Subject: Re: [PATCH v1 2/3] powerpc/mm/radix: Fix PTE/PMD fragment count for early page table mappings

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Tue, 23 Jun 2020 16:07:34 +0530

Bharata B Rao <[email-masked]> writes:

> We can hit the following BUG_ON during memory unplug:
>
> kernel BUG at arch/powerpc/mm/book3s64/pgtable.c:342!
> Oops: Exception in kernel mode, sig: 5 [#1]
> LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA pSeries
> NIP [c000000000093308] pmd_fragment_free+0x48/0xc0
> LR [c00000000147bfec] remove_pagetable+0x578/0x60c
> Call Trace:
> 0xc000008050000000 (unreliable)
> remove_pagetable+0x384/0x60c
> radix__remove_section_mapping+0x18/0x2c
> remove_section_mapping+0x1c/0x3c
> arch_remove_memory+0x11c/0x180
> try_remove_memory+0x120/0x1b0
> __remove_memory+0x20/0x40
> dlpar_remove_lmb+0xc0/0x114
> dlpar_memory+0x8b0/0xb20
> handle_dlpar_errorlog+0xc0/0x190
> pseries_hp_work_fn+0x2c/0x60
> process_one_work+0x30c/0x810
> worker_thread+0x98/0x540
> kthread+0x1c4/0x1d0
> ret_from_kernel_thread+0x5c/0x74
>
> This occurs when unplug is attempted for such memory which has
> been mapped using memblock pages as part of early kernel page
> table setup. We wouldn't have initialized the PMD or PTE fragment
> count for those PMD or PTE pages.
>
> Fixing this includes 3 parts:
>
> - Re-walk the init_mm page tables from mem_init() and initialize
>   the PMD and PTE fragment count to 1.
> - When freeing PUD, PMD and PTE page table pages, check explicitly
>   if they come from memblock and if so free then appropriately.
> - When we do early memblock based allocation of PMD and PUD pages,
>   allocate in PAGE_SIZE granularity so that we are sure the
>   complete page is used as pagetable page.
>
> Since we now do PAGE_SIZE allocations for both PUD table and
> PMD table (Note that PTE table allocation is already of PAGE_SIZE),
> we end up allocating more memory for the same amount of system RAM.
> Here is a comparision of how much more we need for a 64T and 2G
> system after this patch:
>
> 1. 64T system
> -------------
> 64T RAM would need 64G for vmemmap with struct page size being 64B.
>
> 128 PUD tables for 64T memory (1G mappings)
> 1 PUD table and 64 PMD tables for 64G vmemmap (2M mappings)
>
> With default PUD[PMD]_TABLE_SIZE(4K), (128+1+64)*4K=772K
> With PAGE_SIZE(64K) table allocations, (128+1+64)*64K=12352K
>
> 2. 2G system
> ------------
> 2G RAM would need 2M for vmemmap with struct page size being 64B.
>
> 1 PUD table for 2G memory (1G mapping)
> 1 PUD table and 1 PMD table for 2M vmemmap (2M mappings)
>
> With default PUD[PMD]_TABLE_SIZE(4K), (1+1+1)*4K=12K
> With new PAGE_SIZE(64K) table allocations, (1+1+1)*64K=192K

How about we just do

void pmd_fragment_free(unsigned long *pmd)
{
	struct page *page = virt_to_page(pmd);

	/*
	 * Early pmd pages allocated via memblock
	 * allocator need to be freed differently
	 */
	if (PageReserved(page))
		return free_reserved_page(page);

	BUG_ON(atomic_read(&page->pt_frag_refcount) <= 0);
	if (atomic_dec_and_test(&page->pt_frag_refcount)) {
		pgtable_pmd_page_dtor(page);
		__free_page(page);
	}
}

That way we could avoid the fixup_pgtable_fragments completely?

