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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe