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[79.242.60.59]) by smtp.gmail.com with ESMTPSA id o16sm1884830wrx.11.2021.09.29.03.49.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 29 Sep 2021 03:49:58 -0700 (PDT) Subject: Re: [RFC] arm64: mm: update max_pfn after memory hotplug To: Will Deacon Cc: Chris Goldsworthy , Catalin Marinas , Andrew Morton , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Sudarshan Rajagopalan References: <595d09279824faf1f54961cef52b745609b05d97.1632437225.git.quic_cgoldswo@quicinc.com> <20210929101028.GB21057@willie-the-truck> <13f56b37-afc7-bf6f-d544-8d6433588bf9@redhat.com> <20210929104241.GA21395@willie-the-truck> From: David Hildenbrand Organization: Red Hat Message-ID: Date: Wed, 29 Sep 2021 12:49:58 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210929104241.GA21395@willie-the-truck> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 76D58B00008E X-Stat-Signature: 8ecburqdkc61qnkit99p5t5dk84gfddw Authentication-Results: imf25.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=ckeEkcMm; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf25.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=david@redhat.com X-HE-Tag: 1632912602-588766 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 29.09.21 12:42, Will Deacon wrote: > On Wed, Sep 29, 2021 at 12:29:32PM +0200, David Hildenbrand wrote: >> On 29.09.21 12:10, Will Deacon wrote: >>> On Thu, Sep 23, 2021 at 03:54:48PM -0700, Chris Goldsworthy wrote: >>>> From: Sudarshan Rajagopalan >>>> >>>> After new memory blocks have been hotplugged, max_pfn and max_low_pfn >>>> needs updating to reflect on new PFNs being hot added to system. >>>> >>>> Signed-off-by: Sudarshan Rajagopalan >>>> Signed-off-by: Chris Goldsworthy >>>> --- >>>> arch/arm64/mm/mmu.c | 5 +++++ >>>> 1 file changed, 5 insertions(+) >>>> >>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c >>>> index cfd9deb..fd85b51 100644 >>>> --- a/arch/arm64/mm/mmu.c >>>> +++ b/arch/arm64/mm/mmu.c >>>> @@ -1499,6 +1499,11 @@ int arch_add_memory(int nid, u64 start, u64 size, >>>> if (ret) >>>> __remove_pgd_mapping(swapper_pg_dir, >>>> __phys_to_virt(start), size); >>>> + else { >>>> + max_pfn = PFN_UP(start + size); >>>> + max_low_pfn = max_pfn; >>>> + } >>> >>> We use 'max_pfn' as part of the argument to set_max_mapnr(). Does that need >>> updating as well? >>> >>> Do we have sufficient locking to ensure nobody is looking at max_pfn or >>> max_low_pfn while we update them? >> >> Only the write side is protected by memory hotplug locking. The read side is >> lockless -- just like all of the other pfn_to_online_page() machinery. > > Hmm. So the readers can see one of the variables updated but the other one > stale? Yes, just like it has been on x86-64 for a long time: arch/x86/mm/init_64.c:update_end_of_memory_vars() Not sure if anyone really cares about slightly delayed updates while memory is getting hotplugged. The users that I am aware of don't care. -- Thanks, David / dhildenb