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Date: Tue, 4 Feb 2014 04:35:39 -0800
From: Kent Overstreet <kmo@...erainc.com>
To: Hugh Dickins <hughd@...gle.com>
Cc: "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" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] block: Explicitly handle discard/write same segments
Thanks!
On Tue, Feb 4, 2014 at 4:25 AM, Hugh Dickins <hughd@...gle.com> wrote:
> 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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