>
> Signed-off-by: Bharata B Rao <[email-masked]>
> ---
>  arch/powerpc/include/asm/book3s/64/pgalloc.h | 11 ++-
>  arch/powerpc/include/asm/book3s/64/radix.h   |  1 +
>  arch/powerpc/include/asm/sparsemem.h         |  1 +
>  arch/powerpc/mm/book3s64/pgtable.c           | 31 +++++++-
>  arch/powerpc/mm/book3s64/radix_pgtable.c     | 80 +++++++++++++++++++-
>  arch/powerpc/mm/mem.c                        |  5 ++
>  arch/powerpc/mm/pgtable-frag.c               |  9 ++-
>  7 files changed, 129 insertions(+), 9 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/book3s/64/pgalloc.h b/arch/powerpc/include/asm/book3s/64/pgalloc.h
> index 69c5b051734f..56d695f0095c 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgalloc.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgalloc.h
> @@ -109,7 +109,16 @@ static inline pud_t *pud_alloc_one(struct mm_struct *mm, unsigned long addr)
>  
>  static inline void pud_free(struct mm_struct *mm, pud_t *pud)
>  {
> -	kmem_cache_free(PGT_CACHE(PUD_CACHE_INDEX), pud);
> +	struct page *page = virt_to_page(pud);
> +
> +	/*
> +	 * Early pud pages allocated via memblock allocator
> +	 * can't be directly freed to slab
> +	 */
> +	if (PageReserved(page))
> +		free_reserved_page(page);
> +	else
> +		kmem_cache_free(PGT_CACHE(PUD_CACHE_INDEX), pud);
>  }
>  
>  static inline void pud_populate(struct mm_struct *mm, pud_t *pud, pmd_t *pmd)
> diff --git a/arch/powerpc/include/asm/book3s/64/radix.h b/arch/powerpc/include/asm/book3s/64/radix.h
> index 0cba794c4fb8..90f05d52f46d 100644
> --- a/arch/powerpc/include/asm/book3s/64/radix.h
> +++ b/arch/powerpc/include/asm/book3s/64/radix.h
> @@ -297,6 +297,7 @@ static inline unsigned long radix__get_tree_size(void)
>  int radix__create_section_mapping(unsigned long start, unsigned long end,
>  				  int nid, pgprot_t prot);
>  int radix__remove_section_mapping(unsigned long start, unsigned long end);
> +void radix__fixup_pgtable_fragments(void);
>  #endif /* CONFIG_MEMORY_HOTPLUG */
>  #endif /* __ASSEMBLY__ */
>  #endif
> diff --git a/arch/powerpc/include/asm/sparsemem.h b/arch/powerpc/include/asm/sparsemem.h
> index c89b32443cff..d0b22a937a7a 100644
> --- a/arch/powerpc/include/asm/sparsemem.h
> +++ b/arch/powerpc/include/asm/sparsemem.h
> @@ -16,6 +16,7 @@
>  extern int create_section_mapping(unsigned long start, unsigned long end,
>  				  int nid, pgprot_t prot);
>  extern int remove_section_mapping(unsigned long start, unsigned long end);
> +void fixup_pgtable_fragments(void);
>  
>  #ifdef CONFIG_PPC_BOOK3S_64
>  extern int resize_hpt_for_hotplug(unsigned long new_mem_size);
> diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
> index c58ad1049909..ee94a28dc6f9 100644
> --- a/arch/powerpc/mm/book3s64/pgtable.c
> +++ b/arch/powerpc/mm/book3s64/pgtable.c
> @@ -184,6 +184,13 @@ int __meminit remove_section_mapping(unsigned long start, unsigned long end)
>  
>  	return hash__remove_section_mapping(start, end);
>  }
> +
> +void fixup_pgtable_fragments(void)
> +{
