Re: [PATCH v3 07/26] x86/mm: introduce mm-local region

[Yosry Ahmed]

On Sun, Jul 26, 2026 at 10:22:40PM +0000, Brendan Jackman wrote:
> Various security features benefit from having process-local address
> mappings within the kernel. Examples include no-direct-map guest_memfd
> [2] and significant optimizations for ASI [1].
> 
> With the currently envisaged usecases, there will be many situations
> where almost no processes have any need for the mm-local region.
> Therefore, avoid its overhead (memory cost of pagetables, alloc/free
> overhead during fork/exit) for processes that don't use it by requiring
> its users to explicitly initialize it via the new mm_local_* API.
> 
> As pointed out by Andy in [0], x86 already has a PGD entry that is local
> to the mm, which is used for the LDT. In a subsequent patch, the LDT
> remap will be unified with the general mm-local region, but to help
> keep the patch to a manageable size, first just introduce the mm-local
> region.
> 
> On 64-bit, give the mm-local region a whole PGD. On 32-bit, just give it
> one PMD. No investigation has been done into whether it's feasible to
> expand the region on 32-bit. Most likely there is no strong usecase for
> that anyway.
> 
> In order to combine the need for an on-demand mm initialisation, with
> the desire to transparently handle propagating mappings to userspace
> under KPTI, the user and kernel pagetables are shared at the highest
> level possible. For PAE that means the PTE table is shared and for
> 64-bit the P4D/PUD. This is implemented by pre-allocating the first
> shared table when the mm-local region is first initialised.
> 
> [0] https://lore.kernel.org/linux-mm/CALCETrXHbS9VXfZ80kOjiTrreM2EbapYeGp68mvJPbosUtorYA@xxxxxxxxxxxxxx/
> [1] https://linuxasi.dev/
> [2] https://lore.kernel.org/all/20250924151101.2225820-1-patrick.roy@xxxxxxxxxxxxx
> Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx>
> ---
>  arch/x86/Kconfig                        |   2 +
>  arch/x86/include/asm/mmu_context.h      | 118 +++++++++++++++++++++++++++++++-
>  arch/x86/include/asm/pgtable_32_areas.h |   9 ++-
>  arch/x86/include/asm/pgtable_64_types.h |  12 +++-
>  arch/x86/mm/pgtable.c                   |   3 +
>  include/linux/mm.h                      |  13 ++++
>  include/linux/mm_types.h                |   2 +
>  kernel/fork.c                           |   1 +
>  mm/Kconfig                              |   7 ++
>  9 files changed, 161 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index fb298e2191792..3efab3524a6cf 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -132,6 +132,8 @@ config X86
>  	select ARCH_SUPPORTS_LTO_CLANG
>  	select ARCH_SUPPORTS_LTO_CLANG_THIN
>  	select ARCH_SUPPORTS_RT
> +	# LDT remap temporarily clashes with mm-local region, can't have both.
> +	select ARCH_SUPPORTS_MM_LOCAL_REGION	if X86_64 || X86_PAE && !MODIFY_LDT_SYSCALL
>  	select ARCH_USE_BUILTIN_BSWAP
>  	select ARCH_USE_CMPXCHG_LOCKREF
>  	select ARCH_USE_MEMTEST
> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
> index ef5b507de34e2..3d4f54673014f 100644
> --- a/arch/x86/include/asm/mmu_context.h
> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -8,8 +8,10 @@
>  
>  #include <trace/events/tlb.h>
>  
> +#include <asm/tlb.h>
>  #include <asm/tlbflush.h>
>  #include <asm/paravirt.h>
> +#include <asm/pgalloc.h>
>  #include <asm/debugreg.h>
>  #include <asm/gsseg.h>
>  #include <asm/desc.h>
> @@ -223,10 +225,124 @@ static inline int arch_dup_mmap(struct mm_struct *oldmm, struct mm_struct *mm)
>  	return ldt_dup_context(oldmm, mm);
>  }
>  
> +#ifdef CONFIG_MM_LOCAL_REGION
> +static inline void mm_local_region_free(struct mm_struct *mm)
> +{
> +	if (!mm_local_region_used(mm))
> +		return;
> +
> +	struct mmu_gather tlb;
> +	unsigned long start = MM_LOCAL_BASE_ADDR;
> +	unsigned long end = MM_LOCAL_END_ADDR;

These declarations should probably go at the beginning of the function.

