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

From: Ming Lei <ming.lei@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>, Ming Lei <tom.leiming@gmail.com>,
	Eric Biggers <ebiggers@kernel.org>,
	"open list:AIO" <linux-aio@kvack.org>,
	linux-block <linux-block@vger.kernel.org>,
	linux-api@vger.kernel.org, Jeff Moyer <jmoyer@redhat.com>,
	Avi Kivity <avi@scylladb.com>,
	jannh@google.com, Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH 11/19] block: implement bio helper to add iter bvec pages to bio
Date: Fri, 8 Mar 2019 17:12:28 +0800
Message-ID: <20190308091116.GA10956@ming.t460p> (raw)
In-Reply-To: <20190308075522.GA12445@lst.de>

On Fri, Mar 08, 2019 at 08:55:22AM +0100, Christoph Hellwig 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);
> > >>>>
> > > 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.
> 
> FYI, I've got a nice fast path for the driver side in nvme here, but
> I'll need to do some more testing before submitting it:
> 
> http://git.infradead.org/users/hch/block.git/shortlog/refs/heads/nvme-optimize-single-segment-io
> 
> But in the block layer I think one major issue is all the phys_segments
> crap.  What we really should do is to remove bi_phys_segments and all
> the front/back segment crap and only do the calculation of the actual
> per-bio segments once, just before adding the bio to the segment.

I have enabled multi-page bvec for passthrough IO in the following:

https://github.com/ming1/linux/commits/v5.0-blk-for-blk_post_mp

in which .bi_phys_segments becomes same with .bi_vcnt for passthrough bio.

Also intra-bvec merging in one bio has been killed, then only the merge
between bios is required, and seems we still need front/back segment size,
especially some use cases(such as mkfs) may make lots of small mergeable bios.

> 
> And don't bother with it at all unless the driver has weird segment
> size or boundary limitations.

It should be easy to observe that .bv_len is bigger than max segment
size.

Thanks,
Ming