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Date: Thu, 25 Feb 2021 18:54:10 +0100
From: David Hildenbrand <david@...hat.com>
To: Mike Rapoport <rppt@...ux.ibm.com>
Cc: Mike Rapoport <rppt@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Andrea Arcangeli <aarcange@...hat.com>,
Baoquan He <bhe@...hat.com>, Borislav Petkov <bp@...en8.de>,
Chris Wilson <chris@...is-wilson.co.uk>,
"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Łukasz Majczak <lma@...ihalf.com>,
Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
Qian Cai <cai@....pw>,
"Sarvela, Tomi P" <tomi.p.sarvela@...el.com>,
Thomas Gleixner <tglx@...utronix.de>,
Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, stable@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v7 1/1] mm/page_alloc.c: refactor initialization of struct
page for holes in memory layout
On 25.02.21 18:06, Mike Rapoport wrote:
> On Thu, Feb 25, 2021 at 04:59:06PM +0100, David Hildenbrand wrote:
>> On 24.02.21 16:39, Mike Rapoport wrote:
>>> From: Mike Rapoport <rppt@...ux.ibm.com>
>>>
>>> There could be struct pages that are not backed by actual physical memory.
>>> This can happen when the actual memory bank is not a multiple of
>>> SECTION_SIZE or when an architecture does not register memory holes
>>> reserved by the firmware as memblock.memory.
>>>
>>> Such pages are currently initialized using init_unavailable_mem() function
>>> that iterates through PFNs in holes in memblock.memory and if there is a
>>> struct page corresponding to a PFN, the fields of this page are set to
>>> default values and it is marked as Reserved.
>>>
>>> init_unavailable_mem() does not take into account zone and node the page
>>> belongs to and sets both zone and node links in struct page to zero.
>>>
>>> Before commit 73a6e474cb37 ("mm: memmap_init: iterate over memblock regions
>>> rather that check each PFN") the holes inside a zone were re-initialized
>>> during memmap_init() and got their zone/node links right. However, after
>>> that commit nothing updates the struct pages representing such holes.
>>>
>>> On a system that has firmware reserved holes in a zone above ZONE_DMA, for
>>> instance in a configuration below:
>>>
>>> # grep -A1 E820 /proc/iomem
>>> 7a17b000-7a216fff : Unknown E820 type
>>> 7a217000-7bffffff : System RAM
>>>
>>> unset zone link in struct page will trigger
>>>
>>> VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page);
>>>
>>> because there are pages in both ZONE_DMA32 and ZONE_DMA (unset zone link
>>> in struct page) in the same pageblock.
>>>
>>> Interleave initialization of the unavailable pages with the normal
>>> initialization of memory map, so that zone and node information will be
>>> properly set on struct pages that are not backed by the actual memory.
>>>
>>> With this change the pages for holes inside a zone will get proper
>>> zone/node links and the pages that are not spanned by any node will get
>>> links to the adjacent zone/node.
>>>
>>> Fixes: 73a6e474cb37 ("mm: memmap_init: iterate over memblock regions rather that check each PFN")
>>> Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
>>> Reported-by: Qian Cai <cai@....pw>
>>> Reported-by: Andrea Arcangeli <aarcange@...hat.com>
>>> Reviewed-by: Baoquan He <bhe@...hat.com>
>>> ---
>>> mm/page_alloc.c | 147 +++++++++++++++++++++---------------------------
>>> 1 file changed, 64 insertions(+), 83 deletions(-)
>>>
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index 3e93f8b29bae..a11a9acde708 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -6280,12 +6280,60 @@ static void __meminit zone_init_free_lists(struct zone *zone)
>>> }
>>> }
>>> +#if !defined(CONFIG_FLAT_NODE_MEM_MAP)
>>> +/*
>>> + * Only struct pages that correspond to ranges defined by memblock.memory
>>> + * are zeroed and initialized by going through __init_single_page() during
>>> + * memmap_init_zone().
>>> + *
>>> + * But, there could be struct pages that correspond to holes in
>>> + * memblock.memory. This can happen because of the following reasons:
>>> + * - phyiscal memory bank size is not necessarily the exact multiple of the
>>> + * arbitrary section size
>>> + * - early reserved memory may not be listed in memblock.memory
>>> + * - memory layouts defined with memmap= kernel parameter may not align
>>> + * nicely with memmap sections
>>> + *
>>> + * Explicitly initialize those struct pages so that:
>>> + * - PG_Reserved is set
>>> + * - zone and node links point to zone and node that span the page
>>> + */
>>> +static u64 __meminit init_unavailable_range(unsigned long spfn,
>>> + unsigned long epfn,
>>> + int zone, int node)
>>> +{
>>> + unsigned long pfn;
>>> + u64 pgcnt = 0;
>>> +
>>> + for (pfn = spfn; pfn < epfn; pfn++) {
>>> + if (!pfn_valid(ALIGN_DOWN(pfn, pageblock_nr_pages))) {
>>> + pfn = ALIGN_DOWN(pfn, pageblock_nr_pages)
>>> + + pageblock_nr_pages - 1;
>>> + continue;
>>> + }
>>> + __init_single_page(pfn_to_page(pfn), pfn, zone, node);
>>> + __SetPageReserved(pfn_to_page(pfn));
>>> + pgcnt++;
>>> + }
>>> +
>>> + return pgcnt;
>>> +}
>>> +#else
>>> +static inline u64 init_unavailable_range(unsigned long spfn, unsigned long epfn,
>>> + int zone, int node)
>>> +{
>>> + return 0;
>>> +}
>>> +#endif
>>> +
>>> void __meminit __weak memmap_init_zone(struct zone *zone)
>>> {
>>> unsigned long zone_start_pfn = zone->zone_start_pfn;
>>> unsigned long zone_end_pfn = zone_start_pfn + zone->spanned_pages;
>>> int i, nid = zone_to_nid(zone), zone_id = zone_idx(zone);
>>> + static unsigned long hole_pfn = 0;
>>> unsigned long start_pfn, end_pfn;
>>> + u64 pgcnt = 0;
>>> for_each_mem_pfn_range(i, nid, &start_pfn, &end_pfn, NULL) {
>>> start_pfn = clamp(start_pfn, zone_start_pfn, zone_end_pfn);
>>> @@ -6295,7 +6343,23 @@ void __meminit __weak memmap_init_zone(struct zone *zone)
>>> memmap_init_range(end_pfn - start_pfn, nid,
>>> zone_id, start_pfn, zone_end_pfn,
>>> MEMINIT_EARLY, NULL, MIGRATE_MOVABLE);
>>> +
>>> + if (hole_pfn < start_pfn)
>>> + pgcnt += init_unavailable_range(hole_pfn, start_pfn,
>>> + zone_id, nid);
>>> + hole_pfn = end_pfn;
>>> }
>>> +
>>> +#ifdef CONFIG_SPARSEMEM
>>> + end_pfn = round_up(zone_end_pfn, PAGES_PER_SECTION);
>>> + if (hole_pfn < end_pfn)
>>> + pgcnt += init_unavailable_range(hole_pfn, end_pfn,
>>> + zone_id, nid);
>>
>> We might still double-initialize PFNs when two zones overlap within a
>> section, correct?
>
> You mean that a section crosses zones boundary?
> I don't think it's that important.
>
>> This might worth documenting - also, you might want to
>> take some of the original comment the accompanied this code.
>
> The original comment was not exactly right, I believe the comment above
> init_unavailable_range() better describes what's going on there.
Ah, okay - as long as it's documented I'm happy :)
--
Thanks,
David / dhildenb
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