> +
> +	/*
> +	 * Although free_pgd_range() is intended for freeing user
> +	 * page-tables, it also works out for kernel mappings on x86.
> +	 * Use tlb_gather_mmu_fullmm() to avoid confusing the
> +	 * range-tracking logic in __tlb_adjust_range().
> +	 */
> +	tlb_gather_mmu_fullmm(&tlb, mm);
> +	free_pgd_range(&tlb, start, end, start, end);
> +	tlb_finish_mmu(&tlb);
> +
> +	mm_flags_clear(MMF_LOCAL_REGION_USED, mm);
> +}
> +
> +#if defined(CONFIG_MITIGATION_PAGE_TABLE_ISOLATION) && defined(CONFIG_X86_PAE)

Would it be clearer to have nested #ifdefs instead?

#ifdef CONFIG_MITIGATION_PAGE_TABLE_ISOLATION

#ifdef CONFIG_X86_PAE
...
#else /* CONFIG_X86_PAE */
...
#endif /* CONFIG_X86_PAE */

#else /* CONFIG_MITIGATION_PAGE_TABLE_ISOLATION */

#endif /* CONFIG_MITIGATION_PAGE_TABLE_ISOLATION */

Maybe not, just thinking out loud.

> +static inline pmd_t *pgd_to_pmd_walk(pgd_t *pgd, unsigned long va)
> +{
> +	p4d_t *p4d;
> +	pud_t *pud;
> +
> +	if (pgd->pgd == 0)
> +		return NULL;
> +
> +	p4d = p4d_offset(pgd, va);
> +	if (p4d_none(*p4d))
> +		return NULL;
> +
> +	pud = pud_offset(p4d, va);
> +	if (pud_none(*pud))
> +		return NULL;
> +
> +	return pmd_offset(pud, va);
> +}
> +
> +static inline int mm_local_map_to_user(struct mm_struct *mm)
> +{
> +	BUILD_BUG_ON(!PREALLOCATED_PMDS);
> +	pgd_t *k_pgd = pgd_offset(mm, MM_LOCAL_BASE_ADDR);
> +	pgd_t *u_pgd = kernel_to_user_pgdp(k_pgd);
> +	pmd_t *k_pmd, *u_pmd;
> +	int err;
> +
> +	k_pmd = pgd_to_pmd_walk(k_pgd, MM_LOCAL_BASE_ADDR);
> +	u_pmd = pgd_to_pmd_walk(u_pgd, MM_LOCAL_BASE_ADDR);
> +
> +	BUILD_BUG_ON(MM_LOCAL_END_ADDR - MM_LOCAL_BASE_ADDR > PMD_SIZE);
> +
> +	/* Preallocate the PTE table so it can be shared. */
> +	err = pte_alloc(mm, k_pmd);
> +	if (err)
> +		return err;
> +
> +	/* Point the userspace PMD at the same PTE as the kernel PMD. */
> +	set_pmd(u_pmd, *k_pmd);
> +	return 0;
> +}
> +#elif defined(CONFIG_MITIGATION_PAGE_TABLE_ISOLATION)
> +static inline int mm_local_map_to_user(struct mm_struct *mm)
> +{
> +	pgd_t *pgd;
> +	int err;
> +
> +	err = preallocate_sub_pgd(mm, MM_LOCAL_BASE_ADDR);
> +	if (err)
> +		return err;
> +
> +	pgd = pgd_offset(mm, MM_LOCAL_BASE_ADDR);
> +	set_pgd(kernel_to_user_pgdp(pgd), *pgd);
> +	return 0;
> +}

The code above bears a lot of similarity to the LDT code removed in
patch 8, and reviewing them separately is annoying. I realize that they
were a single patch in the previous version and Dave complained that it
was too large.

What if we go a different way:
1. Move the LDT functions that will be repurposed to mmu_context.h.
2. Rename the functions to the mm_local_* domain where needed.
3. Actually perform the switch for LDT to use mm local region.

Maybe (2) and (3) should be combined, depending on what the git diff
looks like.