> +	if (radix_enabled())
> +		radix__fixup_pgtable_fragments();
> +}
> +
>  #endif /* CONFIG_MEMORY_HOTPLUG */
>  
>  void __init mmu_partition_table_init(void)
> @@ -341,13 +348,23 @@ void pmd_fragment_free(unsigned long *pmd)
>  
>  	BUG_ON(atomic_read(&page->pt_frag_refcount) <= 0);
>  	if (atomic_dec_and_test(&page->pt_frag_refcount)) {
> -		pgtable_pmd_page_dtor(page);
> -		__free_page(page);
> +		/*
> +		 * Early pmd pages allocated via memblock
> +		 * allocator wouldn't have called _ctor
> +		 */
> +		if (PageReserved(page))
> +			free_reserved_page(page);
> +		else {
> +			pgtable_pmd_page_dtor(page);
> +			__free_page(page);
> +		}
>  	}
>  }
>  
>  static inline void pgtable_free(void *table, int index)
>  {
> +	struct page *page;
> +
>  	switch (index) {
>  	case PTE_INDEX:
>  		pte_fragment_free(table, 0);
> @@ -356,7 +373,15 @@ static inline void pgtable_free(void *table, int index)
>  		pmd_fragment_free(table);
>  		break;
>  	case PUD_INDEX:
> -		kmem_cache_free(PGT_CACHE(PUD_CACHE_INDEX), table);
> +		page = virt_to_page(table);
> +		/*
> +		 * Early pud pages allocated via memblock
> +		 * allocator need to be freed differently
> +		 */
> +		if (PageReserved(page))
> +			free_reserved_page(page);
> +		else
> +			kmem_cache_free(PGT_CACHE(PUD_CACHE_INDEX), table);
>  		break;
>  #if defined(CONFIG_PPC_4K_PAGES) && defined(CONFIG_HUGETLB_PAGE)
>  		/* 16M hugepd directory at pud level */
> diff --git a/arch/powerpc/mm/book3s64/radix_pgtable.c b/arch/powerpc/mm/book3s64/radix_pgtable.c
> index ffccfe00ca2a..58e42393d5e8 100644
> --- a/arch/powerpc/mm/book3s64/radix_pgtable.c
> +++ b/arch/powerpc/mm/book3s64/radix_pgtable.c
> @@ -37,6 +37,71 @@
>  unsigned int mmu_pid_bits;
>  unsigned int mmu_base_pid;
>  
> +static void fixup_pte_fragments(pmd_t *pmd)
> +{
> +	int i;
> +
> +	for (i = 0; i < PTRS_PER_PMD; i++, pmd++) {
> +		pte_t *pte;
> +		struct page *page;
> +
> +		if (pmd_none(*pmd))
> +			continue;
> +		if (pmd_is_leaf(*pmd))
> +			continue;
> +
> +		pte = pte_offset_kernel(pmd, 0);
> +		page = virt_to_page(pte);
> +		atomic_inc(&page->pt_frag_refcount);
> +	}
> +}
> +
> +static void fixup_pmd_fragments(pud_t *pud)
> +{
> +	int i;
> +
> +	for (i = 0; i < PTRS_PER_PUD; i++, pud++) {
> +		pmd_t *pmd;
> +		struct page *page;
> +
> +		if (pud_none(*pud))
> +			continue;
> +		if (pud_is_leaf(*pud))
> +			continue;
> +
> +		pmd = pmd_offset(pud, 0);
> +		page = virt_to_page(pmd);
> +		atomic_inc(&page->pt_frag_refcount);
> +		fixup_pte_fragments(pmd);
> +	}
> +}
> +
> +/*
> + * Walk the init_mm page tables and fixup the PMD and PTE fragment
> + * counts. This allows the PUD, PMD and PTE pages to be freed
> + * back to buddy allocator properly during memory unplug.
> + */
> +void radix__fixup_pgtable_fragments(void)
> +{
> +	int i;
> +	pgd_t *pgd = pgd_offset_k(0UL);
> +
> +	spin_lock(&init_mm.page_table_lock);
> +	for (i = 0; i < PTRS_PER_PGD; i++, pgd++) {
> +		p4d_t *p4d = p4d_offset(pgd, 0UL);
> +		pud_t *pud;
> +
