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([2600:1700:e321:62f0:329c:23ff:fee3:9d7c]) by smtp.gmail.com with ESMTPSA id tk9-20020a05687189c900b0017ae6741157sm703896oab.4.2023.04.06.06.57.43 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 06 Apr 2023 06:57:43 -0700 (PDT) Sender: Guenter Roeck Message-ID: <83e6bc46-dfc0-0e95-e69c-5b996af1e50b@roeck-us.net> Date: Thu, 6 Apr 2023 06:57:41 -0700 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.9.0 Subject: Re: [PATCH] mm-treewide-redefine-max_order-sanely-fix.txt Content-Language: en-US To: "Kirill A. Shutemov" Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <9460377a-38aa-4f39-ad57-fb73725f92db@roeck-us.net> <20230406072529.vupqyrzqnhyozeyh@box.shutemov.name> From: Guenter Roeck In-Reply-To: <20230406072529.vupqyrzqnhyozeyh@box.shutemov.name> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/6/23 00:25, Kirill A. Shutemov wrote: > On Wed, Apr 05, 2023 at 10:20:26PM -0700, Guenter Roeck wrote: >> Hi, >> >> On Wed, Mar 15, 2023 at 06:38:00PM +0300, Kirill A. Shutemov wrote: >>> fix min() warning >>> >>> Link: https://lkml.kernel.org/r/20230315153800.32wib3n5rickolvh@box >>> Reported-by: kernel test robot >>> Link: https://lore.kernel.org/oe-kbuild-all/202303152343.D93IbJmn-lkp@intel.com/ >>> Signed-off-by: "Kirill A. Shutemov" >>> Cc: "Kirill A. Shutemov" >>> Cc: Zi Yan >>> Signed-off-by: Andrew Morton >> >> This patch results in various boot failures (hang) on arm targets >> in linux-next. Debug messages reveal the reason. >> >> ########### MAX_ORDER=10 start=0 __ffs(start)=-1 min()=10 min_t=-1 >> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ >> >> If start==0, __ffs(start) returns 0xfffffff or (as int) -1, which min_t() >> interprets as such, while min() apparently uses the returned unsigned long >> value. Obviously a negative order isn't received well by the rest of the >> code. > > Actually, __ffs() is not defined for 0. > > Maybe something like this? > > diff --git a/mm/memblock.c b/mm/memblock.c > index 7911224b1ed3..63603b943bd0 100644 > --- a/mm/memblock.c > +++ b/mm/memblock.c > @@ -2043,7 +2043,11 @@ static void __init __free_pages_memory(unsigned long start, unsigned long end) > int order; > > while (start < end) { > - order = min_t(int, MAX_ORDER, __ffs(start)); > + /* __ffs() behaviour is undefined for 0 */ > + if (start) > + order = min_t(int, MAX_ORDER, __ffs(start)); > + else > + order = MAX_ORDER; > Shouldn't that be else order = 0; ? Guenter