Re: [PATCH v1 15/16] mm: Generalize arch_sync_kernel_mappings()

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 2/5/25 20:39, Ryan Roberts wrote:
> arch_sync_kernel_mappings() is an optional hook for arches to allow them
> to synchonize certain levels of the kernel pgtables after modification.
> But arm64 could benefit from a hook similar to this, paired with a call
> prior to starting the batch of modifications.
> 
> So let's introduce arch_update_kernel_mappings_begin() and
> arch_update_kernel_mappings_end(). Both have a default implementation
> which can be overridden by the arch code. The default for the former is
> a nop, and the default for the latter is to call
> arch_sync_kernel_mappings(), so the latter replaces previous
> arch_sync_kernel_mappings() callsites. So by default, the resulting
> behaviour is unchanged.
> 
> To avoid include hell, the pgtbl_mod_mask type and it's associated
> macros are moved to their own header.
> 
> In a future patch, arm64 will opt-in to overriding both functions.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  include/linux/pgtable.h         | 24 +----------------
>  include/linux/pgtable_modmask.h | 32 ++++++++++++++++++++++
>  include/linux/vmalloc.h         | 47 +++++++++++++++++++++++++++++++++
>  mm/memory.c                     |  5 ++--
>  mm/vmalloc.c                    | 15 ++++++-----
>  5 files changed, 92 insertions(+), 31 deletions(-)
>  create mode 100644 include/linux/pgtable_modmask.h
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 94d267d02372..7f70786a73b3 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -4,6 +4,7 @@
>  
>  #include <linux/pfn.h>
>  #include <asm/pgtable.h>
> +#include <linux/pgtable_modmask.h>
>  
>  #define PMD_ORDER	(PMD_SHIFT - PAGE_SHIFT)
>  #define PUD_ORDER	(PUD_SHIFT - PAGE_SHIFT)
> @@ -1786,29 +1787,6 @@ static inline bool arch_has_pfn_modify_check(void)
>  # define PAGE_KERNEL_EXEC PAGE_KERNEL
>  #endif
>  
> -/*
> - * Page Table Modification bits for pgtbl_mod_mask.
> - *
> - * These are used by the p?d_alloc_track*() set of functions an in the generic
> - * vmalloc/ioremap code to track at which page-table levels entries have been
> - * modified. Based on that the code can better decide when vmalloc and ioremap
> - * mapping changes need to be synchronized to other page-tables in the system.
> - */
> -#define		__PGTBL_PGD_MODIFIED	0
> -#define		__PGTBL_P4D_MODIFIED	1
> -#define		__PGTBL_PUD_MODIFIED	2
> -#define		__PGTBL_PMD_MODIFIED	3
> -#define		__PGTBL_PTE_MODIFIED	4
> -
> -#define		PGTBL_PGD_MODIFIED	BIT(__PGTBL_PGD_MODIFIED)
> -#define		PGTBL_P4D_MODIFIED	BIT(__PGTBL_P4D_MODIFIED)
> -#define		PGTBL_PUD_MODIFIED	BIT(__PGTBL_PUD_MODIFIED)
> -#define		PGTBL_PMD_MODIFIED	BIT(__PGTBL_PMD_MODIFIED)
> -#define		PGTBL_PTE_MODIFIED	BIT(__PGTBL_PTE_MODIFIED)
> -
> -/* Page-Table Modification Mask */
> -typedef unsigned int pgtbl_mod_mask;
> -
>  #endif /* !__ASSEMBLY__ */
>  
>  #if !defined(MAX_POSSIBLE_PHYSMEM_BITS) && !defined(CONFIG_64BIT)
> diff --git a/include/linux/pgtable_modmask.h b/include/linux/pgtable_modmask.h
> new file mode 100644
> index 000000000000..5a21b1bb8df3
> --- /dev/null
> +++ b/include/linux/pgtable_modmask.h
> @@ -0,0 +1,32 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_PGTABLE_MODMASK_H
> +#define _LINUX_PGTABLE_MODMASK_H
> +
> +#ifndef __ASSEMBLY__
> +
> +/*
> + * Page Table Modification bits for pgtbl_mod_mask.
> + *
> + * These are used by the p?d_alloc_track*() set of functions an in the generic
> + * vmalloc/ioremap code to track at which page-table levels entries have been
