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Shutemov" Cc: Guenter Roeck , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm-treewide-redefine-max_order-sanely-fix.txt Message-ID: References: <9460377a-38aa-4f39-ad57-fb73725f92db@roeck-us.net> <20230406072529.vupqyrzqnhyozeyh@box.shutemov.name> <83e6bc46-dfc0-0e95-e69c-5b996af1e50b@roeck-us.net> <20230406151015.yndcm24fyxitvqyc@box.shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230406151015.yndcm24fyxitvqyc@box.shutemov.name> X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 81BFA40002 X-Stat-Signature: bjee38oaz3yp3is11kxz1obmhphjrh9a X-HE-Tag: 1680815685-599509 X-HE-Meta: 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 Bo6ZFvmu 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Apr 06, 2023 at 06:10:15PM +0300, Kirill A. Shutemov wrote: > On Thu, Apr 06, 2023 at 06:57:41AM -0700, Guenter Roeck wrote: > > 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; > > ? > > +Mike. > > No. start == 0 is MAX_ORDER-aligned. We want to free the pages in the > largest chunks alignment allows. Right. Before the changes to MAX_ORDER it was order = min(MAX_ORDER - 1UL, __ffs(start)); which would evaluate to 10. I'd just prefer the comment to include the explanation about why we choose MAX_ORDER for start == 0. Say /* * __ffs() behaviour is undefined for 0 and we want to free the * pages in the largest chunks alignment allows, so set order to * MAX_ORDER when start == 0 */ > -- > Kiryl Shutsemau / Kirill A. Shutemov -- Sincerely yours, Mike.