From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-170.mta0.migadu.com (out-170.mta0.migadu.com [91.218.175.170]) (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 25E9B3B19AF; Wed, 22 Jul 2026 10:16:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.170 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784715402; cv=none; b=FZtd3tQSUMcTOgIX0WizYrMdNA4elWbOKJsAKa1SqLMOpDosPqLDK0Fhs3Ia22GO7s/hog0IG779sj0RGXuctRkGe5f84DYEomFMUsT2/0fyd2GLxQj/Jgvlgwo32V4OpEXoXOBvgqMP3lN0aoG9T1KKuZwg3rrvNHzYSGTd9W8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784715402; c=relaxed/simple; bh=NX5iAyJ3e+snnWnA1WdCRJwEZpKuNeQi8eIEWjzNxgc=; h=Message-ID:Date:MIME-Version:Cc:Subject:To:References:From: In-Reply-To:Content-Type; b=gKtm06fjytxIEm1SoXhjVJUYGy/KCU9Owpoiwg8gzdfTfkI/Snr5GElW4pLuz6orQDp6JuNG5DFySqaP6PD/ZA+BEHlzeyPUrsmuMWiririalIgcRbfouhuume/GZ8USxta1T5W6NULR/8uDTwR1j6slNY0CTamayDHk0OCHJT4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=cehKhQic; arc=none smtp.client-ip=91.218.175.170 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="cehKhQic" Message-ID: <36cefa74-bd88-44be-94b2-13b613e50528@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1784715387; 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=lcMeD72NxnTGKJ/lMIJ+2LVnzwq0+08WrYCqZgmK26c=; b=cehKhQicz/6PIkkIv45mtq558rplym/jlIAg27IqhD3cTzWHKZIwYsRuaGcSk+vmKuG8dg 1WuA8mNQuSZUllm1NIDyK6Thx+rOlpHnux2vld7dCpw8WZbew2o9ltA29aR8Y/QMgcNFlv ywwZf12Oz5VfMSDh7ZBlTKk4o/uTlxI= Date: Wed, 22 Jul 2026 18:15:58 +0800 Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Cc: cui.tao@linux.dev, linux-block@vger.kernel.org, josef@toxicpanda.com, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, david.laight.linux@gmail.com, haris.iqbal@linux.dev, Tao Cui Subject: Re: [PATCH v3] blk-throttle: fix divide-by-zero on legacy iops limit of 0 To: yukuai@fygo.io, tj@kernel.org, axboe@kernel.dk References: <20260721040850.90544-1-cui.tao@linux.dev> <3e1616f1-1dc1-4912-8404-4f53b2d1205c@fygo.io> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Tao Cui In-Reply-To: <3e1616f1-1dc1-4912-8404-4f53b2d1205c@fygo.io> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 在 2026/7/22 15:25, yu kuai 写道: > Hi, > > 在 2026/7/21 12:08, Tao Cui 写道: >> From: Tao Cui >> >> Writing a multiple of 2^32 (e.g. 4294967296) to a legacy cgroup v1 >> throttle iops file (blkio.throttle.{read,write}_iops_device) silently >> truncates to 0: tg_set_conf() stores the sscanf-parsed u64 value into >> an unsigned int field with no clamping. The cgroup v2 path, >> tg_set_limit(), already clamps the same kind of value with >> min_t(u64, val, UINT_MAX), but the legacy path never did. Note that >> the "!v -> U64_MAX" mapping only catches an explicit zero and does not >> catch a value that truncates to zero. >> >> With iops stored as 0, tg_update_has_rules() sets has_rules_iops[] and >> the next IO reaches tg_within_iops_limit(), which computes >> >> jiffy_wait = max(jiffy_wait, HZ / iops_limit + 1); >> >> triggering a divide-by-zero oops. The unclamped write in tg_set_conf() >> is long-standing, but it only became a crash once the HZ / iops_limit >> divide was added. >> >> Fix it in two places: >> >> * tg_set_conf(): clamp the value to UINT_MAX, consistent with >> tg_set_limit(). This closes the truncation root cause (and the >> general silent truncation for any value above UINT_MAX). >> >> * tg_dispatch_iops_time(): treat