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

From: Ming Lei <ming.lei@redhat.com>
To: Jens Axboe <axboe@kernel.dk>
Cc: 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, Christoph Hellwig <hch@lst.de>,
	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: Wed, 27 Feb 2019 09:53:37 +0800	[thread overview]
Message-ID: <20190227015336.GD16802@ming.t460p> (raw)
In-Reply-To: <09820845-07cb-5153-e1c5-59ed185db26f@kernel.dk>

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, :-)

Thanks,
Ming