From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-169.mta0.migadu.com [91.218.175.169]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3A57138DC79 for ; Fri, 4 Sep 2026 03:24:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.169 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788492253; cv=none; b=M2XhBK+WXi/bmcV13XztBEJdgTu+qJ4Xw8hxcZQ8qspsScz3jkPhMx9zAUv8mwenTRQlkiYAKniJSMiNYkhbaBXG+lW1wbx7z7mpvZPqKzs2Be8J2mZQaAZzc/aIr6MJnCD+5k0GmupegYQElLzAmnYqEyr7uIRJM3RXtRFRg8A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788492253; c=relaxed/simple; bh=AODDE3qRj81d1tj4XlbGSZLMRB5KcVA6K05GsD7cVN0=; h=Message-ID:Date:MIME-Version:Cc:Subject:To:References:From: In-Reply-To:Content-Type; b=hqQT+70CUvGrHhcoPY0XyJj+HYjJPFDxITqI4oLgQP3Sb1JAEISbdOQoIXglJMkRQbcjjgAUd106kkJkAGsZ5/VzsK5td+50pDBbs/jKNGvv9fTWn1ANVjDvfC268N8Eoi9LUls1DCiUkq/ZUAzL2cpVDPA8ajGNwttQWNcDbCA= 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=KwNnt+4q; arc=none smtp.client-ip=91.218.175.169 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="KwNnt+4q" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=AODDE3qRj81d1tj4XlbGSZLMRB5KcVA6K05GsD7cVN0=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1788492249; v=1; x=1789097049; b=KwNnt+4q6tc8eIOY87WBZXmiVb/1rBad2jOPovbVFxOZSdR1vfFMoseTU7mfShbGVcWocJV/ hGEKFTMwhFIR8VsGRo801fZjLThWzVxqgUKojdxzYpA6zjW7j+RK7C+ImMM4bLXYlNDGBLkXlPL s01bNl8/Nav7nmyc1MggGQfQ= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 1809903fe04b5f0a; Fri, 04 Sep 2026 03:24:09 +0000 X-Mizu-Trace-ID: 1809903fe04b5f0a X-Migadu-Flow: FLOW_OUT Message-ID: <6dc39f3e-e62e-4209-aa93-9308d086bed2@linux.dev> Date: Fri, 4 Sep 2026 11:24:01 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Cc: cui.tao@linux.dev, tj@kernel.org, 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, cuitao@kylinos.cn, Yu Kuai Subject: Re: [PATCH v5] blk-throttle: fix divide-by-zero on legacy iops limit of 0 To: yukuai@fygo.io, axboe@kernel.dk References: <20260904030607.1193800-1-cui.tao@linux.dev> <63b0770e-844a-4249-a64e-85b1e1978fab@fygo.io> From: Tao Cui In-Reply-To: <63b0770e-844a-4249-a64e-85b1e1978fab@fygo.io> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 在 2026/9/4 11:12, yu kuai 写道: > Hi, > > 在 2026/9/4 11:06, 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. >> >> Fix it in tg_set_conf() by clamping the value to UINT_MAX, consistent >> with tg_set_limit(). This closes the truncation root cause: with 0 no >> longer reachable as a stored limit, the HZ / iops_limit divide is never >> hit. >> >> Signed-off-by: Tao Cui >> Reviewed-by: Yu Kuai > This tag is wrong, please use yukuai@fygo.io. Sorry, will update right away. >> >> --- >> Changes in v5: >> - Rebase onto current linux-next head (no code change, context shifted only). >> - Add the Reviewed-by tag collected on v4. >> - Link to v4: https://lore.kernel.org/r/20260722102459.253189-1-cui.tao@linux.dev >> >> Changes in v4: >> - Drop the defensive "iops_limit == 0" check in tg_dispatch_iops_time(): >> with the tg_set_conf() clamp in place, 0 can never be stored as a limit, >> so the runtime check only guards an unreachable state. (Yu Kuai) >> - Drop the Fixes: tag: the unclamped write -- and the iops=0 behavior it >> can produce (calculate_io_allowed() returns 0, so no IO is issued) -- >> long predates the commit that added the HZ / iops_limit divide, so >> attributing it there was incorrect. (Yu Kuai) >> >> 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) >> >> 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 | 6 ++++-- >> 1 file changed, 4 insertions(+), 2 deletions(-) >> >> diff --git a/block/blk-throttle.c b/block/blk-throttle.c >> index ffc3b70065d4..97ad7959d006 100644 >> --- a/block/blk-throttle.c >> +++ b/block/blk-throttle.c >> @@ -1383,10 +1383,12 @@ 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; >