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@xxxxxxxxx> 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@xxxxxxxx>
> >>>>>>>>>>>>>>> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> >>>>>>>>>>>>>>> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> >>>>>>>>>>>>>>> ---
> >>>>>>>>>>>>>>> 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.
Thanks,
Ming
