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From: Christoph Hellwig <hch@lst.de>
To: Jens Axboe <axboe@kernel.dk>
Cc: Ming Lei <ming.lei@redhat.com>, 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: Fri, 8 Mar 2019 08:55:22 +0100
Message-ID: <20190308075522.GA12445@lst.de> (raw)
In-Reply-To: <b54683e8-fbff-01ab-db98-c35de8e00e9f@kernel.dk>

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.

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

  parent reply index

Thread overview: 57+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-11 19:00 [PATCHSET v15] io_uring IO interface Jens Axboe
2019-02-11 19:00 ` [PATCH 01/19] fs: add an iopoll method to struct file_operations Jens Axboe
2019-02-11 19:00 ` [PATCH] io_uring: add io_uring_event cache hit information Jens Axboe
2019-02-11 19:00 ` [PATCH 02/19] block: wire up block device iopoll method Jens Axboe
2019-02-11 19:00 ` [PATCH 03/19] block: add bio_set_polled() helper Jens Axboe
2019-02-11 19:00 ` [PATCH 04/19] iomap: wire up the iopoll method Jens Axboe
2019-02-11 19:00 ` [PATCH 05/19] Add io_uring IO interface Jens Axboe
2019-02-11 19:00 ` [PATCH 06/19] io_uring: add fsync support Jens Axboe
2019-02-11 19:00 ` [PATCH 07/19] io_uring: support for IO polling Jens Axboe
2019-02-11 19:00 ` [PATCH 08/19] fs: add fget_many() and fput_many() Jens Axboe
2019-02-11 19:00 ` [PATCH 09/19] io_uring: use fget/fput_many() for file references Jens Axboe
2019-02-11 19:00 ` [PATCH 10/19] io_uring: batch io_kiocb allocation Jens Axboe
2019-02-11 19:00 ` [PATCH 11/19] block: implement bio helper to add iter bvec pages to bio Jens Axboe
2019-02-20 22:58   ` Ming Lei
2019-02-21 17:45     ` Jens Axboe
2019-02-26  3:46       ` Eric Biggers
2019-02-26  4:34         ` Jens Axboe
2019-02-26 15:54           ` Jens Axboe
2019-02-27  1:21             ` Ming Lei
2019-02-27  1:47               ` Jens Axboe
2019-02-27  1:53                 ` Ming Lei
2019-02-27  1:57                   ` Jens Axboe
2019-02-27  2:21                     ` Ming Lei
2019-02-27  2:28                       ` Jens Axboe
2019-02-27  2:37                         ` Ming Lei
2019-02-27  2:43                           ` Jens Axboe
2019-02-27  3:09                             ` Ming Lei
2019-02-27  3:37                               ` Jens Axboe
2019-02-27  3:43                                 ` Jens Axboe
2019-02-27  3:44                                 ` Ming Lei
2019-02-27  4:05                                   ` Jens Axboe
2019-02-27  4:06                                     ` Jens Axboe
2019-02-27 19:42                                       ` Christoph Hellwig
2019-02-28  8:37                                         ` Ming Lei
2019-02-27 23:35                         ` Ming Lei
2019-03-08  7:55                         ` Christoph Hellwig [this message]
2019-03-08  9:12                           ` Ming Lei
2019-03-08  8:18                     ` Christoph Hellwig
2019-02-11 19:00 ` [PATCH 12/19] io_uring: add support for pre-mapped user IO buffers Jens Axboe
2019-02-19 19:08   ` Jann Horn
2019-02-22 22:29     ` Jens Axboe
2019-02-11 19:00 ` [PATCH 13/19] net: split out functions related to registering inflight socket files Jens Axboe
2019-02-11 19:00 ` [PATCH 14/19] io_uring: add file set registration Jens Axboe
2019-02-19 16:12   ` Jann Horn
2019-02-22 22:29     ` Jens Axboe
2019-02-11 19:00 ` [PATCH 15/19] io_uring: add submission polling Jens Axboe
2019-02-11 19:00 ` [PATCH 16/19] io_uring: add io_kiocb ref count Jens Axboe
2019-02-11 19:00 ` [PATCH 17/19] io_uring: add support for IORING_OP_POLL Jens Axboe
2019-02-11 19:00 ` [PATCH 18/19] io_uring: allow workqueue item to handle multiple buffered requests Jens Axboe
2019-02-11 19:00 ` [PATCH 19/19] io_uring: add io_uring_event cache hit information Jens Axboe
2019-02-21 12:10 ` [PATCHSET v15] io_uring IO interface Marek Majkowski
2019-02-21 17:48   ` Jens Axboe
2019-02-22 15:01     ` Marek Majkowski
2019-02-22 22:32       ` Jens Axboe
  -- strict thread matches above, loose matches on Subject: below --
2019-02-09 21:13 [PATCHSET v14] " Jens Axboe
2019-02-09 21:13 ` [PATCH 11/19] block: implement bio helper to add iter bvec pages to bio Jens Axboe
2019-02-08 17:34 [PATCHSET v13] io_uring IO interface Jens Axboe
2019-02-08 17:34 ` [PATCH 11/19] block: implement bio helper to add iter bvec pages to bio Jens Axboe
2019-02-09  9:45   ` Hannes Reinecke

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