Re: [PATCH v4 2/2] mm: memory_hotplug: make hugetlb_optimize_vmemmap compatible with memmap_on_memory

[Muchun Song]

On Sun, Jun 19, 2022 at 09:38:51PM +0800, Muchun Song wrote:
> For now, the feature of hugetlb_free_vmemmap is not compatible with the
> feature of memory_hotplug.memmap_on_memory, and hugetlb_free_vmemmap
> takes precedence over memory_hotplug.memmap_on_memory. However, someone
> wants to make memory_hotplug.memmap_on_memory takes precedence over
> hugetlb_free_vmemmap since memmap_on_memory makes it more likely to
> succeed memory hotplug in close-to-OOM situations.  So the decision
> of making hugetlb_free_vmemmap take precedence is not wise and elegant.
> The proper approach is to have hugetlb_vmemmap.c do the check whether
> the section which the HugeTLB pages belong to can be optimized.  If
> the section's vmemmap pages are allocated from the added memory block
> itself, hugetlb_free_vmemmap should refuse to optimize the vmemmap,
> otherwise, do the optimization.  Then both kernel parameters are
> compatible.  So this patch introduces VmemmapSelfHosted to mask any
> non-optimizable vmemmap pages. The hugetlb_vmemmap can use this flag
> to detect if a vmemmap page can be optimized.
> 
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> Co-developed-by: Oscar Salvador <osalvador@xxxxxxx>
> Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 22 +++++------
>  Documentation/admin-guide/sysctl/vm.rst         |  5 +--
>  include/linux/memory_hotplug.h                  |  9 -----
>  include/linux/page-flags.h                      | 11 ++++++
>  mm/hugetlb_vmemmap.c                            | 52 +++++++++++++++++++++----
>  mm/memory_hotplug.c                             | 27 ++++++-------
>  6 files changed, 79 insertions(+), 47 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 8090130b544b..d740e2ed0e61 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1722,9 +1722,11 @@
>  			Built with CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON=y,
>  			the default is on.
>  
> -			This is not compatible with memory_hotplug.memmap_on_memory.
> -			If both parameters are enabled, hugetlb_free_vmemmap takes
> -			precedence over memory_hotplug.memmap_on_memory.
> +			Note that the vmemmap pages may be allocated from the added
> +			memory block itself when memory_hotplug.memmap_on_memory is
> +			enabled, those vmemmap pages cannot be optimized even if this
> +			feature is enabled.  Other vmemmap pages not allocated from
> +			the added memory block itself do not be affected.
>  
>  	hung_task_panic=
>  			[KNL] Should the hung task detector generate panics.
> @@ -3069,10 +3071,12 @@
>  			[KNL,X86,ARM] Boolean flag to enable this feature.
>  			Format: {on | off (default)}
>  			When enabled, runtime hotplugged memory will
> -			allocate its internal metadata (struct pages)
> -			from the hotadded memory which will allow to
> -			hotadd a lot of memory without requiring
> -			additional memory to do so.
> +			allocate its internal metadata (struct pages,
> +			those vmemmap pages cannot be optimized even
> +			if hugetlb_free_vmemmap is enabled) from the
> +			hotadded memory which will allow to hotadd a
> +			lot of memory without requiring additional
> +			memory to do so.
>  			This feature is disabled by default because it
>  			has some implication on large (e.g. GB)
>  			allocations in some configurations (e.g. small
> @@ -3082,10 +3086,6 @@
>  			Note that even when enabled, there are a few cases where
>  			the feature is not effective.
>  
> -			This is not compatible with hugetlb_free_vmemmap. If
> -			both parameters are enabled, hugetlb_free_vmemmap takes
> -			precedence over memory_hotplug.memmap_on_memory.
> -
>  	memtest=	[KNL,X86,ARM,M68K,PPC,RISCV] Enable memtest
>  			Format: <integer>
>  			default : 0 <disable>
> diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
> index 5c9aa171a0d3..d7374a1e8ac9 100644
> --- a/Documentation/admin-guide/sysctl/vm.rst
> +++ b/Documentation/admin-guide/sysctl/vm.rst
> @@ -565,9 +565,8 @@ See Documentation/admin-guide/mm/hugetlbpage.rst
>  hugetlb_optimize_vmemmap
>  ========================
>  
> -This knob is not available when memory_hotplug.memmap_on_memory (kernel parameter)
> -is configured or the size of 'struct page' (a structure defined in
> -include/linux/mm_types.h) is not power of two (an unusual system config could
> +This knob is not available when the size of 'struct page' (a structure defined
> +in include/linux/mm_types.h) is not power of two (an unusual system config could
>  result in this).
>  
>  Enable (set to 1) or disable (set to 0) the feature of optimizing vmemmap pages
> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
> index 20d7edf62a6a..e0b2209ab71c 100644
> --- a/include/linux/memory_hotplug.h
> +++ b/include/linux/memory_hotplug.h
> @@ -351,13 +351,4 @@ void arch_remove_linear_mapping(u64 start, u64 size);
>  extern bool mhp_supports_memmap_on_memory(unsigned long size);
>  #endif /* CONFIG_MEMORY_HOTPLUG */
>  
> -#ifdef CONFIG_MHP_MEMMAP_ON_MEMORY
> -bool mhp_memmap_on_memory(void);
> -#else
> -static inline bool mhp_memmap_on_memory(void)
> -{
> -	return false;
> -}
> -#endif
> -
>  #endif /* __LINUX_MEMORY_HOTPLUG_H */
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index e66f7aa3191d..2aa5dcbfe468 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 +633,12 @@ PAGEFLAG_FALSE(SkipKASanPoison, skip_kasan_poison)
>   */
>  __PAGEFLAG(Reported, reported, PF_NO_COMPOUND)
>  
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +PAGEFLAG(VmemmapSelfHosted, vmemmap_self_hosted, PF_ANY)
> +#else
> +PAGEFLAG_FALSE(VmemmapSelfHosted, vmemmap_self_hosted)
> +#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..73bfbb47f6a4 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 +97,54 @@ int hugetlb_vmemmap_alloc(struct hstate *h, struct page *head)
>  	return ret;
>  }
>  
> +static unsigned int vmemmap_optimizable_pages(struct hstate *h,
> +					      struct page *head)
> +{
> +	if (READ_ONCE(vmemmap_optimize_mode) == VMEMMAP_OPTIMIZE_OFF)
> +		return 0;
> +
> +	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG)) {
> +		unsigned long pfn = page_to_pfn(head);
> +
> +		/*
> +		 * Due to HugeTLB alignment requirements and the vmemmap pages
> +		 * being at the start of the hotplugged memory region in
> +		 * memory_hotplug.memmap_on_memory case. Checking the first
> +		 * vmemmap page's vmemmap if it is marked as VmemmapSelfHosted
> +		 * is sufficient.
> +		 *
> +		 * [                  hotplugged memory                  ]
> +		 * [        section        ][...][        section        ]
> +		 * [ vmemmap ][              usable memory               ]
> +		 *   ^   |     |                                        |
> +		 *   +---+     |                                        |
> +		 *     ^       |                                        |
> +		 *     +-------+                                        |
> +		 *          ^                                           |
> +		 *          +-------------------------------------------+
> +		 *
> +		 * Hotplugged memory block never has non-present sections, while
> +		 * boot memory block can have one or more. So pfn_valid() is
> +		 * used to filter out the non-present section which also cannot
> +		 * be memmap_on_memory.
> +		 */
> +		pfn = ALIGN_DOWN(pfn, PHYS_PFN(memory_block_size_bytes()));
> +		if (pfn_valid(pfn) && PageVmemmapSelfHosted(pfn_to_page(pfn)))

Although it works, I think PageVmemmapSelfHosted() check for the 1st pfn's
vmemmap page is not always reliable.  Since we reused PG_owner_priv_1
as PG_vmemmap_self_hosted, the test is noly reliable for vmemmap page's
vmemmap page.  Other non-vmemmap page can be flagged with PG_owner_priv_1.
So this check can be false-positive. Maybe the following code snippet is
the solution.

Any thoughts? Oscar or David.

+       if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG)) {
+               pmd_t *pmdp, pmd;
+               struct page *vmemmap_page;
+               unsigned long vaddr = (unsigned long)head;
+
+               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_leaf(pmd))
+                       vmemmap_page = pmd_page(pmd) + pte_index(vaddr);
+               else
+                       vmemmap_page = pte_page(*pte_offset_kernel(pmdp, vaddr));
+               /*
+                * Due to HugeTLB alignment requirements and the vmemmap pages
+                * being at the start of the hotplugged memory region in
+                * memory_hotplug.memmap_on_memory case. Checking any vmemmap
+                * page's vmemmap page if it is marked as VmemmapSelfHosted is
+                * sufficient.
+                *
+                * [                  hotplugged memory                  ]
+                * [        section        ][...][        section        ]
+                * [ vmemmap ][              usable memory               ]
+                *   ^   |     |                                        |
+                *   +---+     |                                        |
+                *     ^       |                                        |
+                *     +-------+                                        |
+                *          ^                                           |
+                *          +-------------------------------------------+
+                */
+               if (PageVmemmapSelfHosted(vmemmap_page))
+                       return 0;
+       }

> +			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 = vmemmap_optimizable_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 +235,10 @@ 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 6662b86e9e64..3a59d4e97c03 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -43,30 +43,22 @@
>  #include "shuffle.h"
>  
>  #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);
> -}
> -
> -static const struct kernel_param_ops memmap_on_memory_ops = {
> -	.flags	= KERNEL_PARAM_OPS_FL_NOARG,
> -	.set	= memmap_on_memory_set,
> -	.get	= param_get_bool,
> -};
> -
>  /*
>   * memory_hotplug.memmap_on_memory parameter
>   */
>  static bool memmap_on_memory __ro_after_init;
> -module_param_cb(memmap_on_memory, &memmap_on_memory_ops, &memmap_on_memory, 0444);
> +module_param(memmap_on_memory, bool, 0444);
>  MODULE_PARM_DESC(memmap_on_memory, "Enable memmap on memory for memory hotplug");
>  
> -bool mhp_memmap_on_memory(void)
> +static inline bool mhp_memmap_on_memory(void)
>  {
>  	return memmap_on_memory;
>  }
> +#else
> +static inline bool mhp_memmap_on_memory(void)
> +{
> +	return false;
> +}
>  #endif
>  
>  enum {
> @@ -1035,7 +1027,7 @@ int mhp_init_memmap_on_memory(unsigned long pfn, unsigned long nr_pages,
>  			      struct zone *zone)
>  {
>  	unsigned long end_pfn = pfn + nr_pages;
> -	int ret;
> +	int ret, i;
>  
>  	ret = kasan_add_zero_shadow(__va(PFN_PHYS(pfn)), PFN_PHYS(nr_pages));
>  	if (ret)
> @@ -1043,6 +1035,9 @@ 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);
>  
> +	for (i = 0; i < nr_pages; i++)
> +		SetPageVmemmapSelfHosted(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
> -- 
> 2.11.0
> 
>