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From: Ming Lei <ming.lei@redhat.com>
To: Jens Axboe <axboe@kernel.dk>
Cc: Ming Lei <tom.leiming@gmail.com>,
	Eric Biggers <ebiggers@kernel.org>,
	"open list:AIO" <linux-aio@kvack.org>,
	linux-block <linux-block@vger.kernel.org>,
	linux-api@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
	Jeff Moyer <jmoyer@redhat.com>, Avi Kivity <avi@scylladb.com>,
	jannh@google.com, Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH 11/19] block: implement bio helper to add iter bvec pages to bio
Date: Wed, 27 Feb 2019 11:09:45 +0800
Message-ID: <20190227030944.GG16802@ming.t460p> (raw)
In-Reply-To: <9ce37f86-dd8b-ef51-7507-d457585a892a@kernel.dk>

On Tue, Feb 26, 2019 at 07:43:32PM -0700, Jens Axboe wrote:
> On 2/26/19 7:37 PM, Ming Lei wrote:
> > On Tue, Feb 26, 2019 at 07:28:54PM -0700, Jens Axboe wrote:
> >> On 2/26/19 7:21 PM, Ming Lei wrote:
> >>> On Tue, Feb 26, 2019 at 06:57:16PM -0700, Jens Axboe wrote:
> >>>> On 2/26/19 6:53 PM, Ming Lei wrote:
> >>>>> On Tue, Feb 26, 2019 at 06:47:54PM -0700, Jens Axboe wrote:
> >>>>>> On 2/26/19 6:21 PM, Ming Lei wrote:
> >>>>>>> On Tue, Feb 26, 2019 at 11:56 PM Jens Axboe <axboe@kernel.dk> wrote:
> >>>>>>>>
> >>>>>>>> On 2/25/19 9:34 PM, Jens Axboe wrote:
> >>>>>>>>> On 2/25/19 8:46 PM, Eric Biggers wrote:
> >>>>>>>>>> Hi Jens,
> >>>>>>>>>>
> >>>>>>>>>> On Thu, Feb 21, 2019 at 10:45:27AM -0700, Jens Axboe wrote:
> >>>>>>>>>>> On 2/20/19 3:58 PM, Ming Lei wrote:
> >>>>>>>>>>>> On Mon, Feb 11, 2019 at 12:00:41PM -0700, Jens Axboe wrote:
> >>>>>>>>>>>>> For an ITER_BVEC, we can just iterate the iov and add the pages
> >>>>>>>>>>>>> to the bio directly. This requires that the caller doesn't releases
> >>>>>>>>>>>>> the pages on IO completion, we add a BIO_NO_PAGE_REF flag for that.
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> The current two callers of bio_iov_iter_get_pages() are updated to
> >>>>>>>>>>>>> check if they need to release pages on completion. This makes them
> >>>>>>>>>>>>> work with bvecs that contain kernel mapped pages already.
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Reviewed-by: Hannes Reinecke <hare@suse.com>
> >>>>>>>>>>>>> Reviewed-by: Christoph Hellwig <hch@lst.de>
> >>>>>>>>>>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> >>>>>>>>>>>>> ---
> >>>>>>>>>>>>>  block/bio.c               | 59 ++++++++++++++++++++++++++++++++-------
> >>>>>>>>>>>>>  fs/block_dev.c            |  5 ++--
> >>>>>>>>>>>>>  fs/iomap.c                |  5 ++--
> >>>>>>>>>>>>>  include/linux/blk_types.h |  1 +
> >>>>>>>>>>>>>  4 files changed, 56 insertions(+), 14 deletions(-)
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> diff --git a/block/bio.c b/block/bio.c
> >>>>>>>>>>>>> index 4db1008309ed..330df572cfb8 100644
> >>>>>>>>>>>>> --- a/block/bio.c
> >>>>>>>>>>>>> +++ b/block/bio.c
> >>>>>>>>>>>>> @@ -828,6 +828,23 @@ int bio_add_page(struct bio *bio, struct page *page,
> >>>>>>>>>>>>>  }
> >>>>>>>>>>>>>  EXPORT_SYMBOL(bio_add_page);
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> +static int __bio_iov_bvec_add_pages(struct bio *bio, struct iov_iter *iter)
> >>>>>>>>>>>>> +{
> >>>>>>>>>>>>> + const struct bio_vec *bv = iter->bvec;
> >>>>>>>>>>>>> + unsigned int len;
> >>>>>>>>>>>>> + size_t size;
> >>>>>>>>>>>>> +
> >>>>>>>>>>>>> + len = min_t(size_t, bv->bv_len, iter->count);
> >>>>>>>>>>>>> + size = bio_add_page(bio, bv->bv_page, len,
> >>>>>>>>>>>>> +                         bv->bv_offset + iter->iov_offset);
> >>>>>>>>>>>>
> >>>>>>>>>>>> iter->iov_offset needs to be subtracted from 'len', looks
> >>>>>>>>>>>> the following delta change[1] is required, otherwise memory corruption
> >>>>>>>>>>>> can be observed when running xfstests over loop/dio.
> >>>>>>>>>>>
> >>>>>>>>>>> Thanks, I folded this in.
> >>>>>>>>>>>
> >>>>>>>>>>> --
> >>>>>>>>>>> Jens Axboe
> >>>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>> syzkaller started hitting a crash on linux-next starting with this commit, and
> >>>>>>>>>> it still occurs even with your latest version that has Ming's fix folded in.
> >>>>>>>>>> Specifically, commit a566653ab5ab80a from your io_uring branch with commit date
> >>>>>>>>>> Sun Feb 24 08:20:53 2019 -0700.
> >>>>>>>>>>
> >>>>>>>>>> Reproducer:
> >>>>>>>>>>
> >>>>>>>>>> #define _GNU_SOURCE
> >>>>>>>>>> #include <fcntl.h>
> >>>>>>>>>> #include <linux/loop.h>
