Immutable biovecs changed the way biovecs are interpreted - drivers no longer use bi_vcnt, they have to go by bi_iter.bi_size (to allow for using part of an existing segment without modifying it).
This breaks with discards and write_same bios, since for those bi_size has nothing to do with segments in the biovec. So for now, we need a fairly gross hack - we fortunately know that there will never be more than one segment for the entire request, so we can special case discard/write_same. Signed-off-by: Kent Overstreet <[email protected]> --- On Fri, Jan 31, 2014 at 09:17:25AM -0800, Hugh Dickins wrote: > On Thu, 16 Jan 2014, Kent Overstreet wrote: > > > > Ok, I reread the code and figured it out - the analagous change also has to > > be > > made in __blk_segment_map_sg(). I'll mail out a patch for this tomorrow > > after > > I've stared at the code more and had less beer. > > I'd been hoping for a patch to try, but now your changes have hit Linus's > tree: so today we have discard broken there too, crashing as originally > reported on the NULL struct page pointer in __blk_recalc_rq_segments()'s > page_to_pfn(bv.bv_page). > > How to reproduce it? I hope you'll find easier ways, but I get it with > swapping to SSD (remember "swapon -d" to enable discard). I'm just doing > what I've done for years, running a pair of make -j20 kbuilds to tmpfs in > limited RAM (I use mem=700M with 1.5G of swap: but that would be far too > little RAM for a general config of current tree), to get plenty of fairly > chaotic swapping but good forward progress nonetheless (if the sizes are > too small, then it'll just thrash abysmally or be OOM-killed). > > But please do send me a patch and I'll give it a try - thanks. Hugh - can you give this patch a try? Passes my tests but I was never able to reproduce your crash, unfortunately. block/blk-merge.c | 91 +++++++++++++++++++++++++++++++++++++------------------ 1 file changed, 62 insertions(+), 29 deletions(-) diff --git a/block/blk-merge.c b/block/blk-merge.c index 8f8adaa954..6c583f9c5b 100644 --- a/block/blk-merge.c +++ b/block/blk-merge.c @@ -21,6 +21,16 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q, if (!bio) return 0; + /* + * This should probably be returning 0, but blk_add_request_payload() + * (Christoph!!!!) + */ + if (bio->bi_rw & REQ_DISCARD) + return 1; + + if (bio->bi_rw & REQ_WRITE_SAME) + return 1; + fbio = bio; cluster = blk_queue_cluster(q); seg_size = 0; @@ -161,30 +171,60 @@ new_segment: *bvprv = *bvec; } -/* - * 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) +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 req_iterator iter; - struct scatterlist *sg; + struct bvec_iter iter; int nsegs, cluster; nsegs = 0; cluster = blk_queue_cluster(q); - /* - * for each bio in rq - */ - sg = NULL; - rq_for_each_segment(bvec, rq, iter) { - __blk_segment_map_sg(q, &bvec, sglist, &bvprv, &sg, - &nsegs, &cluster); - } /* segments in rq */ + if (bio->bi_rw & REQ_DISCARD) { + /* + * This is a hack - drivers should be neither modifying the + * biovec, nor relying on bi_vcnt - but because of + * blk_add_request_payload(), a discard bio may or may not have + * a payload we need to set up here (thank you Christoph) and + * bi_vcnt is really the only way of telling if we need to. + */ + + if (bio->bi_vcnt) + goto single_segment; + + return 0; + } + + if (bio->bi_rw & REQ_WRITE_SAME) { +single_segment: + *sg = sglist; + bvec = bio_iovec(bio); + sg_set_page(*sg, bvec.bv_page, bvec.bv_len, bvec.bv_offset); + return 1; + } + + for_each_bio(bio) + bio_for_each_segment(bvec, bio, iter) + __blk_segment_map_sg(q, &bvec, sglist, &bvprv, sg, + &nsegs, &cluster); + 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 + */ +int blk_rq_map_sg(struct request_queue *q, struct request *rq, + struct scatterlist *sglist) +{ + struct scatterlist *sg = NULL; + int nsegs = 0; + + if (rq->bio) + nsegs = __blk_bios_map_sg(q, rq->bio, sglist, &sg); if (unlikely(rq->cmd_flags & REQ_COPY_USER) && (blk_rq_bytes(rq) & q->dma_pad_mask)) { @@ -230,20 +270,13 @@ EXPORT_SYMBOL(blk_rq_map_sg); int blk_bio_map_sg(struct request_queue *q, struct bio *bio, struct scatterlist *sglist) { - struct bio_vec bvec, bvprv = { NULL }; - struct scatterlist *sg; - int nsegs, cluster; - struct bvec_iter iter; - - nsegs = 0; - cluster = blk_queue_cluster(q); - - sg = NULL; - bio_for_each_segment(bvec, bio, iter) { - __blk_segment_map_sg(q, &bvec, sglist, &bvprv, &sg, - &nsegs, &cluster); - } /* segments in bio */ + struct scatterlist *sg = NULL; + int nsegs; + struct bio *next = bio->bi_next; + bio->bi_next = NULL; + nsegs = __blk_bios_map_sg(q, bio, sglist, &sg); + bio->bi_next = next; if (sg) sg_mark_end(sg); -- 1.9.rc1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
