Re: [PATCH 2/2] Revert "arm64: Enable vmalloc-huge with ptdump"

[Dev Jain]

On 10/07/26 4:19 pm, Lorenzo Stoakes wrote:
> This partially reverts commit fa93b45fd397 ("arm64: Enable vmalloc-huge
> with ptdump"), retaining vmalloc-huge support but eliminating the now
> redundant mitigation against a race between huge vmap page table freeing
> and ptdump, as this issue has now been fixed at core.
> 
> We also simultaneously remove the arm64 ifdeffery when acquiring the mmap
> read lock upon vmap huge page table promotion as it is no longer required.
> 
> Signed-off-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> ---

As pointed out by Mike, the subject line doesn't fit. Perhaps just start with
"Partially revert ..."


Reviewed-by: Dev Jain <dev.jain@xxxxxxx>

>  arch/arm64/include/asm/ptdump.h |  2 --
>  arch/arm64/mm/mmu.c             | 43 ++++-------------------------------------
>  arch/arm64/mm/ptdump.c          | 11 ++---------
>  mm/vmalloc.c                    | 15 +++-----------
>  4 files changed, 9 insertions(+), 62 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/ptdump.h b/arch/arm64/include/asm/ptdump.h
> index 5b374a6ab34a..50a195eda8ed 100644
> --- a/arch/arm64/include/asm/ptdump.h
> +++ b/arch/arm64/include/asm/ptdump.h
> @@ -7,8 +7,6 @@
>  
>  #include <linux/ptdump.h>
>  
> -DECLARE_STATIC_KEY_FALSE(arm64_ptdump_lock_key);
> -
>  #ifdef CONFIG_PTDUMP
>  
>  #include <linux/mm_types.h>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index f2be501468ce..f723bcf68174 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -49,8 +49,6 @@
>  #define NO_CONT_MAPPINGS	BIT(1)
>  #define NO_EXEC_MAPPINGS	BIT(2)	/* assumes FEAT_HPDS is not used */
>  
> -DEFINE_STATIC_KEY_FALSE(arm64_ptdump_lock_key);
> -
>  u64 kimage_voffset __ro_after_init;
>  EXPORT_SYMBOL(kimage_voffset);
>  
> @@ -1857,8 +1855,7 @@ int pmd_clear_huge(pmd_t *pmdp)
>  	return 1;
>  }
>  
> -static int __pmd_free_pte_page(pmd_t *pmdp, unsigned long addr,
> -			       bool acquire_mmap_lock)
> +int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr)
>  {
>  	pte_t *table;
>  	pmd_t pmd;
> @@ -1870,25 +1867,13 @@ static int __pmd_free_pte_page(pmd_t *pmdp, unsigned long addr,
>  		return 1;
>  	}
>  
> -	/* See comment in pud_free_pmd_page for static key logic */
>  	table = pte_offset_kernel(pmdp, addr);
>  	pmd_clear(pmdp);
>  	__flush_tlb_kernel_pgtable(addr);
> -	if (static_branch_unlikely(&arm64_ptdump_lock_key) && acquire_mmap_lock) {
> -		mmap_read_lock(&init_mm);
> -		mmap_read_unlock(&init_mm);
> -	}
> -
>  	pte_free_kernel(NULL, table);
>  	return 1;
>  }
>  
> -int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr)
> -{
> -	/* If ptdump is walking the pagetables, acquire init_mm.mmap_lock */
> -	return __pmd_free_pte_page(pmdp, addr, /* acquire_mmap_lock = */ true);
> -}
> -
>  int pud_free_pmd_page(pud_t *pudp, unsigned long addr)
>  {
>  	pmd_t *table;
> @@ -1904,36 +1889,16 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr)
>  	}
>  
>  	table = pmd_offset(pudp, addr);
> -
> -	/*
> -	 * Our objective is to prevent ptdump from reading a PMD table which has
> -	 * been freed. In this race, if pud_free_pmd_page observes the key on
> -	 * (which got flipped by ptdump) then the mmap lock sequence here will,
> -	 * as a result of the mmap write lock/unlock sequence in ptdump, give
> -	 * us the correct synchronization. If not, this means that ptdump has
> -	 * yet not started walking the pagetables - the sequence of barriers
