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Mon, 7 Dec 2020 04:38:57 +0000 (UTC) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 89F2B1042; Sun, 6 Dec 2020 20:38:56 -0800 (PST) Received: from [10.163.86.92] (unknown [10.163.86.92]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 22C713F718; Sun, 6 Dec 2020 20:38:53 -0800 (PST) Subject: Re: [RFC V2 3/3] s390/mm: Define arch_get_mappable_range() To: David Hildenbrand , Heiko Carstens Cc: linux-mm@kvack.org, akpm@linux-foundation.org, linux-arm-kernel@lists.infradead.org, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, Vasily Gorbik References: <1606706992-26656-1-git-send-email-anshuman.khandual@arm.com> <1606706992-26656-4-git-send-email-anshuman.khandual@arm.com> <20201202203233.GB11274@osiris> <24905c32-f6c1-97a0-000f-f822b9870ea5@arm.com> <20201203115133.GB9994@osiris> <4d6c9ec4-f1be-46b9-5d67-5c53f5afedc5@redhat.com> From: Anshuman Khandual Message-ID: <62c60c9e-20d6-25bd-94d0-78bfed0f2476@arm.com> Date: Mon, 7 Dec 2020 10:08:49 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <4d6c9ec4-f1be-46b9-5d67-5c53f5afedc5@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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 12/3/20 5:31 PM, David Hildenbrand wrote: > On 03.12.20 12:51, Heiko Carstens wrote: >> On Thu, Dec 03, 2020 at 06:03:00AM +0530, Anshuman Khandual wrote: >>>>> diff --git a/arch/s390/mm/extmem.c b/arch/s390/mm/extmem.c >>>>> index 5060956b8e7d..cc055a78f7b6 100644 >>>>> --- a/arch/s390/mm/extmem.c >>>>> +++ b/arch/s390/mm/extmem.c >>>>> @@ -337,6 +337,11 @@ __segment_load (char *name, int do_nonshared, unsigned long *addr, unsigned long >>>>> goto out_free_resource; >>>>> } >>>>> >>>>> + if (seg->end + 1 > VMEM_MAX_PHYS || seg->end + 1 < seg->start_addr) { >>>>> + rc = -ERANGE; >>>>> + goto out_resource; >>>>> + } >>>>> + >>>>> rc = vmem_add_mapping(seg->start_addr, seg->end - seg->start_addr + 1); >>>>> if (rc) >>>>> goto out_resource; >>>>> diff --git a/arch/s390/mm/vmem.c b/arch/s390/mm/vmem.c >>>>> index b239f2ba93b0..06dddcc0ce06 100644 >>>>> --- a/arch/s390/mm/vmem.c >>>>> +++ b/arch/s390/mm/vmem.c >>>>> @@ -532,14 +532,19 @@ void vmem_remove_mapping(unsigned long start, unsigned long size) >>>>> mutex_unlock(&vmem_mutex); >>>>> } >>>>> >>>>> +struct range arch_get_mappable_range(void) >>>>> +{ >>>>> + struct range memhp_range; >>>>> + >>>>> + memhp_range.start = 0; >>>>> + memhp_range.end = VMEM_MAX_PHYS; >>>>> + return memhp_range; >>>>> +} >>>>> + >>>>> int vmem_add_mapping(unsigned long start, unsigned long size) >>>>> { >>>>> int ret; >>>>> >>>>> - if (start + size > VMEM_MAX_PHYS || >>>>> - start + size < start) >>>>> - return -ERANGE; >>>>> - >>>> >>>> I really fail to see how this could be considered an improvement for >>>> s390. Especially I do not like that the (central) range check is now >>>> moved to the caller (__segment_load). Which would mean potential >>>> additional future callers would have to duplicate that code as well. >>> >>> The physical range check is being moved to the generic hotplug code >>> via arch_get_mappable_range() instead, making the existing check in >>> vmem_add_mapping() redundant. Dropping the check there necessitates >>> adding back a similar check in __segment_load(). Otherwise there >>> will be a loss of functionality in terms of range check. >>> >>> May be we could just keep this existing check in vmem_add_mapping() >>> as well in order avoid this movement but then it would be redundant >>> check in every hotplug path. >>> >>> So I guess the choice is to either have redundant range checks in >>> all hotplug paths or future internal callers of vmem_add_mapping() >>> take care of the range check. >> >> The problem I have with this current approach from an architecture >> perspective: we end up having two completely different methods which >> are doing the same and must be kept in sync. This might be obvious >> looking at this patch, but I'm sure this will go out-of-sync (aka >> broken) sooner or later. > > Exactly, there should be one function only that was the whole idea of > arch_get_mappable_range(). > >> >> Therefore I would really like to see a single method to do the range >> checking. Maybe you could add a callback into architecture code, so >> that such an architecture specific function could also be used >> elsewhere. Dunno. >> > > I think we can just switch to using "memhp_range_allowed()" here then > after implementing arch_get_mappable_range(). > > Doesn't hurt to double check in vmem_add_mapping() - especially to keep > extmem working without changes. At least for callers of memory hotplug > it's then clear which values actually won't fail deep down in arch code. But there is a small problem here. memhp_range_allowed() is now defined and available with CONFIG_MEMORY_HOTPLUG where as vmem_add_mapping() and __segment_load() are generally available without any config dependency. So if CONFIG_MEMORY_HOTPLUG is not enabled there will be a build failure in vmem_add_mapping() for memhp_range_allowed() symbol. We could just move VM_BUG_ON(!memhp_range_allowed(start, size, 1)) check from vmem_add_mapping() to arch_add_memory() like on arm64 platform. But then __segment_load() would need that additional new check to compensate as proposed earlier. Also leaving vmem_add_mapping() and __segment_load() unchanged will cause the address range check to be called three times on the hotplug path i.e 1. register_memory_resource() 2. arch_add_memory() 3. vmem_add_mapping() Moving memhp_range_allowed() check inside arch_add_memory() seems better and consistent with arm64. Also in the future, any platform which choose to override arch_get_mappable() will have this additional VM_BUG_ON() in their arch_add_memory().