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From: Ming Lei <ming.lei@redhat.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: Jens Axboe <axboe@kernel.dk>, 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: Thu, 28 Feb 2019 16:37:20 +0800
Message-ID: <20190228083719.GA24857@ming.t460p> (raw)
In-Reply-To: <20190227194241.GA3860@infradead.org>

On Wed, Feb 27, 2019 at 11:42:41AM -0800, Christoph Hellwig wrote:
> On Tue, Feb 26, 2019 at 09:06:23PM -0700, Jens Axboe wrote:
> > On 2/26/19 9:05 PM, Jens Axboe wrote:
> > > On 2/26/19 8:44 PM, Ming Lei wrote:
> > >> On Tue, Feb 26, 2019 at 08:37:05PM -0700, Jens Axboe wrote:
> > >>> On 2/26/19 8:09 PM, Ming Lei wrote:
> > >>>> 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?
> > >>>
> > >>> A bit, it's up 1% with this patch. I'm going to try without the
> > >>> get_page/put_page that we had earlier, to see where we are in regards to
> > >>> the old baseline.
> > >>
> > >> OK, today I will test io_uring over null_blk on one real machine and see
> > >> if something can be improved.
> > > 
> > > For reference, I'm running the default t/io_uring from fio, which is
> > > QD=128, fixed files/buffers, and polled. Running it on two devices to
> > > max out the CPU core:
> > > 
> > > sudo taskset -c 0 t/io_uring /dev/nvme1n1 /dev/nvme5n1
> > 
> > Forgot to mention, this is loading nvme with 12 poll queues, which is of
> > course important to get good performance on this test case.
> 
> Btw, is your nvme device SGL capable?  There is some low hanging fruit
> in that IFF a device has SGL support we can basically dumb down
> blk_mq_map_sg to never split in this case ever because we don't have
> any segment size limits.

Indeed.

In case of SGL, big sg list may not be needed and blk_rq_map_sg() can be
skipped if proper DMA mapping interface is to return the dma address
for each segment. That can be one big improvement.

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
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 [this message]
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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