iops_limit == 0 as unlimited so the >> divide in tg_within_iops_limit() is never reached, defending >> against any future path that could produce a zero limit. >> >> Fixes: 1beabab88ece ("blk-throttle: fix lower control under super low iops limit") > Hi Kuai, Thanks for the review, and you're right on both points. > I think the root cause is that iops_limit set to 0, and what's the behavior of throttle > with iops=0? Since calculate_io_allowed() will always return 0, it's likely that no IO > can be issued. On the root cause: yes, iops=0 is the real root cause and it predates 1beabab88ece. With iops_limit = 0, calculate_io_allowed() returns 0 (0 * jiffy_elapsed), so no IO can be issued at all; 1beabab88ece only turned that pre-existing iops=0 hang into a divide-by-zero via the new HZ / iops_limit. Attributing it to 1beabab88ece was therefore wrong -- my apologies. I'll drop the Fixes: tag in v4. > >> Signed-off-by: Tao Cui >> >> --- >> Changes in v3: >> - Drop the (u64) cast on UINT_MAX: the kernel's type-checked min() accepts >> two unsigned types of different width (both >= 4 bytes), so >> min(v, UINT_MAX) compiles clean. (David Laight) >> - Add a Fixes: tag pointing at the commit that introduced the >> HZ / iops_limit divide, which is also where the oops became reachable. >> >> Changes in v2: >> - Use a "void *field" local for the config write so the assignment reads >> *(u64 *)field / *(unsigned int *)field instead of the >> (type *)((void *)tg + of_cft(of)->private) casts. >> - Use min(v, UINT_MAX) instead of min_t(u64, v, UINT_MAX). >> --- >> block/blk-throttle.c | 13 ++++++++++--- >> 1 file changed, 10 insertions(+), 3 deletions(-) >> >> diff --git a/block/blk-throttle.c b/block/blk-throttle.c >> index ffc3b70065d4..e894852c3142 100644 >> --- a/block/blk-throttle.c >> +++ b/block/blk-throttle.c >> @@ -883,7 +883,12 @@ static unsigned long tg_dispatch_iops_time(struct throtl_grp *tg, struct bio *bi >> u32 iops_limit = tg_iops_limit(tg, rw); >> unsigned long iops_wait; >> >> - if (iops_limit == UINT_MAX || tg->flags & THROTL_TG_CANCELING) >> + /* >> + * iops_limit == 0 is not a valid limit. Treat it as unlimited so we >> + * never reach the HZ / iops_limit divide in tg_within_iops_limit(). >> + */ >> + if (iops_limit == UINT_MAX || iops_limit == 0 || >> + tg->flags & THROTL_TG_CANCELING) >> return 0; > > Is the above still needed with the tg_set_conf() changes? 0 is not a valid limit, and > it should not exist in the first place, so I don't like the above new checking. On the iops_limit == 0 check: agreed, dropping it. With the tg_set_conf() clamp in place, 0 can never be stored as a limit (explicit 0 maps to U64_MAX, anything that would truncate to 0 is clamped to UINT_MAX, and the init value is UINT_MAX), so the runtime check only guards an unreachable state. v4 keeps only the tg_set_conf() clamp. Will send v4 shortly. Thanks, Tao > >> >> tg_update_slice(tg, rw); >> @@ -1383,10 +1388,12 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of, >> tg = blkg_to_tg(ctx.blkg); >> tg_update_carryover(tg); >> >> + void *field = (void *)tg + of_cft(of)->private; >> + >> if (is_u64) >> - *(u64 *)((void *)tg + of_cft(of)->private) = v; >> + *(u64 *)field = v; >> else >> - *(unsigned int *)((void *)tg + of_cft(of)->private) = v; >> + *(unsigned int *)field = min(v, UINT_MAX); >> >> tg_conf_updated(tg, false); >> ret = 0; >