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[66.29.188.166]) by smtp.gmail.com with ESMTPSA id d26sm18155313pfn.86.2019.02.26.20.05.14 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 26 Feb 2019 20:05:16 -0800 (PST) Subject: Re: [PATCH 11/19] block: implement bio helper to add iter bvec pages to bio To: Ming Lei Cc: Ming Lei , Eric Biggers , "open list:AIO" , linux-block , linux-api@vger.kernel.org, Christoph Hellwig , Jeff Moyer , Avi Kivity , jannh@google.com, Al Viro References: <09820845-07cb-5153-e1c5-59ed185db26f@kernel.dk> <20190227015336.GD16802@ming.t460p> <20190227022144.GE16802@ming.t460p> <20190227023719.GF16802@ming.t460p> <9ce37f86-dd8b-ef51-7507-d457585a892a@kernel.dk> <20190227030944.GG16802@ming.t460p> <7a1f4293-8f88-81e0-9464-cc27e39184cb@kernel.dk> <20190227034409.GH16802@ming.t460p> From: Jens Axboe Message-ID: <2a48c5da-7eea-13a1-9939-9e182e11dc9b@kernel.dk> Date: Tue, 26 Feb 2019 21:05:14 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1 MIME-Version: 1.0 In-Reply-To: <20190227034409.GH16802@ming.t460p> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 2/26/19 8:44 PM, Ming Lei wrote: > On Tue, Feb 26, 2019 at 08:37:05PM -0700, Jens Axboe wrote: >> On 2/26/19 8:09 PM, Ming Lei wrote: >>> On Tue, Feb 26, 2019 at 07:43:32PM -0700, Jens Axboe wrote: >>>> On 2/26/19 7:37 PM, Ming Lei 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 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 >>>>>>>>>>>>>>>>> Reviewed-by: Christoph Hellwig >>>>>>>>>>>>>>>>> Signed-off-by: Jens Axboe >>>>>>>>>>>>>>>>> --- >>>>>>>>>>>>>>>>> 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 >>>>>>>>>>>>>> #include >>>>>>>>>>>>>> #include >>>>>>>>>>>>>> #include >>>>>>>>>>>>>> #include >>>>>>>>>>>>>> #include >>>>>>>>>>>>>> >>>>>>>>>>>>>> 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? >>>>> >>>>> You mentioned nth_page() costs much in bio_for_each_bvec(), but which >>>>> shouldn't call into nth_page(). I will look into it first. >>>> >>>> I'll check on the test box tomorrow, I lost connectivity before. I'll >>>> double check in the morning. >>>> >>>> I'd focus on the blk_rq_map_sg() path, since that's the biggest cycle >>>> consumer. >>> >>> Hi Jens, >>> >>> Could you test the following patch which may improve on the 4k randio >>> test case? >> >> A bit, it's up 1% with this patch. I'm going to try without the >> get_page/put_page that we had earlier, to see where we are in regards to >> the old baseline. > > OK, today I will test io_uring over null_blk on one real machine and see > if something can be improved. For reference, I'm running the default t/io_uring from fio, which is QD=128, fixed files/buffers, and polled. Running it on two devices to max out the CPU core: sudo taskset -c 0 t/io_uring /dev/nvme1n1 /dev/nvme5n1 since nvme1n1 tops out at 1164K 4k rand reads (hardware limit). Just tried with null_blk, since I haven't done that before, and I get about 1875K from a single device with the same test case. Using 2 devices yields the same result, so we're CPU core bound at that point. We don't get the sg walk with null_blk though, but I do see about 4% blk_queue_split() time with that. -- Jens Axboe