> + * modified. Based on that the code can better decide when vmalloc and ioremap
> + * mapping changes need to be synchronized to other page-tables in the system.
> + */
> +#define		__PGTBL_PGD_MODIFIED	0
> +#define		__PGTBL_P4D_MODIFIED	1
> +#define		__PGTBL_PUD_MODIFIED	2
> +#define		__PGTBL_PMD_MODIFIED	3
> +#define		__PGTBL_PTE_MODIFIED	4
> +
> +#define		PGTBL_PGD_MODIFIED	BIT(__PGTBL_PGD_MODIFIED)
> +#define		PGTBL_P4D_MODIFIED	BIT(__PGTBL_P4D_MODIFIED)
> +#define		PGTBL_PUD_MODIFIED	BIT(__PGTBL_PUD_MODIFIED)
> +#define		PGTBL_PMD_MODIFIED	BIT(__PGTBL_PMD_MODIFIED)
> +#define		PGTBL_PTE_MODIFIED	BIT(__PGTBL_PTE_MODIFIED)
> +
> +/* Page-Table Modification Mask */
> +typedef unsigned int pgtbl_mod_mask;
> +
> +#endif /* !__ASSEMBLY__ */
> +
> +#endif /* _LINUX_PGTABLE_MODMASK_H */
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 16dd4cba64f2..cb5d8f1965a1 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -11,6 +11,7 @@
>  #include <asm/page.h>		/* pgprot_t */
>  #include <linux/rbtree.h>
>  #include <linux/overflow.h>
> +#include <linux/pgtable_modmask.h>
>  
>  #include <asm/vmalloc.h>
>  
> @@ -213,6 +214,26 @@ extern int remap_vmalloc_range(struct vm_area_struct *vma, void *addr,
>  int vmap_pages_range(unsigned long addr, unsigned long end, pgprot_t prot,
>  		     struct page **pages, unsigned int page_shift);
>  
> +#ifndef arch_update_kernel_mappings_begin
> +/**
> + * arch_update_kernel_mappings_begin - A batch of kernel pgtable mappings are
> + * about to be updated.
> + * @start: Virtual address of start of range to be updated.
> + * @end: Virtual address of end of range to be updated.
> + *
> + * An optional hook to allow architecture code to prepare for a batch of kernel
> + * pgtable mapping updates. An architecture may use this to enter a lazy mode
> + * where some operations can be deferred until the end of the batch.
> + *
> + * Context: Called in task context and may be preemptible.
> + */
> +static inline void arch_update_kernel_mappings_begin(unsigned long start,
> +						     unsigned long end)
> +{
> +}
> +#endif
> +
> +#ifndef arch_update_kernel_mappings_end
>  /*
>   * Architectures can set this mask to a combination of PGTBL_P?D_MODIFIED values
>   * and let generic vmalloc and ioremap code know when arch_sync_kernel_mappings()
> @@ -229,6 +250,32 @@ int vmap_pages_range(unsigned long addr, unsigned long end, pgprot_t prot,
>   */
>  void arch_sync_kernel_mappings(unsigned long start, unsigned long end);
>  
> +/**
> + * arch_update_kernel_mappings_end - A batch of kernel pgtable mappings have
> + * been updated.
> + * @start: Virtual address of start of range that was updated.
> + * @end: Virtual address of end of range that was updated.
> + *
> + * An optional hook to inform architecture code that a batch update is complete.
> + * This balances a previous call to arch_update_kernel_mappings_begin().
> + *
> + * An architecture may override this for any purpose, such as exiting a lazy
> + * mode previously entered with arch_update_kernel_mappings_begin() or syncing
> + * kernel mappings to a secondary pgtable. The default implementation calls an
> + * arch-provided arch_sync_kernel_mappings() if any arch-defined pgtable level
> + * was updated.
> + *
> + * Context: Called in task context and may be preemptible.
> + */
> +static inline void arch_update_kernel_mappings_end(unsigned long start,
> +						   unsigned long end,
> +						   pgtbl_mod_mask mask)
> +{
> +	if (mask & ARCH_PAGE_TABLE_SYNC_MASK)
> +		arch_sync_kernel_mappings(start, end);
> +}