> -	 * issued by __flush_tlb_kernel_pgtable() guarantees that ptdump will
> -	 * observe an empty PUD.
> -	 */
> -	pud_clear(pudp);
> -	__flush_tlb_kernel_pgtable(addr);
> -	if (static_branch_unlikely(&arm64_ptdump_lock_key)) {
> -		mmap_read_lock(&init_mm);
> -		mmap_read_unlock(&init_mm);
> -	}
> -
>  	pmdp = table;
>  	next = addr;
>  	end = addr + PUD_SIZE;
>  	do {
>  		if (pmd_present(pmdp_get(pmdp)))
> -			/*
> -			 * PMD has been isolated, so ptdump won't see it. No
> -			 * need to acquire init_mm.mmap_lock.
> -			 */
> -			__pmd_free_pte_page(pmdp, next, /* acquire_mmap_lock = */ false);
> +			pmd_free_pte_page(pmdp, next);
>  	} while (pmdp++, next += PMD_SIZE, next != end);
>  
> +	pud_clear(pudp);
> +	__flush_tlb_kernel_pgtable(addr);
>  	pmd_free(NULL, table);
>  	return 1;
>  }
> diff --git a/arch/arm64/mm/ptdump.c b/arch/arm64/mm/ptdump.c
> index 1c20144700d7..5a76c59b5ada 100644
> --- a/arch/arm64/mm/ptdump.c
> +++ b/arch/arm64/mm/ptdump.c
> @@ -283,13 +283,6 @@ void note_page_flush(struct ptdump_state *pt_st)
>  	note_page(pt_st, 0, -1, pte_val(pte_zero));
>  }
>  
> -static void arm64_ptdump_walk_pgd(struct ptdump_state *st, struct mm_struct *mm)
> -{
> -	static_branch_inc(&arm64_ptdump_lock_key);
> -	ptdump_walk_pgd(st, mm, NULL);
> -	static_branch_dec(&arm64_ptdump_lock_key);
> -}
> -
>  void ptdump_walk(struct seq_file *s, struct ptdump_info *info)
>  {
>  	unsigned long end = ~0UL;
> @@ -318,7 +311,7 @@ void ptdump_walk(struct seq_file *s, struct ptdump_info *info)
>  		}
>  	};
>  
> -	arm64_ptdump_walk_pgd(&st.ptdump, info->mm);
> +	ptdump_walk_pgd(&st.ptdump, info->mm, NULL);
>  }
>  
>  static void __init ptdump_initialize(void)
> @@ -360,7 +353,7 @@ bool ptdump_check_wx(void)
>  		}
>  	};
>  
> -	arm64_ptdump_walk_pgd(&st.ptdump, &init_mm);
> +	ptdump_walk_pgd(&st.ptdump, &init_mm, NULL);
>  
>  	if (st.wx_pages || st.uxn_pages) {
>  		pr_warn("Checked W+X mappings: FAILED, %lu W+X pages found, %lu non-UXN pages found\n",
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 9d0f1fdd6af3..537ff7b3c412 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -170,10 +170,7 @@ static int vmap_try_huge_pmd(pmd_t *pmd, unsigned long addr, unsigned long end,
>  	 * Therefore, acquire the mmap read lock to prevent use-after-free when
>  	 * freeing page tables.
>  	 */
> -#ifndef CONFIG_ARM64
> -	scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm)
> -#endif
> -	{
> +	scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm) {
>  		if (!pmd_free_pte_page(pmd, addr))
>  			return 0;
>  		return pmd_set_huge(pmd, phys_addr, prot);
> @@ -230,10 +227,7 @@ static int vmap_try_huge_pud(pud_t *pud, unsigned long addr, unsigned long end,
>  		return pud_set_huge(pud, phys_addr, prot);
>  
>  	/* See comment in vmap_try_huge_pmd(). */
> -#ifndef CONFIG_ARM64
> -	scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm)
> -#endif
> -	{
> +	scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm) {
>  		if (!pud_free_pmd_page(pud, addr))
>  			return 0;
>  		return pud_set_huge(pud, phys_addr, prot);
> @@ -290,10 +284,7 @@ static int vmap_try_huge_p4d(p4d_t *p4d, unsigned long addr, unsigned long end,
>  		return p4d_set_huge(p4d, phys_addr, prot);
>  
>  	/* See comment in vmap_try_huge_pmd(). */
> -#ifndef CONFIG_ARM64
> -	scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm)
> -#endif
> -	{
> +	scoped_cond_guard(mmap_read_lock_try, return 0, &init_mm) {
>  		if (!p4d_free_pud_page(p4d, addr))
>  			return 0;
>  		return p4d_set_huge(p4d, phys_addr, prot);
>