> >>>>>>>>>> #include <sys/ioctl.h>
> >>>>>>>>>> #include <sys/sendfile.h>
> >>>>>>>>>> #include <sys/syscall.h>
> >>>>>>>>>> #include <unistd.h>
> >>>>>>>>>>
> >>>>>>>>>> int main(void)
> >>>>>>>>>> {
> >>>>>>>>>>         int memfd, loopfd;
> >>>>>>>>>>
> >>>>>>>>>>         memfd = syscall(__NR_memfd_create, "foo", 0);
> >>>>>>>>>>
> >>>>>>>>>>         pwrite(memfd, "\xa8", 1, 4096);
> >>>>>>>>>>
> >>>>>>>>>>         loopfd = open("/dev/loop0", O_RDWR|O_DIRECT);
> >>>>>>>>>>
> >>>>>>>>>>         ioctl(loopfd, LOOP_SET_FD, memfd);
> >>>>>>>>>>
> >>>>>>>>>>         sendfile(loopfd, loopfd, NULL, 1000000);
> >>>>>>>>>> }
> >>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>> Crash:
> >>>>>>>>>>
> >>>>>>>>>> page:ffffea0001a6aab8 count:0 mapcount:0 mapping:0000000000000000 index:0x0
> >>>>>>>>>> flags: 0x100000000000000()
> >>>>>>>>>> raw: 0100000000000000 ffffea0001ad2c50 ffff88807fca49d0 0000000000000000
> >>>>>>>>>> raw: 0000000000000000 0000000000000000 00000000ffffffff
> >>>>>>>>>> page dumped because: VM_BUG_ON_PAGE(page_ref_count(page) == 0)
> >>>>>>>>>
> >>>>>>>>> I see what this is, I'll cut a fix for this tomorrow.
> >>>>>>>>
> >>>>>>>> Folded in a fix for this, it's in my current io_uring branch and my for-next
> >>>>>>>> branch.
> >>>>>>>
> >>>>>>> Hi Jens,
> >>>>>>>
> >>>>>>> I saw the following change is added:
> >>>>>>>
> >>>>>>> + if (size == len) {
> >>>>>>> + /*
> >>>>>>> + * For the normal O_DIRECT case, we could skip grabbing this
> >>>>>>> + * reference and then not have to put them again when IO
> >>>>>>> + * completes. But this breaks some in-kernel users, like
> >>>>>>> + * splicing to/from a loop device, where we release the pipe
> >>>>>>> + * pages unconditionally. If we can fix that case, we can
> >>>>>>> + * get rid of the get here and the need to call
> >>>>>>> + * bio_release_pages() at IO completion time.
> >>>>>>> + */
> >>>>>>> + get_page(bv->bv_page);
> >>>>>>>
> >>>>>>> Now the 'bv' may point to more than one page, so the following one may be
> >>>>>>> needed:
> >>>>>>>
> >>>>>>> int i;
> >>>>>>> struct bvec_iter_all iter_all;
> >>>>>>> struct bio_vec *tmp;
> >>>>>>>
> >>>>>>> mp_bvec_for_each_segment(tmp, bv, i, iter_all)
> >>>>>>>       get_page(tmp->bv_page);
> >>>>>>
> >>>>>> I guess that would be the safest, even if we don't currently have more
> >>>>>> than one page in there. I'll fix it up.
> >>>>>
> >>>>> It is easy to see multipage bvec from loop, :-)
> >>>>
> >>>> Speaking of this, I took a quick look at why we've now regressed a lot
> >>>> on IOPS perf with the multipage work. It looks like it's all related to
> >>>> the (much) fatter setup around iteration, which is related to this very
> >>>> topic too.
> >>>>
> >>>> Basically setup of things like bio_for_each_bvec() and indexing through
> >>>> nth_page() is MUCH slower than before.
> >>>
> >>> But bio_for_each_bvec() needn't nth_page(), and only bio_for_each_segment()
> >>> needs that. However, bio_for_each_segment() isn't called from
> >>> blk_queue_split() and blk_rq_map_sg().
> >>>
> >>> One issue is that bio_for_each_bvec() still advances by page size
> >>> instead of bvec->len, I guess that is the problem, will cook a patch
> >>> for your test.
> >>
> >> Probably won't make a difference for my test case...
> >>
> >>>> We need to do something about this, it's like tossing out months of
> >>>> optimizations.
> >>>
> >>> Some following optimization can be done, such as removing
> >>> biovec_phys_mergeable() from blk_bio_segment_split().
> >>
> >> I think we really need a fast path for <= PAGE_SIZE IOs, to the extent
> >> that it is possible. But iteration startup cost is a problem in a lot of
> >> spots, and a split fast path will only help a bit for that specific
> >> case.
> >>
> >> 5% regressions is HUGE. I know I've mentioned this before, I just want
> >> to really stress how big of a deal that is. It's enough to make me
> >> consider just reverting it again, which sucks, but I don't feel great
> >> shipping something that is known that much slower.
> >>
> >> Suggestions?
> > 
> > You mentioned nth_page() costs much in bio_for_each_bvec(), but which
> > shouldn't call into nth_page(). I will look into it first.
> 
> I'll check on the test box tomorrow, I lost connectivity before. I'll
> double check in the morning.
> 
> I'd focus on the blk_rq_map_sg() path, since that's the biggest cycle
> consumer.

