Re: [PATCH] powerpc/64s/radix/kfence: map __kfence_pool at page granularity

[Aboorva Devarajan <aboorvad@linux.ibm.com>]

On Mon, 2025-09-29 at 07:07 +0530, Aboorva Devarajan wrote:
> On Wed, 2025-09-10 at 16:32 +0530, Aboorva Devarajan wrote:
> 
> 
> > From: Hari Bathini <hbathini@linux.ibm.com>
> > 
> > When KFENCE is enabled, total system memory is mapped at page level
> > granularity. But in radix MMU mode, ~3GB additional memory is needed
> > to map 100GB of system memory at page level granularity when compared
> > to using 2MB direct mapping.This is not desired considering KFENCE is
> > designed to be enabled in production kernels [1].
> > 
> > Mapping only the memory allocated for KFENCE pool at page granularity is
> > sufficient to enable KFENCE support. So, allocate __kfence_pool during
> > bootup and map it at page granularity instead of mapping all system
> > memory at page granularity.
> > 
> > Without patch:
> >   # cat /proc/meminfo
> >   MemTotal:       101201920 kB
> > 
> > With patch:
> >   # cat /proc/meminfo
> >   MemTotal:       104483904 kB
> > 
> > Note that enabling KFENCE at runtime is disabled for radix MMU for now,
> > as it depends on the ability to split page table mappings and such APIs
> > are not currently implemented for radix MMU.
> > 
> > All kfence_test.c testcases passed with this patch.
> > 
> > [1] https://lore.kernel.org/all/20201103175841.3495947-2-elver@google.com/
> > 
> > Fixes: a5edf9815dd7 ("powerpc/64s: Enable KFENCE on book3s64")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
> > Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> > Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
> > Link: https://msgid.link/20240701130021.578240-1-hbathini@linux.ibm.com
> > 
> > ---
> > 
> > Upstream commit 353d7a84c214 ("powerpc/64s/radix/kfence: map __kfence_pool at page granularity")
> > 
> > This has already been merged upstream and is required in stable kernels as well.
> > 
> > ---
> >  arch/powerpc/include/asm/kfence.h        | 11 +++-
> >  arch/powerpc/mm/book3s64/radix_pgtable.c | 84 ++++++++++++++++++++++--
> >  arch/powerpc/mm/init-common.c            |  3 +
> >  3 files changed, 93 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/kfence.h b/arch/powerpc/include/asm/kfence.h
> > index 424ceef82ae615..fab124ada1c7f2 100644
> > --- a/arch/powerpc/include/asm/kfence.h
> > +++ b/arch/powerpc/include/asm/kfence.h
> > @@ -15,10 +15,19 @@
> >  #define ARCH_FUNC_PREFIX "."
> >  #endif
> >  
> > +#ifdef CONFIG_KFENCE
> > +extern bool kfence_disabled;
> > +
> > +static inline void disable_kfence(void)
> > +{
> > +	kfence_disabled = true;
> > +}
> > +
> >  static inline bool arch_kfence_init_pool(void)
> >  {
> > -	return true;
> > +	return !kfence_disabled;
> >  }
> > +#endif
> >  
> >  #ifdef CONFIG_PPC64
> >  static inline bool kfence_protect_page(unsigned long addr, bool protect)
> > diff --git a/arch/powerpc/mm/book3s64/radix_pgtable.c b/arch/powerpc/mm/book3s64/radix_pgtable.c
> > index 15e88f1439ec20..b0d927009af83c 100644
> > --- a/arch/powerpc/mm/book3s64/radix_pgtable.c
> > +++ b/arch/powerpc/mm/book3s64/radix_pgtable.c
> > @@ -17,6 +17,7 @@
> >  #include <linux/hugetlb.h>
> >  #include <linux/string_helpers.h>
> >  #include <linux/memory.h>
> > +#include <linux/kfence.h>
> >  
> >  #include <asm/pgalloc.h>
> >  #include <asm/mmu_context.h>
> > @@ -31,6 +32,7 @@
> >  #include <asm/uaccess.h>
> >  #include <asm/ultravisor.h>
> >  #include <asm/set_memory.h>
> > +#include <asm/kfence.h>
> >  
> >  #include <trace/events/thp.h>
> >  
> > @@ -293,7 +295,8 @@ static unsigned long next_boundary(unsigned long addr, unsigned long end)
> >  
> >  static int __meminit create_physical_mapping(unsigned long start,
> >  					     unsigned long end,
> > -					     int nid, pgprot_t _prot)
> > +					     int nid, pgprot_t _prot,
> > +					     unsigned long mapping_sz_limit)
> >  {
> >  	unsigned long vaddr, addr, mapping_size = 0;
> >  	bool prev_exec, exec = false;
> > @@ -301,7 +304,10 @@ static int __meminit create_physical_mapping(unsigned long start,
> >  	int psize;
> >  	unsigned long max_mapping_size = memory_block_size;
> >  
> > -	if (debug_pagealloc_enabled_or_kfence())
> > +	if (mapping_sz_limit < max_mapping_size)
