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.
>
> 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().
Thanks,
Ming
