Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754126AbaBDMfn (ORCPT ); Tue, 4 Feb 2014 07:35:43 -0500 Received: from mail-oa0-f42.google.com ([209.85.219.42]:38966 "EHLO mail-oa0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751513AbaBDMfk (ORCPT ); Tue, 4 Feb 2014 07:35:40 -0500 MIME-Version: 1.0 In-Reply-To: References: <20140114023346.GN9037@kmo> <20140114044841.GO9037@kmo> <20140114222424.GA7333@moria.home.lan> <20140117012105.GS9037@kmo> <20140204101748.GA12440@kmo-pixel> Date: Tue, 4 Feb 2014 04:35:39 -0800 Message-ID: Subject: Re: [PATCH] block: Explicitly handle discard/write same segments From: Kent Overstreet To: Hugh Dickins Cc: "Martin K. Petersen" , Jens Axboe , Shaohua Li , Andrew Morton , "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Thanks! On Tue, Feb 4, 2014 at 4:25 AM, Hugh Dickins 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 >> --- >> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/