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.
Thanks,
Ming
