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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe