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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Tue, Aug 12, 2025 at 05:59:02PM +0900, Harry Yoo wrote:
> On Mon, Aug 11, 2025 at 12:46:32PM +0100, Lorenzo Stoakes wrote:
> > 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.
>
> Yeah, that's indeed confusing and agree that it's worth mentioning.
> Will do. The new one:
>
> Define ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings() to
> ensure page tables are properly synchronized when calling
> p*d_populate_kernel().
>
> For 5-level paging, synchronization is performed via pgd_populate_kernel().
> In 4-level paging, pgd_populate() is a no-op, so synchronization is instead
> performed at the P4D level via p4d_populate_kernel().

That's great thanks!

>
> > > 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@vger.kernel.org>
> > > Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> > > Closes: https://lore.kernel.org/linux-mm/20250311114420.240341-1-gwan-gyeong.mun@intel.com [1]
> > > Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> > > Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> >
> > Other than nitty comments, this looks good to me, so:
> >
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>
> Thanks!
>
> > > ---
> > >  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'?
>
> Maybe:
>
> /*
>  * Make kernel mappings visible in all page tables in the system.
>  * This is necessary except when the init task populates kernel mappings
>  * during the boot process. In that case, all processes originating from
>  * the init task copies the kernel mappings, so there is no issue.
>  * Otherwise, missing synchronization could lead to kernel crashes due
>  * to missing page table entries for certain kernel mappings.
>  *
>  * Synchronization is performed at the top level, which is the PGD in
>  * 5-level paging systems. But in 4-level paging systems, however,
>  * pgd_populate() is a no-op, so synchronization is done at P4D level instead.
>  * sync_global_pgds() handles this difference between paging levels.
>  */
>

That's great also, thanks!

> --
> Cheers,
> Harry / Hyeonggon
>
> > > +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
> > >
>