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Wed, 26 Mar 2025 21:49:51 -0400 X-MC-Unique: plQgOVqaM2ONbHP5apBjJw-1 X-Mimecast-MFC-AGG-ID: plQgOVqaM2ONbHP5apBjJw_1743040190 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id A0A161903081; Thu, 27 Mar 2025 01:49:49 +0000 (UTC) Received: from fedora (unknown [10.72.120.3]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 6A57E1801762; Thu, 27 Mar 2025 01:49:44 +0000 (UTC) Date: Thu, 27 Mar 2025 09:49:39 +0800 From: Ming Lei To: Caleb Sander Mateos Cc: Jens Axboe , linux-block@vger.kernel.org, Keith Busch , Uday Shankar Subject: Re: [PATCH 4/8] ublk: add segment parameter Message-ID: References: <20250324134905.766777-1-ming.lei@redhat.com> <20250324134905.766777-5-ming.lei@redhat.com> Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 On Wed, Mar 26, 2025 at 09:43:13AM -0700, Caleb Sander Mateos wrote: > On Tue, Mar 25, 2025 at 7:17 PM Ming Lei wrote: > > > > On Tue, Mar 25, 2025 at 12:43:26PM -0700, Caleb Sander Mateos wrote: > > > On Mon, Mar 24, 2025 at 6:16 PM Ming Lei wrote: > > > > > > > > On Mon, Mar 24, 2025 at 03:26:06PM -0700, Caleb Sander Mateos wrote: > > > > > On Mon, Mar 24, 2025 at 6:49 AM Ming Lei wrote: > > > > > > > > > > > > IO split is usually bad in io_uring world, since -EAGAIN is caused and > > > > > > IO handling may have to fallback to io-wq, this way does hurt performance. > > > > > > > > > > > > ublk starts to support zero copy recently, for avoiding unnecessary IO > > > > > > split, ublk driver's segment limit should be aligned with backend > > > > > > device's segment limit. > > > > > > > > > > > > Another reason is that io_buffer_register_bvec() needs to allocate bvecs, > > > > > > which number is aligned with ublk request segment number, so that big > > > > > > memory allocation can be avoided by setting reasonable max_segments limit. > > > > > > > > > > > > So add segment parameter for providing ublk server chance to align > > > > > > segment limit with backend, and keep it reasonable from implementation > > > > > > viewpoint. > > > > > > > > > > > > Signed-off-by: Ming Lei > > > > > > --- > > > > > > drivers/block/ublk_drv.c | 15 ++++++++++++++- > > > > > > include/uapi/linux/ublk_cmd.h | 9 +++++++++ > > > > > > 2 files changed, 23 insertions(+), 1 deletion(-) > > > > > > > > > > > > diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c > > > > > > index acb6aed7be75..53a463681a41 100644 > > > > > > --- a/drivers/block/ublk_drv.c > > > > > > +++ b/drivers/block/ublk_drv.c > > > > > > @@ -74,7 +74,7 @@ > > > > > > #define UBLK_PARAM_TYPE_ALL \ > > > > > > (UBLK_PARAM_TYPE_BASIC | UBLK_PARAM_TYPE_DISCARD | \ > > > > > > UBLK_PARAM_TYPE_DEVT | UBLK_PARAM_TYPE_ZONED | \ > > > > > > - UBLK_PARAM_TYPE_DMA_ALIGN) > > > > > > + UBLK_PARAM_TYPE_DMA_ALIGN | UBLK_PARAM_TYPE_SEGMENT) > > > > > > > > > > > > struct ublk_rq_data { > > > > > > struct kref ref; > > > > > > @@ -580,6 +580,13 @@ static int ublk_validate_params(const struct ublk_device *ub) > > > > > > return -EINVAL; > > > > > > } > > > > > > > > > > > > + if (ub->params.types & UBLK_PARAM_TYPE_SEGMENT) { > > > > > > + const struct ublk_param_segment *p = &ub->params.seg; > > > > > > + > > > > > > + if (!is_power_of_2(p->seg_boundary_mask + 1)) > > > > > > + return -EINVAL; > > > > > > > > > > Looking at blk_validate_limits(), it seems like there are some > > > > > additional requirements? Looks like seg_boundary_mask has to be at > > > > > least PAGE_SIZE - 1 > > > > > > > > Yeah, it isn't done in ublk because block layer runs the check, and it > > > > will be failed when starting the device. That said we take block layer's > > > > default setting, which isn't good from UAPI viewpoint, since block > > > > layer may change the default setting. > > > > > > Even though blk_validate_limits() rejects it, it appears to log a > > > warning. That seems undesirable for something controllable from > > > userspace. > > > /* > > > * By default there is no limit on the segment boundary alignment, > > > * but if there is one it can't be smaller than the page size as > > > * that would break all the normal I/O patterns. > > > */ > > > if (!lim->seg_boundary_mask) > > > lim->seg_boundary_mask = BLK_SEG_BOUNDARY_MASK; > > > if (WARN_ON_ONCE(lim->seg_boundary_mask < BLK_MIN_SEGMENT_SIZE - 1)) > > > return -EINVAL; > > > > Yes, it has