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?
--
Jens Axboe
