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Thu, 06 Feb 2020 09:37:45 -0500 X-MC-Unique: ZR9i1TwBNkyYRKkLT3fC0g-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 0BCED106BCBA; Thu, 6 Feb 2020 14:37:43 +0000 (UTC) Received: from [10.36.118.128] (unknown [10.36.118.128]) by smtp.corp.redhat.com (Postfix) with ESMTP id 488FD48; Thu, 6 Feb 2020 14:37:41 +0000 (UTC) Subject: Re: [PATCH] mm/sparsemem: pfn_to_page is not valid yet on SPARSEMEM To: Baoquan He Cc: Wei Yang , akpm@linux-foundation.org, osalvador@suse.de, dan.j.williams@intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20200206125343.9070-1-richardw.yang@linux.intel.com> <6d9e36cb-ee4a-00c8-447b-9b75a0262c3a@redhat.com> <20200206135016.GA25537@MiWiFi-R3L-srv> <87bb4563-481d-cce9-b916-50a098558210@redhat.com> <20200206140703.GB25537@MiWiFi-R3L-srv> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.1 MIME-Version: 1.0 In-Reply-To: <20200206140703.GB25537@MiWiFi-R3L-srv> Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Content-Transfer-Encoding: quoted-printable Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06.02.20 15:07, Baoquan He wrote: > On 02/06/20 at 02:55pm, David Hildenbrand wrote: >> On 06.02.20 14:50, Baoquan He wrote: >>> On 02/06/20 at 02:28pm, David Hildenbrand wrote: >>>> On 06.02.20 13:53, Wei Yang wrote: >>>>> When we use SPARSEMEM instead of SPARSEMEM_VMEMMAP, pfn_to_page() >>>>> doesn't work before sparse_init_one_section() is called. This leads= to a >>>>> crash when hotplug memory. >>>>> >>>>> We should use memmap as it did. >>>>> >>>>> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug") >>>>> Signed-off-by: Wei Yang >>>>> CC: Dan Williams >>>>> --- >>>>> mm/sparse.c | 2 +- >>>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>>> >>>>> diff --git a/mm/sparse.c b/mm/sparse.c >>>>> index 5a8599041a2a..2efb24ff8f96 100644 >>>>> --- a/mm/sparse.c >>>>> +++ b/mm/sparse.c >>>>> @@ -882,7 +882,7 @@ int __meminit sparse_add_section(int nid, unsig= ned long start_pfn, >>>>> * Poison uninitialized struct pages in order to catch invalid fl= ags >>>>> * combinations. >>>>> */ >>>>> - page_init_poison(pfn_to_page(start_pfn), sizeof(struct page) * nr= _pages); >>>>> + page_init_poison(memmap, sizeof(struct page) * nr_pages); >>>> >>>> If you add sub-sections that don't fall onto the start of the sectio= n, >>>> >>>> pfn_to_page(start_pfn) !=3D memmap >>>> >>>> and your patch would break that under SPARSEMEM_VMEMMAP if I am not = wrong. >>> >>> It returns the pfn_to_page(pfn) from __populate_section_memmap() and >>> assign to memmap in vmemmap case, how come it breaks anything. Correc= t >>> me if I was wrong. >> >> I'm sorry, I can't follow :) Can you elaborate? >> >> Was your comment targeted at why the old code cannot be broken or why >> this patch cannot be broken? >=20 > Sorry for the confusion :-) the latter. I mean the returned memmap has = been > at the pfn_to_page(start_pfn) in SPARSEMEM_VMEMMAP case. Yeah, at least for SPARSEMEM_VMEMMAP it is indeed right. Thanks :) Now, about SPARSEMEM: populate_section_memmap() does not care about nr_pages and will allocate a memmap for the whole section. So, whenever we add sub-sections to a section, we allocate a new memmap for the whole section. And we do overwrite the memmap pointer in our section. ( sparse_add_section() ) That makes me assume that sub-section hot-add under SPARSEMEM is either a) never enabled and only works with SPARSEMEM_VMEMMAP b) horribly broken And I think a) applies (looking at pfn_section_valid()). Therefore, we don't have to care about sub-section hot-add specifics (and I would be broken already) Acked-by: David Hildenbrand --=20 Thanks, David / dhildenb