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Wed, 24 Jun 2020 04:51:12 -0400 X-MC-Unique: GUl-l0hxNu2P6x608axcLg-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id CBF76805EE2; Wed, 24 Jun 2020 08:51:10 +0000 (UTC) Received: from [10.36.113.65] (ovpn-113-65.ams2.redhat.com [10.36.113.65]) by smtp.corp.redhat.com (Postfix) with ESMTP id 296065C660; Wed, 24 Jun 2020 08:51:09 +0000 (UTC) Subject: Re: [PATCH] mm/spase: never partially remove memmap for early section To: Wei Yang , Baoquan He Cc: Dan Williams , Andrew Morton , Oscar Salvador , Linux MM , Linux Kernel Mailing List References: <20200623094258.6705-1-richard.weiyang@linux.alibaba.com> <20200624014737.GG3346@MiWiFi-R3L-srv> <20200624034638.GA10687@L-31X9LVDL-1304.local> <20200624035236.GI3346@MiWiFi-R3L-srv> <20200624035622.GA10774@L-31X9LVDL-1304.local> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: <20200624035622.GA10774@L-31X9LVDL-1304.local> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Rspamd-Queue-Id: 0446D180D0180 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam05 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 24.06.20 05:56, Wei Yang wrote: > On Wed, Jun 24, 2020 at 11:52:36AM +0800, Baoquan He wrote: >> On 06/24/20 at 11:46am, Wei Yang wrote: >>> On Wed, Jun 24, 2020 at 09:47:37AM +0800, Baoquan He wrote: >>>> On 06/23/20 at 05:21pm, Dan Williams wrote: >>>>> On Tue, Jun 23, 2020 at 2:43 AM Wei Yang >>>>> wrote: >>>>>> >>>>>> For early sections, we assumes its memmap will never be partially >>>>>> removed. But current behavior breaks this. >>>>> >>>>> Where do we assume that? >>>>> >>>>> The primary use case for this was mapping pmem that collides with >>>>> System-RAM in the same 128MB section. That collision will certainly be >>>>> depopulated on-demand depending on the state of the pmem device. So, >>>>> I'm not understanding the problem or the benefit of this change. >>>> >>>> I was also confused when review this patch, the patch log is a little >>>> short and simple. From the current code, with SPARSE_VMEMMAP enabled, we >>>> do build memmap for the whole memory section during boot, even though >>>> some of them may be partially populated. We just mark the subsection map >>>> for present pages. >>>> >>>> Later, if pmem device is mapped into the partially boot memory section, >>>> we just fill the relevant subsection map, do return directly, w/o building >>>> the memmap for it, in section_activate(). Because the memmap for the >>>> unpresent RAM part have been there. I guess this is what Wei is trying to >>>> do to keep the behaviour be consistent for pmem device adding, or >>>> pmem device removing and later adding again. >>>> >>>> Please correct me if I am wrong. >>> >>> You are right here. >>> >>>> >>>> To me, fixing it looks good. But a clear doc or code comment is >>>> necessary so that people can understand the code with less time. >>>> Leaving it as is doesn't cause harm. I personally tend to choose >>>> the former. >>>> >>> >>> The former is to add a clear doc? >> >> Sorry for the confusion. The former means the fix in your patch. Maybe a >> improved log and some code comment adding can make it more perfect. >> > > Sure, I would try to add more log and comments, in case you have some good > suggestion, just let me know :) > We have documented this is section_activate() and pfn_valid() sufficiently. Maybe add a pointer like /* * The memmap of early sections is always fully populated. See * section_activate() and pfn_valid() . */ -- Thanks, David / dhildenb