Re: [PATCH V4 mm-hotfixes 3/3] x86/mm/64: define ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings()

[Lorenzo Stoakes]

On Mon, Aug 11, 2025 at 02:34:20PM +0900, Harry Yoo wrote:
> Define ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings() to ensure
> page tables are properly synchronized when calling p*d_populate_kernel().
> It is inteneded to synchronize page tables via pgd_pouplate_kernel() when
> 5-level paging is in use and via p4d_pouplate_kernel() when 4-level paging
> is used.
>

I think it's worth mentioning here that pgd_populate() is a no-op in 4-level
systems, so the sychronisation must occur at the P4D level, just to make this
clear.

> This fixes intermittent boot failures on systems using 4-level paging
> and a large amount of persistent memory:
>
>   BUG: unable to handle page fault for address: ffffe70000000034
>   #PF: supervisor write access in kernel mode
>   #PF: error_code(0x0002) - not-present page
>   PGD 0 P4D 0
>   Oops: 0002 [#1] SMP NOPTI
>   RIP: 0010:__init_single_page+0x9/0x6d
>   Call Trace:
>    <TASK>
>    __init_zone_device_page+0x17/0x5d
>    memmap_init_zone_device+0x154/0x1bb
>    pagemap_range+0x2e0/0x40f
>    memremap_pages+0x10b/0x2f0
>    devm_memremap_pages+0x1e/0x60
>    dev_dax_probe+0xce/0x2ec [device_dax]
>    dax_bus_probe+0x6d/0xc9
>    [... snip ...]
>    </TASK>
>
> It also fixes a crash in vmemmap_set_pmd() caused by accessing vmemmap
> before sync_global_pgds() [1]:
>
>   BUG: unable to handle page fault for address: ffffeb3ff1200000
>   #PF: supervisor write access in kernel mode
>   #PF: error_code(0x0002) - not-present page
>   PGD 0 P4D 0
>   Oops: Oops: 0002 [#1] PREEMPT SMP NOPTI
>   Tainted: [W]=WARN
>   RIP: 0010:vmemmap_set_pmd+0xff/0x230
>    <TASK>
>    vmemmap_populate_hugepages+0x176/0x180
>    vmemmap_populate+0x34/0x80
>    __populate_section_memmap+0x41/0x90
>    sparse_add_section+0x121/0x3e0
>    __add_pages+0xba/0x150
>    add_pages+0x1d/0x70
>    memremap_pages+0x3dc/0x810
>    devm_memremap_pages+0x1c/0x60
>    xe_devm_add+0x8b/0x100 [xe]
>    xe_tile_init_noalloc+0x6a/0x70 [xe]
>    xe_device_probe+0x48c/0x740 [xe]
>    [... snip ...]
>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> Closes: https://lore.kernel.org/linux-mm/20250311114420.240341-1-gwan-gyeong.mun@xxxxxxxxx [1]
> Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Signed-off-by: Harry Yoo <harry.yoo@xxxxxxxxxx>

Other than nitty comments, this looks good to me, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

> ---
>  arch/x86/include/asm/pgtable_64_types.h | 3 +++
>  arch/x86/mm/init_64.c                   | 5 +++++
>  2 files changed, 8 insertions(+)
>
> diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
> index 4604f924d8b8..7eb61ef6a185 100644
> --- a/arch/x86/include/asm/pgtable_64_types.h
> +++ b/arch/x86/include/asm/pgtable_64_types.h
> @@ -36,6 +36,9 @@ static inline bool pgtable_l5_enabled(void)
>  #define pgtable_l5_enabled() cpu_feature_enabled(X86_FEATURE_LA57)
>  #endif /* USE_EARLY_PGTABLE_L5 */
>
> +#define ARCH_PAGE_TABLE_SYNC_MASK \
> +	(pgtable_l5_enabled() ? PGTBL_PGD_MODIFIED : PGTBL_P4D_MODIFIED)
> +
>  extern unsigned int pgdir_shift;
>  extern unsigned int ptrs_per_p4d;
>
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 76e33bd7c556..a78b498c0dc3 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -223,6 +223,11 @@ static void sync_global_pgds(unsigned long start, unsigned long end)
>  		sync_global_pgds_l4(start, end);
>  }
>

Worth a comment to say 'if 4-level, then we synchronise at P4D level by
convention, however the same sync_global_pgds() applies'?

> +void arch_sync_kernel_mappings(unsigned long start, unsigned long end)
> +{
> +	sync_global_pgds(start, end);
> +}
> +
>  /*
>   * NOTE: This function is marked __ref because it calls __init function
>   * (alloc_bootmem_pages). It's safe to do it ONLY when after_bootmem == 0.
> --
> 2.43.0
>