> +		if (p4d_none(*p4d))
> +			continue;
> +		if (p4d_is_leaf(*p4d))
> +			continue;
> +
> +		pud = pud_offset(p4d, 0);
> +		fixup_pmd_fragments(pud);
> +	}
> +	spin_unlock(&init_mm.page_table_lock);
> +}
> +
>  static __ref void *early_alloc_pgtable(unsigned long size, int nid,
>  			unsigned long region_start, unsigned long region_end)
>  {
> @@ -58,6 +123,13 @@ static __ref void *early_alloc_pgtable(unsigned long size, int nid,
>  	return ptr;
>  }
>  
> +/*
> + * When allocating pud or pmd pointers, we allocate a complete page
> + * of PAGE_SIZE rather than PUD_TABLE_SIZE or PMD_TABLE_SIZE. This
> + * is to ensure that the page obtained from the memblock allocator
> + * can be completely used as page table page and can be freed
> + * correctly when the page table entries are removed.
> + */
>  static int early_map_kernel_page(unsigned long ea, unsigned long pa,
>  			  pgprot_t flags,
>  			  unsigned int map_page_size,
> @@ -74,8 +146,8 @@ static int early_map_kernel_page(unsigned long ea, unsigned long pa,
>  	pgdp = pgd_offset_k(ea);
>  	p4dp = p4d_offset(pgdp, ea);
>  	if (p4d_none(*p4dp)) {
> -		pudp = early_alloc_pgtable(PUD_TABLE_SIZE, nid,
> -						region_start, region_end);
> +		pudp = early_alloc_pgtable(PAGE_SIZE, nid,
> +					   region_start, region_end);
>  		p4d_populate(&init_mm, p4dp, pudp);
>  	}
>  	pudp = pud_offset(p4dp, ea);
> @@ -84,8 +156,8 @@ static int early_map_kernel_page(unsigned long ea, unsigned long pa,
>  		goto set_the_pte;
>  	}
>  	if (pud_none(*pudp)) {
> -		pmdp = early_alloc_pgtable(PMD_TABLE_SIZE, nid,
> -						region_start, region_end);
> +		pmdp = early_alloc_pgtable(PAGE_SIZE, nid, region_start,
> +					   region_end);
>  		pud_populate(&init_mm, pudp, pmdp);
>  	}
>  	pmdp = pmd_offset(pudp, ea);
> diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
> index 5f7fe13211e9..b8ea004c3ebf 100644
> --- a/arch/powerpc/mm/mem.c
> +++ b/arch/powerpc/mm/mem.c
> @@ -54,6 +54,10 @@
>  
>  #include <mm/mmu_decl.h>
>  
> +void __weak fixup_pgtable_fragments(void)
> +{
> +}
> +
>  #ifndef CPU_FTR_COHERENT_ICACHE
>  #define CPU_FTR_COHERENT_ICACHE	0	/* XXX for now */
>  #define CPU_FTR_NOEXECUTE	0
> @@ -301,6 +305,7 @@ void __init mem_init(void)
>  
>  	memblock_free_all();
>  
> +	fixup_pgtable_fragments();
>  #ifdef CONFIG_HIGHMEM
>  	{
>  		unsigned long pfn, highmem_mapnr;
> diff --git a/arch/powerpc/mm/pgtable-frag.c b/arch/powerpc/mm/pgtable-frag.c
> index ee4bd6d38602..16213c09896a 100644
> --- a/arch/powerpc/mm/pgtable-frag.c
> +++ b/arch/powerpc/mm/pgtable-frag.c
> @@ -114,6 +114,13 @@ void pte_fragment_free(unsigned long *table, int kernel)
>  	if (atomic_dec_and_test(&page->pt_frag_refcount)) {
>  		if (!kernel)
>  			pgtable_pte_page_dtor(page);
> -		__free_page(page);
> +		/*
> +		 * Early pte pages allocated via memblock
> +		 * allocator need to be freed differently
> +		 */
> +		if (PageReserved(page))
> +			free_reserved_page(page);
> +		else
> +			__free_page(page);
>  	}
>  }
> -- 
> 2.21.3