From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christophe Leroy Date: Fri, 25 Oct 2019 08:52:54 +0000 Subject: Re: [PATCH V7] mm/debug: Add tests validating architecture page table helpers Message-Id: List-Id: References: <69256008-2235-4AF1-A3BA-0146C82CCB93@lca.pw> <3cfec421-4006-4159-ca32-313ff5196ff9@c-s.fr> <763d58b4-f532-0bba-bf2b-71433ac514fb@arm.com> In-Reply-To: <763d58b4-f532-0bba-bf2b-71433ac514fb@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit To: Anshuman Khandual , Qian Cai Cc: Mark Rutland , linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org, Peter Zijlstra , James Hogan , Tetsuo Handa , Heiko Carstens , Michal Hocko , linux-mm@kvack.org, Dave Hansen , Paul Mackerras , sparclinux@vger.kernel.org, Thomas Gleixner , linux-s390@vger.kernel.org, Michael Ellerman , x86@kernel.org, Russell King - ARM Linux , Matthew Wilcox , Steven Price , Jason Gunthorpe , Gerald Schaefer , linux-snps-arc@lists.infradead.org, Ingo Molnar , Kees Cook , Masahiro Yamada , Mark Brown , "Kirill A . Shutemov" , Dan Williams , Vlastimil Babka , linux-arm-kernel@lists.infradead.org, Sri Krishna chowdary , Ard Biesheuvel , Greg Kroah-Hartman , linux-mips@vger.kernel.org, Ralf Baechle , linux-kernel@vger.kernel.org, Paul Burton , Mike Rapoport , Vineet Gupta , Martin Schwidefsky , Andrew Morton , linuxppc-dev@lists.ozlabs.org, "David S. Miller" , Mike Kravetz Le 25/10/2019 à 10:24, Anshuman Khandual a écrit : > > > On 10/25/2019 12:41 PM, Christophe Leroy wrote: >> >> >> Le 25/10/2019 à 07:52, Qian Cai a écrit : >>> >>> >>>> On Oct 24, 2019, at 11:45 PM, Anshuman Khandual wrote: >>>> >>>> Nothing specific. But just tested this with x86 defconfig with relevant configs >>>> which are required for this test. Not sure if it involved W=1. >>> >>> No, it will not. It needs to run like, >>> >>> make W=1 -j 64 2>/tmp/warns >>> >> >> Are we talking about this peace of code ? >> >> +static unsigned long __init get_random_vaddr(void) >> +{ >> +    unsigned long random_vaddr, random_pages, total_user_pages; >> + >> +    total_user_pages = (TASK_SIZE - FIRST_USER_ADDRESS) / PAGE_SIZE; >> + >> +    random_pages = get_random_long() % total_user_pages; >> +    random_vaddr = FIRST_USER_ADDRESS + random_pages * PAGE_SIZE; >> + >> +    WARN_ON((random_vaddr > TASK_SIZE) || >> +        (random_vaddr < FIRST_USER_ADDRESS)); >> +    return random_vaddr; >> +} >> + >> >> ramdom_vaddr is unsigned, >> random_pages is unsigned and lower than total_user_pages >> >> So the max value random_vaddr can get is FIRST_USER_ADDRESS + ((TASK_SIZE - FIRST_USER_ADDRESS - 1) / PAGE_SIZE) * PAGE_SIZE = TASK_SIZE - 1 >> And the min value random_vaddr can get is FIRST_USER_ADDRESS (that's when random_pages = 0) > > That's right. > >> >> So the WARN_ON() is just unneeded, isn't it ? > > It is just a sanity check on possible vaddr values before it's corresponding > page table mappings could be created. If it's worth to drop this in favor of > avoiding these unwanted warning messages on x86, will go ahead with it as it > is not super important. > But you are checking what ? That the compiler does calculation correctly or what ? As mentionned just above, based on the calculation done, what you are testing cannot happen, so I'm having a hard time understanding what kind of sanity check it can be. Can you give an exemple of a situation which could trigger the warning ? Christophe