been addressed in my local version, and we need to make it > > a hw/sw interface. > > > > > > > > > > > > > Also it is bad to associate device property with PAGE_SIZE which is > > > > a variable actually. The latest kernel has replaced PAGE_SIZE with 4096 > > > > for segment limits. > > > > > > > > I think we can take 4096 for validation here. > > > > > > > > > and max_segment_size has to be at least PAGE_SIZE > > > > > if virt_boundary_mask is set? > > > > > > > > If virt_boundary_mask is set, max_segment_size will be ignored usually > > > > except for some stacking devices. > > > > > > Sorry, I had it backwards. The requirement is if virt_boundary_mask is > > > *not* set: > > > /* > > > * Stacking device may have both virtual boundary and max segment > > > * size limit, so allow this setting now, and long-term the two > > > * might need to move out of stacking limits since we have immutable > > > * bvec and lower layer bio splitting is supposed to handle the two > > > * correctly. > > > */ > > > if (lim->virt_boundary_mask) { > > > if (!lim->max_segment_size) > > > lim->max_segment_size = UINT_MAX; > > > } else { > > > /* > > > * The maximum segment size has an odd historic 64k default that > > > * drivers probably should override. Just like the I/O size we > > > * require drivers to at least handle a full page per segment. > > > */ > > > if (!lim->max_segment_size) > > > lim->max_segment_size = BLK_MAX_SEGMENT_SIZE; > > > if (WARN_ON_ONCE(lim->max_segment_size < BLK_MIN_SEGMENT_SIZE)) > > > return -EINVAL; > > > } > > > > Right. > > > > Please feel free to see if the revised patch is good: > > > > > > From 0718b9f130b3bc9b9b06907c687fb5b9eea172f7 Mon Sep 17 00:00:00 2001 > > From: Ming Lei > > Date: Mon, 24 Mar 2025 12:33:59 +0000 > > Subject: [PATCH V2 3/8] ublk: add segment parameter > > > > IO split is usually bad in io_uring world, since -EAGAIN is caused and > > IO handling may have to fallback to io-wq, this way does hurt performance. > > > > ublk starts to support zero copy recently, for avoiding unnecessary IO > > split, ublk driver's segment limit should be aligned with backend > > device's segment limit. > > > > Another reason is that io_buffer_register_bvec() needs to allocate bvecs, > > which number is aligned with ublk request segment number, so that big > > memory allocation can be avoided by setting reasonable max_segments limit. > > > > So add segment parameter for providing ublk server chance to align > > segment limit with backend, and keep it reasonable from implementation > > viewpoint. > > > > Signed-off-by: Ming Lei > > --- > > drivers/block/ublk_drv.c | 20 +++++++++++++++++++- > > include/uapi/linux/ublk_cmd.h | 21 +++++++++++++++++++++ > > 2 files changed, 40 insertions(+), 1 deletion(-) > > > > diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c > > index 6fa1384c6436..6367476cef2b 100644 > > --- a/drivers/block/ublk_drv.c > > +++ b/drivers/block/ublk_drv.c > > @@ -74,7 +74,7 @@ > > #define UBLK_PARAM_TYPE_ALL \ > > (UBLK_PARAM_TYPE_BASIC | UBLK_PARAM_TYPE_DISCARD | \ > > UBLK_PARAM_TYPE_DEVT | UBLK_PARAM_TYPE_ZONED | \ > > - UBLK_PARAM_TYPE_DMA_ALIGN) > > + UBLK_PARAM_TYPE_DMA_ALIGN | UBLK_PARAM_TYPE_SEGMENT) > > > > struct ublk_rq_data { > > struct kref ref; > > @@ -580,6 +580,18 @@ static int ublk_validate_params(const struct ublk_device *ub) > > return -EINVAL; > > } > > > > + if (ub->params.types & UBLK_PARAM_TYPE_SEGMENT) { > > + const struct ublk_param_segment *p = &ub->params.seg; > > + > > + if (!is_power_of_2(p->seg_boundary_mask + 1)) > > + return -EINVAL; > > + > > + if (p->seg_boundary_mask + 1 < UBLK_MIN_SEGMENT_SIZE) > > + return -EINVAL; > > + if (p->max_segment_size < UBLK_MIN_SEGMENT_SIZE) > > + return -EINVAL; > > These checks look good, except they don't allow omitting > seg_boundary_mask or max_segment_size. I can imagine a ublk server > might want to set only some of the segment limits and leave the others > as 0? Any unset field in `ublk_param_segment` should be undefined behavior(or either rejected or one default value is taken), we can document it. Thanks for raising the issue. `seg_boundary_mask` is very similar with `max_segment_size`, so if either one of the two are set, the other one should get one default value if it is unset. If either one of the above two are set, 'max_segments' can't be zero. Thanks, Ming