From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 37DD1C43381 for ; Wed, 27 Feb 2019 02:39:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0664B20643 for ; Wed, 27 Feb 2019 02:39:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729384AbfB0Chi (ORCPT ); Tue, 26 Feb 2019 21:37:38 -0500 Received: from mx1.redhat.com ([209.132.183.28]:57622 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729276AbfB0Chh (ORCPT ); Tue, 26 Feb 2019 21:37:37 -0500 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 3674630BA2EB; Wed, 27 Feb 2019 02:37:37 +0000 (UTC) Received: from ming.t460p (ovpn-8-21.pek2.redhat.com [10.72.8.21]) by smtp.corp.redhat.com (Postfix) with ESMTPS id DB00D5DD6E; Wed, 27 Feb 2019 02:37:26 +0000 (UTC) Date: Wed, 27 Feb 2019 10:37:21 +0800 From: Ming Lei To: Jens Axboe 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 Subject: Re: [PATCH 11/19] block: implement bio helper to add iter bvec pages to bio Message-ID: <20190227023719.GF16802@ming.t460p> References: <20190226034613.GA676@sol.localdomain> <1652577e-787b-638e-625d-c200fb144a9d@kernel.dk> <09820845-07cb-5153-e1c5-59ed185db26f@kernel.dk> <20190227015336.GD16802@ming.t460p> <20190227022144.GE16802@ming.t460p> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.1 (2017-09-22) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.47]); Wed, 27 Feb 2019 02:37:37 +0000 (UTC) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org 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. Thanks, Ming From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ming Lei Subject: Re: [PATCH 11/19] block: implement bio helper to add iter bvec pages to bio Date: Wed, 27 Feb 2019 10:37:21 +0800 Message-ID: <20190227023719.GF16802@ming.t460p> References: <20190226034613.GA676@sol.localdomain> <1652577e-787b-638e-625d-c200fb144a9d@kernel.dk> <09820845-07cb-5153-e1c5-59ed185db26f@kernel.dk> <20190227015336.GD16802@ming.t460p> <20190227022144.GE16802@ming.t460p> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: owner-linux-aio@kvack.org To: Jens Axboe 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 List-Id: linux-api@vger.kernel.org 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. Thanks, Ming -- To unsubscribe, send a message with 'unsubscribe linux-aio' in the body to majordomo@kvack.org. For more info on Linux AIO, see: http://www.kvack.org/aio/ Don't email: aart@kvack.org