Re: [PATCH v7 1/1] mm/page_alloc.c: refactor initialization of struct page for holes in memory layout
From: Mike Rapoport
Date: Thu Feb 25 2021 - 12:09:17 EST
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@xxxxxxxxxxxxx>
>
>
>
> 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@xxxxxxxxxxxxx>
>
> Reported-by: Qian Cai <cai@xxxxxx>
>
> Reported-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
>
> Reviewed-by: Baoquan He <bhe@xxxxxxxxxx>
>
> ---
>
> 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:
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> + * - 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))) {
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> + 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)
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> memmap_init_range(end_pfn - start_pfn, nid,
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> 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.
>
You should also document (in the patch description?) that node/zone spans
>
are not properly handled yet for such hole pfns and that this might require
>
care in the future.
I think Link: will suffice for this.
>
I played a little with weird setups and expected the memap state using
>
page-types (well, I can't inspect the node/zone that way but at least have a
>
look if the memmap was initialized). No surprises.
Great, thanks!
--
Sincerely yours,
Mike.