Hi Jens,

Could you test the following patch which may improve on the 4k randio
test case?

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 066b66430523..c1ad8abbd9d6 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -447,7 +447,7 @@ static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio,
 	return biovec_phys_mergeable(q, &end_bv, &nxt_bv);
 }
 
-static struct scatterlist *blk_next_sg(struct scatterlist **sg,
+static inline struct scatterlist *blk_next_sg(struct scatterlist **sg,
 		struct scatterlist *sglist)
 {
 	if (!*sg)
@@ -483,7 +483,7 @@ static unsigned blk_bvec_map_sg(struct request_queue *q,
 
 		offset = (total + bvec->bv_offset) % PAGE_SIZE;
 		idx = (total + bvec->bv_offset) / PAGE_SIZE;
-		pg = nth_page(bvec->bv_page, idx);
+		pg = bvec_nth_page(bvec->bv_page, idx);
 
 		sg_set_page(*sg, pg, seg_size, offset);
 
@@ -512,7 +512,12 @@ __blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
 		(*sg)->length += nbytes;
 	} else {
 new_segment:
-		(*nsegs) += blk_bvec_map_sg(q, bvec, sglist, sg);
+		if (bvec->bv_offset + bvec->bv_len <= PAGE_SIZE) {
+			*sg = blk_next_sg(sg, sglist);
+			sg_set_page(*sg, bvec->bv_page, nbytes, bvec->bv_offset);
+			(*nsegs) += 1;
+		} else
+			(*nsegs) += blk_bvec_map_sg(q, bvec, sglist, sg);
 	}
 	*bvprv = *bvec;
 }
diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index 30a57b68d017..4376f683c08a 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -51,6 +51,11 @@ struct bvec_iter_all {
 	unsigned	done;
 };
 
+static inline struct page *bvec_nth_page(struct page *page, int idx)
+{
+	return idx == 0 ? page : nth_page(page, idx);
+}
+
 /*
  * various member access, note that bio_data should of course not be used
  * on highmem page vectors
@@ -87,8 +92,8 @@ struct bvec_iter_all {
 	      PAGE_SIZE - bvec_iter_offset((bvec), (iter)))
 
 #define bvec_iter_page(bvec, iter)				\
-	nth_page(mp_bvec_iter_page((bvec), (iter)),		\
-		 mp_bvec_iter_page_idx((bvec), (iter)))
+	bvec_nth_page(mp_bvec_iter_page((bvec), (iter)),		\
+		      mp_bvec_iter_page_idx((bvec), (iter)))
 
 #define bvec_iter_bvec(bvec, iter)				\
 ((struct bio_vec) {						\
@@ -171,7 +176,7 @@ static inline void mp_bvec_last_segment(const struct bio_vec *bvec,
 	unsigned total = bvec->bv_offset + bvec->bv_len;
 	unsigned last_page = (total - 1) / PAGE_SIZE;
 
-	seg->bv_page = nth_page(bvec->bv_page, last_page);
+	seg->bv_page = bvec_nth_page(bvec->bv_page, last_page);
 
