[PATCH] mm: make 'highmem' symbol ro_after_init

[PATCH] mm: make 'highmem' symbol ro_after_init

[Ard Biesheuvel]

The 'highmem' variable is only set at boot, so we can make it
ro_after_init and prevent it from being corrupted inadvertently, or from
ending up in a contended cacheline.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 mm/memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index c125c4969913..50e82cb94ccc 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -106,7 +106,7 @@ EXPORT_SYMBOL(mem_map);
  * highstart_pfn must be the same; there must be no gap between ZONE_NORMAL
  * and ZONE_HIGHMEM.
  */
-void *high_memory;
+void *high_memory __ro_after_init;
 EXPORT_SYMBOL(high_memory);
 
 /*
-- 
2.30.2

Re: [PATCH] mm: make 'highmem' symbol ro_after_init

[David Rientjes]

On Mon, 24 Jan 2022, Ard Biesheuvel wrote:

> diff --git a/mm/memory.c b/mm/memory.c
> index c125c4969913..50e82cb94ccc 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -106,7 +106,7 @@ EXPORT_SYMBOL(mem_map);
>   * highstart_pfn must be the same; there must be no gap between ZONE_NORMAL
>   * and ZONE_HIGHMEM.
>   */
> -void *high_memory;
> +void *high_memory __ro_after_init;
>  EXPORT_SYMBOL(high_memory);
>  
>  /*

Same should be true for the mm/nommu.c variant?

Re: [PATCH] mm: make 'highmem' symbol ro_after_init

[Matthew Wilcox]

On Mon, Jan 24, 2022 at 06:05:55PM +0100, Ard Biesheuvel wrote:
> The 'highmem' variable is only set at boot, so we can make it
> ro_after_init and prevent it from being corrupted inadvertently, or from
> ending up in a contended cacheline.

I'm not against this patch, but it'd be nice to go further and remove
it entirely for !CONFIG_HIGHMEM builds?  Adding something like

static inline bool is_high_pfn(unsigned long pfn)
{
#ifdef CONFIG_HIGHMEM
	return pfn <= max_low_pfn;
#else
	return false;
#endif
}

static inline bool is_high_phys(phys_addr_t pa)
{
	return is_high_pfn(pa / PAGE_SIZE);
}

static inline bool is_high_addr(void *addr)
{
	return is_high_phys(virt_to_phys(addr));
}

should be all the primitives you need, from a quick grep.

> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  mm/memory.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index c125c4969913..50e82cb94ccc 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -106,7 +106,7 @@ EXPORT_SYMBOL(mem_map);
>   * highstart_pfn must be the same; there must be no gap between ZONE_NORMAL
>   * and ZONE_HIGHMEM.
>   */
> -void *high_memory;
> +void *high_memory __ro_after_init;
>  EXPORT_SYMBOL(high_memory);
>  
>  /*
> -- 
> 2.30.2
> 
> 

Re: [PATCH] mm: make 'highmem' symbol ro_after_init

[Mike Rapoport]

On Mon, Jan 24, 2022 at 06:05:55PM +0100, Ard Biesheuvel wrote:
> The 'highmem' variable is only set at boot, so we can make it
> ro_after_init and prevent it from being corrupted inadvertently, or from
> ending up in a contended cacheline.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Acked-by: Mike Rapoport <rppt@linux.ibm.com>

> ---
>  mm/memory.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index c125c4969913..50e82cb94ccc 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -106,7 +106,7 @@ EXPORT_SYMBOL(mem_map);
>   * highstart_pfn must be the same; there must be no gap between ZONE_NORMAL
>   * and ZONE_HIGHMEM.
>   */
> -void *high_memory;
> +void *high_memory __ro_after_init;
>  EXPORT_SYMBOL(high_memory);
>  
>  /*
> -- 
> 2.30.2
> 
> 

-- 
Sincerely yours,
Mike.

Re: [PATCH] mm: make 'highmem' symbol ro_after_init

[Ard Biesheuvel]

On Wed, 26 Jan 2022 at 22:38, Mike Rapoport <rppt@kernel.org> wrote:
>
> On Mon, Jan 24, 2022 at 06:05:55PM +0100, Ard Biesheuvel wrote:
> > The 'highmem' variable is only set at boot, so we can make it
> > ro_after_init and prevent it from being corrupted inadvertently, or from
> > ending up in a contended cacheline.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
>
> Acked-by: Mike Rapoport <rppt@linux.ibm.com>
>
> > ---
> >  mm/memory.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index c125c4969913..50e82cb94ccc 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -106,7 +106,7 @@ EXPORT_SYMBOL(mem_map);
> >   * highstart_pfn must be the same; there must be no gap between ZONE_NORMAL
> >   * and ZONE_HIGHMEM.
> >   */
> > -void *high_memory;
> > +void *high_memory __ro_after_init;
> >  EXPORT_SYMBOL(high_memory);
> >
> >  /*
> > --
> > 2.30.2
> >
> >
>


This turns out to break CONFIG_MEMORY_HOTPLUG on x86, as it updates
max_pfn, max_low_pfn and high_memory after init.

So probably best to make all of those __read_mostly instead.