Re: [PATCH 11/19] block: implement bio helper to add iter bvec pages to bio

[Christoph Hellwig <hch@infradead.org>]

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.

PRPs only unforturtunately are a little dumb and could lead to all kinds
of whacky splitting.