One arch call back calling yet another arch call back sounds bit odd. Also
should not ARCH_PAGE_TABLE_SYNC_MASK be checked both for __begin and __end
callbacks in case a platform subscribes into this framework. Instead the
following changes sound more reasonable, but will also require some more
updates for the current platforms using arch_sync_kernel_mappings().

if (mask & ARCH_PAGE_TABLE_SYNC_MASK)
	arch_update_kernel_mappings_begin()

if (mask & ARCH_PAGE_TABLE_SYNC_MASK)
	arch_update_kernel_mappings_end()

Basically when any platform defines ARCH_PAGE_TABLE_SYNC_MASK and subscribes
this framework, it will also provide arch_update_kernel_mappings_begin/end()
callbacks as required.

> +#endif
> +
>  /*
>   *	Lowlevel-APIs (not for driver use!)
>   */
> diff --git a/mm/memory.c b/mm/memory.c
> index a15f7dd500ea..f80930bc19f6 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3035,6 +3035,8 @@ static int __apply_to_page_range(struct mm_struct *mm, unsigned long addr,
>  	if (WARN_ON(addr >= end))
>  		return -EINVAL;
>  
> +	arch_update_kernel_mappings_begin(start, end);
> +
>  	pgd = pgd_offset(mm, addr);
>  	do {
>  		next = pgd_addr_end(addr, end);
> @@ -3055,8 +3057,7 @@ static int __apply_to_page_range(struct mm_struct *mm, unsigned long addr,
>  			break;
>  	} while (pgd++, addr = next, addr != end);
>  
> -	if (mask & ARCH_PAGE_TABLE_SYNC_MASK)
> -		arch_sync_kernel_mappings(start, start + size);
> +	arch_update_kernel_mappings_end(start, end, mask);
>  
>  	return err;
>  }
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 50fd44439875..c5c51d86ef78 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -312,10 +312,10 @@ int vmap_page_range(unsigned long addr, unsigned long end,
>  	pgtbl_mod_mask mask = 0;
>  	int err;
>  
> +	arch_update_kernel_mappings_begin(addr, end);
>  	err = vmap_range_noflush(addr, end, phys_addr, pgprot_nx(prot),
>  				 ioremap_max_page_shift, &mask);
> -	if (mask & ARCH_PAGE_TABLE_SYNC_MASK)
> -		arch_sync_kernel_mappings(addr, end);
> +	arch_update_kernel_mappings_end(addr, end, mask);
>  
>  	flush_cache_vmap(addr, end);
>  	if (!err)
> @@ -463,6 +463,9 @@ void __vunmap_range_noflush(unsigned long start, unsigned long end)
>  	pgtbl_mod_mask mask = 0;
>  
>  	BUG_ON(addr >= end);
> +
> +	arch_update_kernel_mappings_begin(start, end);
> +
>  	pgd = pgd_offset_k(addr);
>  	do {
>  		next = pgd_addr_end(addr, end);
> @@ -473,8 +476,7 @@ void __vunmap_range_noflush(unsigned long start, unsigned long end)
>  		vunmap_p4d_range(pgd, addr, next, &mask);
>  	} while (pgd++, addr = next, addr != end);
>  
> -	if (mask & ARCH_PAGE_TABLE_SYNC_MASK)
> -		arch_sync_kernel_mappings(start, end);
> +	arch_update_kernel_mappings_end(start, end, mask);
>  }
>  
>  void vunmap_range_noflush(unsigned long start, unsigned long end)
> @@ -625,6 +627,8 @@ int __vmap_pages_range_noflush(unsigned long addr, unsigned long end,
>  
>  	WARN_ON(page_shift < PAGE_SHIFT);
>  
> +	arch_update_kernel_mappings_begin(start, end);
> +
>  	if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMALLOC) ||
>  			page_shift == PAGE_SHIFT) {
>  		err = vmap_small_pages_range_noflush(addr, end, prot, pages,
> @@ -642,8 +646,7 @@ int __vmap_pages_range_noflush(unsigned long addr, unsigned long end,
>  		}
>  	}
>  
> -	if (mask & ARCH_PAGE_TABLE_SYNC_MASK)
> -		arch_sync_kernel_mappings(start, end);
> +	arch_update_kernel_mappings_end(start, end, mask);
>  
>  	return err;
>  }