From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E770434D3BD for ; Wed, 5 Nov 2025 01:48:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762307305; cv=none; b=RdqxTnUVyiH4A556rU0+jrWmDzwsCFgiAb5x5x0u13HkCDWtb/PSnDTl2B6XVz4hshgjuD7MzhlrvcB4ARrLce9ooq2gl46ha58ABWT5Ya6cMCHORgNl9OQLpgV2iI5t2DYjY1HadHNi0l1LNHN/raHMWzhiu9463j3Y0IgH8LQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762307305; c=relaxed/simple; bh=k3rauIZ5CGXgfrJkKcA12x7VQSg277A/Wjd7cYzgaOQ=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=fz94PHwgi7wzKVWgNXhWwlJkR9Rbly7yAMU2OTBzrG7CozgawP77jjDs1ADC8+0jVggeC1b43Gcc9fZ0xLpPduBefDRwsAU4YtBgCkYWB7dh9EBcmfLdu8pZX//xNDKxBnMHTaiqDeKPt21nXxHZm5dpKFOrt/Xn4+Qe67GyuWM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=ZgKSGdlh; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="ZgKSGdlh" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1762307302; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=sMbwjC+PDOMjlfFNbA9ncfkYxb/LczwezWdontvo8i0=; b=ZgKSGdlhkHKuZDEfxrIpJyjoFtV6rsm5AZ62xT17PN99qeG4O5QZUUgcM6lGdJpOedlrIl ChzCmc850uO7y5TlA6IUi77nV+Fj9eNSRmUWn6ftSrN0mkjUAQMvFoN6MOhh9/YtGsQigv 8/ESE8qI3EdEJAzfTMN2yBOIdTOgcJ4= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-534-dQ_x-NeQNsSSKyQw_rO_Ug-1; Tue, 04 Nov 2025 20:48:20 -0500 X-MC-Unique: dQ_x-NeQNsSSKyQw_rO_Ug-1 X-Mimecast-MFC-AGG-ID: dQ_x-NeQNsSSKyQw_rO_Ug_1762307299 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (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-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 4A52D19560B0; Wed, 5 Nov 2025 01:48:19 +0000 (UTC) Received: from fedora (unknown [10.72.120.17]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 9C89A19560A2; Wed, 5 Nov 2025 01:48:15 +0000 (UTC) Date: Wed, 5 Nov 2025 09:48:09 +0800 From: Ming Lei To: Caleb Sander Mateos Cc: Jens Axboe , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] ublk: use copy_{to,from}_iter() for user copy Message-ID: References: <20251031010522.3509499-1-csander@purestorage.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 3.0 on 10.30.177.12 On Mon, Nov 03, 2025 at 08:40:30AM -0800, Caleb Sander Mateos wrote: > On Fri, Oct 31, 2025 at 4:04 PM Ming Lei wrote: > > > > On Fri, Oct 31, 2025 at 09:02:48AM -0700, Caleb Sander Mateos wrote: > > > On Thu, Oct 30, 2025 at 8:45 PM Ming Lei wrote: > > > > > > > > On Thu, Oct 30, 2025 at 07:05:21PM -0600, Caleb Sander Mateos wrote: > > > > > ublk_copy_user_pages()/ublk_copy_io_pages() currently uses > > > > > iov_iter_get_pages2() to extract the pages from the iov_iter and > > > > > memcpy()s between the bvec_iter and the iov_iter's pages one at a time. > > > > > Switch to using copy_to_iter()/copy_from_iter() instead. This avoids the > > > > > user page reference count increments and decrements and needing to split > > > > > the memcpy() at user page boundaries. It also simplifies the code > > > > > considerably. > > > > > > > > > > Signed-off-by: Caleb Sander Mateos > > > > > --- > > > > > drivers/block/ublk_drv.c | 62 +++++++++------------------------------- > > > > > 1 file changed, 14 insertions(+), 48 deletions(-) > > > > > > > > > > diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c > > > > > index 0c74a41a6753..852350e639d6 100644 > > > > > --- a/drivers/block/ublk_drv.c > > > > > +++ b/drivers/block/ublk_drv.c > > > > > @@ -912,58 +912,47 @@ static const struct block_device_operations ub_fops = { > > > > > .open = ublk_open, > > > > > .free_disk = ublk_free_disk, > > > > > .report_zones = ublk_report_zones, > > > > > }; > > > > > > > > > > -#define UBLK_MAX_PIN_PAGES 32 > > > > > - > > > > > struct ublk_io_iter { > > > > > - struct page *pages[UBLK_MAX_PIN_PAGES]; > > > > > struct bio *bio; > > > > > struct bvec_iter iter; > > > > > }; > > > > > > > > ->pages[] is actually for pinning user io pages in batch, so killing it may cause > > > > perf drop. > > > > > > As far as I can tell, copy_to_iter()/copy_from_iter() avoids the page > > > pinning entirely. It calls copy_to_user_iter() for each contiguous > > > user address range: > > > > > > size_t _copy_to_iter(const void *addr, size_t bytes, struct iov_iter *i) > > > { > > > if (WARN_ON_ONCE(i->data_source)) > > > return 0; > > > if (user_backed_iter(i)) > > > might_fault(); > > > return iterate_and_advance(i, bytes, (void *)addr, > > > copy_to_user_iter, memcpy_to_iter); > > > } > > > > > > Which just checks that the address range doesn't include any kernel > > > addresses and then memcpy()s directly via the userspace virtual > > > addresses: > > > > > > static __always_inline > > > size_t copy_to_user_iter(void __user *iter_to, size_t progress, > > > size_t len, void *from, void *priv2) > > > { > > > if (should_fail_usercopy()) > > > return len; > > > if (access_ok(iter_to, len)) { > > > from += progress; > > > instrument_copy_to_user(iter_to, from, len); > > > len = raw_copy_to_user(iter_to, from, len); > > > } > > > return len; > > > } > > > > > > static __always_inline __must_check unsigned long > > > raw_copy_to_user(void __user *dst, const void *src, unsigned long size) > > > { > > > return copy_user_generic((__force void *)dst, src, size); > > > } > > > > > > static __always_inline __must_check unsigned long > > > copy_user_generic(void *to, const void *from, unsigned long len) > > > { > > > stac(); > > > /* > > > * If CPU has FSRM feature, use 'rep movs'. > > > * Otherwise, use rep_movs_alternative. > > > */ > > > asm volatile( > > > "1:\n\t" > > > ALTERNATIVE("rep movsb", > > > "call rep_movs_alternative", > > > ALT_NOT(X86_FEATURE_FSRM)) > > > "2:\n" > > > _ASM_EXTABLE_UA(1b, 2b) > > > :"+c" (len), "+D" (to), "+S" (from), ASM_CALL_CONSTRAINT > > > : : "memory", "rax"); > > > clac(); > > > return len; > > > } > > > > > > Am I missing something? > > > > page is allocated & mapped in page fault handler. > > Right, physical pages certainly need to be allocated for the virtual > address range being copied to/from. But that would have happened > previously in iov_iter_get_pages2(), so this isn't a new cost. And as > you point out, in the common case that the virtual pages are already > mapped to physical pages, the copy won't cause any page faults. > > > > > However, in typical cases, pages in io buffer shouldn't be swapped out > > frequently, so this cleanup may be good, I will run some perf test. > > Thanks for testing. `fio/t/io_uring` shows 40% improvement on `./kublk -t null -q 2` with this patch in my test VM, so looks very nice improvement. Also it works well by forcing to pass IOSQE_ASYNC on the ublk uring_cmd, and this change is correct because the copy is guaranteed to be done in ublk daemon context. > > > > > Also copy_page_from_iter()/copy_page_to_iter() can be used for avoiding > > bvec_kmap_local(), and the two helper can handle one whole bvec instead > > of single page. > > Yes, that's a good idea. Thanks, I didn't know about that. > > > > > Then rq_for_each_bvec() can be used directly, and `ublk_io_iter` may be > > killed. > > Hmm, we still need a way to offset into the request (i.e. what > ublk_advance_io_iter() does currently). Are you thinking of a single > rq_for_each_bvec() loop that would skip bvecs until the offset is > reached and then copy until reaching the end of the user iterator? Yeah, that is basically what ublk_advance_io_iter() does. Thanks, Ming