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Date: Fri, 17 Jun 2022 15:28:10 +0800
From: Muchun Song <songmuchun@...edance.com>
To: Oscar Salvador <osalvador@...e.de>
Cc: David Hildenbrand <david@...hat.com>, corbet@....net,
akpm@...ux-foundation.org, paulmck@...nel.org,
mike.kravetz@...cle.com, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
duanxiongchun@...edance.com, smuchun@...il.com
Subject: Re: [PATCH v2 2/2] mm: memory_hotplug: introduce
SECTION_CANNOT_OPTIMIZE_VMEMMAP
On Fri, Jun 17, 2022 at 07:46:53AM +0200, Oscar Salvador wrote:
> On Thu, Jun 16, 2022 at 09:30:33AM +0200, David Hildenbrand wrote:
> > IIRC, that was used to skip these patches on the offlining path before
> > we provided the ranges to offline_pages().
>
> Yeah, it was designed for that purpose back then.
>
> > I'd not mess with PG_reserved, and give them a clearer name, to not
> > confuse them with other, ordinary, vmemmap pages that are not
> > self-hosted (maybe in the future we might want to flag all vmemmap pages
> > with a new type?).
>
> Not sure whether a new type is really needed, or to put it another way, I
> cannot see the benefit.
>
> >
> > I'd just try reusing the flag PG_owner_priv_1. And eventually, flag all
> > (v)memmap pages with a type PG_memmap. However, the latter would be
> > optional and might not be strictly required
> >
> >
> > So what think could make sense is
> >
> > /* vmemmap pages that are self-hosted and cannot be optimized/freed. */
> > PG_vmemmap_self_hosted = PG_owner_priv_1,
>
> Sure, I just lightly tested the below, and seems to work, but not sure
> whether that is what you are referring to.
> @Munchun: thoughts?
>
I think it works and fits my requirement.
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index e66f7aa3191d..a4556afd7bda 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -193,6 +193,11 @@ enum pageflags {
>
> /* Only valid for buddy pages. Used to track pages that are reported */
> PG_reported = PG_uptodate,
> +
> +#ifdef CONFIG_MEMORY_HOTPLUG
> + /* For self-hosted memmap pages */
> + PG_vmemmap_self_hosted = PG_owner_priv_1,
> +#endif
> };
>
> #define PAGEFLAGS_MASK ((1UL << NR_PAGEFLAGS) - 1)
> @@ -628,6 +633,10 @@ PAGEFLAG_FALSE(SkipKASanPoison, skip_kasan_poison)
> */
> __PAGEFLAG(Reported, reported, PF_NO_COMPOUND)
>
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +PAGEFLAG(Vmemmap_self_hosted, vmemmap_self_hosted, PF_ANY)
> +#endif
> +
> /*
> * On an anonymous page mapped into a user virtual memory area,
> * page->mapping points to its anon_vma, not to a struct address_space;
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 1089ea8a9c98..e2de7ed27e9e 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -101,6 +101,14 @@ void hugetlb_vmemmap_free(struct hstate *h, struct page *head)
> {
> unsigned long vmemmap_addr = (unsigned long)head;
> unsigned long vmemmap_end, vmemmap_reuse, vmemmap_pages;
> + struct mem_section *ms = __pfn_to_section(page_to_pfn(head));
> + struct page *memmap;
> +
> + memmap = sparse_decode_mem_map(ms->section_mem_map,
> + pfn_to_section_nr(page_to_pfn(head)));
> +
> + if (PageVmemmap_self_hosted(memmap))
> + return;
I think here needs a loop if it is a 1GB page (spans multiple sections).
Right? Here is an implementation based on another approach. But I think
your implementation is more simpler and efficient. Would you mind me
squash your diff into my patch and with your "Co-developed-by"?
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index fcd9f7872064..46d637acc15e 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -10,7 +10,7 @@
*/
#define pr_fmt(fmt) "HugeTLB: " fmt
-#include <linux/memory_hotplug.h>
+#include <linux/memory.h>
#include "hugetlb_vmemmap.h"
/*
@@ -97,18 +97,79 @@ int hugetlb_vmemmap_alloc(struct hstate *h, struct page *head)
return ret;
}
+/*
+ * The vmemmap of the first page of hotplugged memory falls onto itself when
+ * memory_hotplug.memmap_on_memory is enabled, and the vmemmap pages cannot be
+ * optimized in this case. We can simply lookup the first page and test if
+ * the vmemmap maps to itself to detect if memory_hotplug.memmap_on_memory is
+ * enabled for this memory block.
+ *
+ * [ hotplugged memory ]
+ * [ vmemmap ][ usable memory ]
+ * ^ | | |
+ * +---+ | |
+ * ^ | |
+ * +--------+ |
+ * ^ |
+ * +-----------------+
+ */
+static bool memory_block_vmemmap_optimizable(unsigned long start_pfn)
+{
+ pmd_t *pmdp, pmd;
+ unsigned long pfn, vaddr;
+
+ vaddr = (unsigned long)pfn_to_page(start_pfn);
+ pmdp = pmd_off_k(vaddr);
+ /*
+ * The READ_ONCE() is used to stabilize *pmdp in a register or on
+ * the stack so that it will stop changing under the code.
