linux-kernel - Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added memory range

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Message-ID: <YFy+olsdS4iwrovN@dhcp22.suse.cz>
Date:   Thu, 25 Mar 2021 17:47:30 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pavel Tatashin <pasha.tatashin@...een.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added
 memory range

On Thu 25-03-21 17:36:22, Michal Hocko wrote:
> If all it takes is to make pfn_to_online_page work (and my
> previous attempt is incorrect because it should consult block rather
> than section pfn range)

This should work.

diff --git a/drivers/base/memory.c b/drivers/base/memory.c
index 9697acfe96eb..e50d685be8ab 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -510,6 +510,23 @@ static struct memory_block *find_memory_block_by_id(unsigned long block_id)
 	return mem;
 }
 
+struct page *is_vmemmap_page(unsigned long pfn)
+{
+	unsigned long nr = pfn_to_section_nr(pfn);
+	struct memory_block *mem;
+	unsigned long block_pfn;
+
+	mem = find_memory_block_by_id(memory_block_id(nr));
+	if (!mem || !mem->nr_vmemmap_pages)
+		return NULL;
+
+	block_pfn = section_nr_to_pfn(mem->start_section_nr);
+	if (pfn - block_pfn > mem->nr_vmemmap_pages)
+		return NULL;
+
+	return pfn_to_page(pfn);
+}
+
 /*
  * Called under device_hotplug_lock.
  */
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 754026a9164d..760bf3ad48d5 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -304,8 +304,16 @@ struct page *pfn_to_online_page(unsigned long pfn)
 		return NULL;
 
 	ms = __nr_to_section(nr);
-	if (!online_section(ms))
+	if (!online_section(ms)) {
+		/*
+		 * vmemmap reserved space can eat up a whole section which then
+		 * never gets onlined because it doesn't contain any memory to
+		 * online.
+		 */
+		if (memmap_on_memory)
+			return is_vmemmap_page(pfn);
 		return NULL;
+	}
 
 	/*
 	 * Save some code text when online_section() +
-- 
Michal Hocko
SUSE Labs