 	/* the whole segment is inside the last page */
 	if (bvec->bv_offset >= last_page * PAGE_SIZE) {

thanks,
Ming

  reply index

Thread overview: 57+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-11 19:00 [PATCHSET v15] io_uring IO interface Jens Axboe
2019-02-11 19:00 ` [PATCH 01/19] fs: add an iopoll method to struct file_operations Jens Axboe
2019-02-11 19:00 ` [PATCH] io_uring: add io_uring_event cache hit information Jens Axboe
2019-02-11 19:00 ` [PATCH 02/19] block: wire up block device iopoll method Jens Axboe
2019-02-11 19:00 ` [PATCH 03/19] block: add bio_set_polled() helper Jens Axboe
2019-02-11 19:00 ` [PATCH 04/19] iomap: wire up the iopoll method Jens Axboe
2019-02-11 19:00 ` [PATCH 05/19] Add io_uring IO interface Jens Axboe
2019-02-11 19:00 ` [PATCH 06/19] io_uring: add fsync support Jens Axboe
2019-02-11 19:00 ` [PATCH 07/19] io_uring: support for IO polling Jens Axboe
2019-02-11 19:00 ` [PATCH 08/19] fs: add fget_many() and fput_many() Jens Axboe
2019-02-11 19:00 ` [PATCH 09/19] io_uring: use fget/fput_many() for file references Jens Axboe
2019-02-11 19:00 ` [PATCH 10/19] io_uring: batch io_kiocb allocation Jens Axboe
2019-02-11 19:00 ` [PATCH 11/19] block: implement bio helper to add iter bvec pages to bio Jens Axboe
2019-02-20 22:58   ` Ming Lei
2019-02-21 17:45     ` Jens Axboe
2019-02-26  3:46       ` Eric Biggers
2019-02-26  4:34         ` Jens Axboe
2019-02-26 15:54           ` Jens Axboe
2019-02-27  1:21             ` Ming Lei
2019-02-27  1:47               ` Jens Axboe
2019-02-27  1:53                 ` Ming Lei
2019-02-27  1:57                   ` Jens Axboe
2019-02-27  2:21                     ` Ming Lei
2019-02-27  2:28                       ` Jens Axboe
2019-02-27  2:37                         ` Ming Lei
2019-02-27  2:43                           ` Jens Axboe
2019-02-27  3:09                             ` Ming Lei [this message]
2019-02-27  3:37                               ` Jens Axboe
2019-02-27  3:43                                 ` Jens Axboe
2019-02-27  3:44                                 ` Ming Lei
2019-02-27  4:05                                   ` Jens Axboe
2019-02-27  4:06                                     ` Jens Axboe
2019-02-27 19:42                                       ` Christoph Hellwig
2019-02-28  8:37                                         ` Ming Lei
2019-02-27 23:35                         ` Ming Lei
2019-03-08  7:55                         ` Christoph Hellwig
2019-03-08  9:12                           ` Ming Lei
2019-03-08  8:18                     ` Christoph Hellwig
2019-02-11 19:00 ` [PATCH 12/19] io_uring: add support for pre-mapped user IO buffers Jens Axboe
2019-02-19 19:08   ` Jann Horn
2019-02-22 22:29     ` Jens Axboe
2019-02-11 19:00 ` [PATCH 13/19] net: split out functions related to registering inflight socket files Jens Axboe
2019-02-11 19:00 ` [PATCH 14/19] io_uring: add file set registration Jens Axboe
2019-02-19 16:12   ` Jann Horn
2019-02-22 22:29     ` Jens Axboe
2019-02-11 19:00 ` [PATCH 15/19] io_uring: add submission polling Jens Axboe
2019-02-11 19:00 ` [PATCH 16/19] io_uring: add io_kiocb ref count Jens Axboe
2019-02-11 19:00 ` [PATCH 17/19] io_uring: add support for IORING_OP_POLL Jens Axboe
2019-02-11 19:00 ` [PATCH 18/19] io_uring: allow workqueue item to handle multiple buffered requests Jens Axboe
2019-02-11 19:00 ` [PATCH 19/19] io_uring: add io_uring_event cache hit information Jens Axboe
2019-02-21 12:10 ` [PATCHSET v15] io_uring IO interface Marek Majkowski
2019-02-21 17:48   ` Jens Axboe
2019-02-22 15:01     ` Marek Majkowski
2019-02-22 22:32       ` Jens Axboe
  -- strict thread matches above, loose matches on Subject: below --
2019-02-09 21:13 [PATCHSET v14] " Jens Axboe
2019-02-09 21:13 ` [PATCH 11/19] block: implement bio helper to add iter bvec pages to bio Jens Axboe
2019-02-08 17:34 [PATCHSET v13] io_uring IO interface Jens Axboe
2019-02-08 17:34 ` [PATCH 11/19] block: implement bio helper to add iter bvec pages to bio Jens Axboe
2019-02-09  9:45   ` Hannes Reinecke

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