+ */
+ pmd = READ_ONCE(*pmdp);
+
+ if (pmd_large(pmd))
+ pfn = pmd_pfn(pmd);
+ else
+ pfn = pte_pfn(*pte_offset_kernel(pmdp, vaddr));
+
+ return pfn != start_pfn;
+}
+
+static unsigned int optimizable_vmemmap_pages(struct hstate *h,
+ struct page *head)
+{
+ unsigned long size = memory_block_size_bytes();
+ unsigned long pfn = page_to_pfn(head);
+ unsigned long start = ALIGN_DOWN(pfn, size);
+ unsigned long end = start + pages_per_huge_page(h);
+
+ if (READ_ONCE(vmemmap_optimize_mode) == VMEMMAP_OPTIMIZE_OFF)
+ return 0;
+
+ for (; start < end; start += size) {
+ /*
+ * Fast path. The early section is always optimizable since the
+ * early section's vmemmap pages do not allocated from the added
+ * memory block itself.
+ */
+ if (early_section(__pfn_to_section(start + (pfn & PAGE_SECTION_MASK))))
+ continue;
+
+ if (!memory_block_vmemmap_optimizable(start))
+ return 0;
+ }
+
+ return hugetlb_optimize_vmemmap_pages(h);
+}
+
void hugetlb_vmemmap_free(struct hstate *h, struct page *head)
{
unsigned long vmemmap_addr = (unsigned long)head;
unsigned long vmemmap_end, vmemmap_reuse, vmemmap_pages;
- vmemmap_pages = hugetlb_optimize_vmemmap_pages(h);
+ vmemmap_pages = optimizable_vmemmap_pages(h, head);
if (!vmemmap_pages)
return;
- if (READ_ONCE(vmemmap_optimize_mode) == VMEMMAP_OPTIMIZE_OFF)
- return;
-
static_branch_inc(&hugetlb_optimize_vmemmap_key);
vmemmap_addr += RESERVE_VMEMMAP_SIZE;
@@ -199,10 +260,10 @@ static struct ctl_table hugetlb_vmemmap_sysctls[] = {
static __init int hugetlb_vmemmap_sysctls_init(void)
{
/*
- * If "memory_hotplug.memmap_on_memory" is enabled or "struct page"
- * crosses page boundaries, the vmemmap pages cannot be optimized.
+ * If "struct page" crosses page boundaries, the vmemmap pages cannot
+ * be optimized.
*/
- if (!mhp_memmap_on_memory() && is_power_of_2(sizeof(struct page)))
+ if (is_power_of_2(sizeof(struct page)))
register_sysctl_init("vm", hugetlb_vmemmap_sysctls);
return 0;
>
> vmemmap_pages = hugetlb_optimize_vmemmap_pages(h);
> if (!vmemmap_pages)
> @@ -199,10 +207,10 @@ static struct ctl_table hugetlb_vmemmap_sysctls[] = {
> static __init int hugetlb_vmemmap_sysctls_init(void)
> {
> /*
> - * If "memory_hotplug.memmap_on_memory" is enabled or "struct page"
> - * crosses page boundaries, the vmemmap pages cannot be optimized.
> + * If "struct page" crosses page boundaries, the vmemmap pages cannot
> + * be optimized.
> */
> - if (!mhp_memmap_on_memory() && is_power_of_2(sizeof(struct page)))
> + if (is_power_of_2(sizeof(struct page)))
> register_sysctl_init("vm", hugetlb_vmemmap_sysctls);
>
> return 0;
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 1213d0c67a53..863966c2c6f1 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -45,8 +45,6 @@
> #ifdef CONFIG_MHP_MEMMAP_ON_MEMORY
> static int memmap_on_memory_set(const char *val, const struct kernel_param *kp)
> {
> - if (hugetlb_optimize_vmemmap_enabled())
> - return 0;
> return param_set_bool(val, kp);
> }
>
> @@ -1032,6 +1030,7 @@ int mhp_init_memmap_on_memory(unsigned long pfn, unsigned long nr_pages,
> {
> unsigned long end_pfn = pfn + nr_pages;
> int ret;
> + int i;
>
> ret = kasan_add_zero_shadow(__va(PFN_PHYS(pfn)), PFN_PHYS(nr_pages));
> if (ret)
> @@ -1039,6 +1038,12 @@ int mhp_init_memmap_on_memory(unsigned long pfn, unsigned long nr_pages,
>
> move_pfn_range_to_zone(zone, pfn, nr_pages, NULL, MIGRATE_UNMOVABLE);
>
> + /*
> + * Let us flag self-hosted memmap
> + */
> + for (i = 0; i < nr_pages; i++)
> + SetPageVmemmap_self_hosted(pfn_to_page(pfn + i));
> +
> /*
> * It might be that the vmemmap_pages fully span sections. If that is
> * the case, mark those sections online here as otherwise they will be
>
>
> --
> Oscar Salvador
> SUSE Labs
>
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