I think this will make the diffs much clearer, for example
mm_local_map_to_user() mainly differ from map_ldt_struct_to_user() in
preallocation.

> +#else
> +static inline int mm_local_map_to_user(struct mm_struct *mm)
> +{
> +	WARN_ONCE(1, "mm_local_map_to_user() not implemented");
> +	return -EINVAL;
> +}
> +#endif
[..]
> diff --git a/arch/x86/include/asm/pgtable_32_areas.h b/arch/x86/include/asm/pgtable_32_areas.h
> index 921148b429676..7fccb887f8b33 100644
> --- a/arch/x86/include/asm/pgtable_32_areas.h
> +++ b/arch/x86/include/asm/pgtable_32_areas.h
> @@ -30,9 +30,14 @@ extern bool __vmalloc_start_set; /* set once high_memory is set */
>  #define CPU_ENTRY_AREA_BASE	\
>  	((FIXADDR_TOT_START - PAGE_SIZE*(CPU_ENTRY_AREA_PAGES+1)) & PMD_MASK)
>  
> -#define LDT_BASE_ADDR		\
> -	((CPU_ENTRY_AREA_BASE - PAGE_SIZE) & PMD_MASK)
> +/*
> + * On 32-bit the mm-local region is currently completely consumed by the LDT
> + * remap.
> + */
> +#define MM_LOCAL_BASE_ADDR	((CPU_ENTRY_AREA_BASE - PAGE_SIZE) & PMD_MASK)
> +#define MM_LOCAL_END_ADDR	(MM_LOCAL_BASE_ADDR + PMD_SIZE)
>  
> +#define LDT_BASE_ADDR		MM_LOCAL_BASE_ADDR
>  #define LDT_END_ADDR		(LDT_BASE_ADDR + PMD_SIZE)
>  
>  #define PKMAP_BASE		\
> diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
> index 7eb61ef6a185f..1181565966405 100644
> --- a/arch/x86/include/asm/pgtable_64_types.h
> +++ b/arch/x86/include/asm/pgtable_64_types.h
> @@ -5,8 +5,11 @@
>  #include <asm/sparsemem.h>
>  
>  #ifndef __ASSEMBLER__
> +#include <linux/build_bug.h>
>  #include <linux/types.h>
>  #include <asm/kaslr.h>
> +#include <asm/page_types.h>
> +#include <uapi/asm/ldt.h>
>  
>  /*
>   * These are used to make use of C type-checking..
> @@ -100,9 +103,12 @@ extern unsigned int ptrs_per_p4d;
>  #define GUARD_HOLE_BASE_ADDR	(GUARD_HOLE_PGD_ENTRY << PGDIR_SHIFT)
>  #define GUARD_HOLE_END_ADDR	(GUARD_HOLE_BASE_ADDR + GUARD_HOLE_SIZE)
>  
> -#define LDT_PGD_ENTRY		-240UL
> -#define LDT_BASE_ADDR		(LDT_PGD_ENTRY << PGDIR_SHIFT)
> -#define LDT_END_ADDR		(LDT_BASE_ADDR + PGDIR_SIZE)
> +#define MM_LOCAL_PGD_ENTRY	-240UL
> +#define MM_LOCAL_BASE_ADDR	(MM_LOCAL_PGD_ENTRY << PGDIR_SHIFT)
> +#define MM_LOCAL_END_ADDR	((MM_LOCAL_PGD_ENTRY + 1) << PGDIR_SHIFT)

Any reason not keep the current formula (i.e. MM_LOCAL_BASE_ADDR +
PGDIR_SIZE)?

> +
> +#define LDT_BASE_ADDR		MM_LOCAL_BASE_ADDR
> +#define LDT_END_ADDR		(LDT_BASE_ADDR + PMD_SIZE)

Looks like the LDT area was silently changed to PMD_SIZE here. I assume
this is to give the rest of the pgd-mapped address space to the mermap,
but maybe we should call this out explicitly, or do it when the mermap
is introduced (or separately)?

>  
>  #define __VMALLOC_BASE_L4	0xffffc90000000000UL
>  #define __VMALLOC_BASE_L5 	0xffa0000000000000UL
[..]