> > +		max_mapping_size = mapping_sz_limit;
> > +
> > +	if (debug_pagealloc_enabled())
> >  		max_mapping_size = PAGE_SIZE;
> >  
> >  	start = ALIGN(start, PAGE_SIZE);
> > @@ -356,8 +362,74 @@ static int __meminit create_physical_mapping(unsigned long start,
> >  	return 0;
> >  }
> >  
> > +#ifdef CONFIG_KFENCE
> > +static bool __ro_after_init kfence_early_init = !!CONFIG_KFENCE_SAMPLE_INTERVAL;
> > +
> > +static int __init parse_kfence_early_init(char *arg)
> > +{
> > +	int val;
> > +
> > +	if (get_option(&arg, &val))
> > +		kfence_early_init = !!val;
> > +	return 0;
> > +}
> > +early_param("kfence.sample_interval", parse_kfence_early_init);
> > +
> > +static inline phys_addr_t alloc_kfence_pool(void)
> > +{
> > +	phys_addr_t kfence_pool;
> > +
> > +	/*
> > +	 * TODO: Support to enable KFENCE after bootup depends on the ability to
> > +	 *       split page table mappings. As such support is not currently
> > +	 *       implemented for radix pagetables, support enabling KFENCE
> > +	 *       only at system startup for now.
> > +	 *
> > +	 *       After support for splitting mappings is available on radix,
> > +	 *       alloc_kfence_pool() & map_kfence_pool() can be dropped and
> > +	 *       mapping for __kfence_pool memory can be
> > +	 *       split during arch_kfence_init_pool().
> > +	 */
> > +	if (!kfence_early_init)
> > +		goto no_kfence;
> > +
> > +	kfence_pool = memblock_phys_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
> > +	if (!kfence_pool)
> > +		goto no_kfence;
> > +
> > +	memblock_mark_nomap(kfence_pool, KFENCE_POOL_SIZE);
> > +	return kfence_pool;
> > +
> > +no_kfence:
> > +	disable_kfence();
> > +	return 0;
> > +}
> > +
> > +static inline void map_kfence_pool(phys_addr_t kfence_pool)
> > +{
> > +	if (!kfence_pool)
> > +		return;
> > +
> > +	if (create_physical_mapping(kfence_pool, kfence_pool + KFENCE_POOL_SIZE,
> > +				    -1, PAGE_KERNEL, PAGE_SIZE))
> > +		goto err;
> > +
> > +	memblock_clear_nomap(kfence_pool, KFENCE_POOL_SIZE);
> > +	__kfence_pool = __va(kfence_pool);
> > +	return;
> > +
> > +err:
> > +	memblock_phys_free(kfence_pool, KFENCE_POOL_SIZE);
> > +	disable_kfence();
> > +}
> > +#else
> > +static inline phys_addr_t alloc_kfence_pool(void) { return 0; }
> > +static inline void map_kfence_pool(phys_addr_t kfence_pool) { }
> > +#endif
> > +
> >  static void __init radix_init_pgtable(void)
> >  {
> > +	phys_addr_t kfence_pool;
> >  	unsigned long rts_field;
> >  	phys_addr_t start, end;
> >  	u64 i;
> > @@ -365,6 +437,8 @@ static void __init radix_init_pgtable(void)
> >  	/* We don't support slb for radix */
> >  	slb_set_size(0);
> >  
> > +	kfence_pool = alloc_kfence_pool();
> > +
> >  	/*
> >  	 * Create the linear mapping
> >  	 */
> > @@ -381,9 +455,11 @@ static void __init radix_init_pgtable(void)
> >  		}
> >  
> >  		WARN_ON(create_physical_mapping(start, end,
> > -						-1, PAGE_KERNEL));
> > +						-1, PAGE_KERNEL, ~0UL));
> >  	}
> >  
> > +	map_kfence_pool(kfence_pool);
> > +
> >  	if (!cpu_has_feature(CPU_FTR_HVMODE) &&
> >  			cpu_has_feature(CPU_FTR_P9_RADIX_PREFETCH_BUG)) {
> >  		/*
> > @@ -875,7 +951,7 @@ int __meminit radix__create_section_mapping(unsigned long start,
> >  	}
> >  
> >  	return create_physical_mapping(__pa(start), __pa(end),
> > -				       nid, prot);
> > +				       nid, prot, ~0UL);
> >  }
> >  
> >  int __meminit radix__remove_section_mapping(unsigned long start, unsigned long end)
> > diff --git a/arch/powerpc/mm/init-common.c b/arch/powerpc/mm/init-common.c
> > index d3a7726ecf512c..21131b96d20901 100644
> > --- a/arch/powerpc/mm/init-common.c
> > +++ b/arch/powerpc/mm/init-common.c
> > @@ -31,6 +31,9 @@ EXPORT_SYMBOL_GPL(kernstart_virt_addr);
> >  
> >  bool disable_kuep = !IS_ENABLED(CONFIG_PPC_KUEP);
> >  bool disable_kuap = !IS_ENABLED(CONFIG_PPC_KUAP);
> > +#ifdef CONFIG_KFENCE
> > +bool __ro_after_init kfence_disabled;
> > +#endif
> >  
> >  static int __init parse_nosmep(char *p)
> >  {
> 
> 
> Hi,
> 
> Just a gentle reminder, this patch is required in the stable kernels.
> 
> Please let me know if there are any comments.
> 
> Thanks,
> Aboorva



CC'ing Greg to check whether this patch can be backported to the stable
kernels.


Regards,
Aboorva