk_segment_map_sg_merge(q, &bvec, &bvprv, sg))
goto next_bvec;
if (bvec.bv_offset + bvec.bv_len <= PAGE_SIZE)
nsegs += __blk_bvec_map_sg(bvec, sglist, sg);
else
nsegs += blk_bvec_map_sg(q, &bvec, sglist, sg);
next_bvec:
new_bio = false;
}
if (likely(bio->bi_iter.bi_size)) {
bvprv = bvec;
new_bio = true;
}
}
return nsegs;
}
/*
* map a request to scatterlist, return number of sg entries setup. Caller
* must make sure sg can hold rq->nr_phys_segments entries
* Map a request to scatterlist, return number of sg entries setup. Caller
* must make sure sg can hold rq->nr_phys_segments entries.
*/
int __blk_rq_map_sg(struct request_queue *q, struct request *rq,
struct scatterlist *sglist, struct scatterlist **last_sg)
{
struct req_iterator iter = {
.bio = rq->bio,
.iter = rq->bio->bi_iter,
};
struct phys_vec vec;
int nsegs = 0;
if (rq->rq_flags & RQF_SPECIAL_PAYLOAD)
nsegs = __blk_bvec_map_sg(rq->special_vec, sglist, last_sg);
else if (rq->bio)
nsegs = __blk_bios_map_sg(q, rq->bio, sglist, last_sg);
while (blk_map_iter_next(rq, &iter, &vec)) {
*last_sg = blk_next_sg(last_sg, sglist);
sg_set_page(*last_sg, phys_to_page(vec.paddr), vec.len,
offset_in_page(vec.paddr));
nsegs++;
}
if (*last_sg)
sg_mark_end(*last_sg);