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Date: Tue, 4 Feb 2014 04:25:24 -0800 (PST)
From: Hugh Dickins <hughd@...gle.com>
To: Kent Overstreet <kmo@...erainc.com>
cc: Hugh Dickins <hughd@...gle.com>,
"Martin K. Petersen" <martin.petersen@...cle.com>,
Jens Axboe <axboe@...nel.dk>, Shaohua Li <shli@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: Explicitly handle discard/write same segments
On Tue, 4 Feb 2014, Kent Overstreet wrote:
> 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 <kmo@...erainc.com>
> ---
> 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.
Thanks a lot, Kent: I'm glad to report, this is working fine for me.
Hugh
>
> 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
--
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