Re: [Patch V6 12/16] mm: provide early_memremap_ro to establish read-only mapping

[Vlastimil Babka]

On 07/17/2015 06:51 AM, Juergen Gross wrote:
> During early boot as Xen pv domain the kernel needs to map some page
> tables supplied by the hypervisor read only. This is needed to be
> able to relocate some data structures conflicting with the physical
> memory map especially on systems with huge RAM (above 512GB).
>
> Provide the function early_memremap_ro() to provide this read only
> mapping.
>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> Acked-by: Konrad Rzeszutek Wilk <Konrad.wilk@xxxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-arch@xxxxxxxxxxxxxxx
> ---
>   include/asm-generic/early_ioremap.h |  2 ++
>   include/asm-generic/fixmap.h        |  3 +++
>   mm/early_ioremap.c                  | 12 ++++++++++++
>   3 files changed, 17 insertions(+)
>
> diff --git a/include/asm-generic/early_ioremap.h b/include/asm-generic/early_ioremap.h
> index a5de55c..316bd04 100644
> --- a/include/asm-generic/early_ioremap.h
> +++ b/include/asm-generic/early_ioremap.h
> @@ -11,6 +11,8 @@ extern void __iomem *early_ioremap(resource_size_t phys_addr,
>   				   unsigned long size);
>   extern void *early_memremap(resource_size_t phys_addr,
>   			    unsigned long size);
> +extern void *early_memremap_ro(resource_size_t phys_addr,
> +			       unsigned long size);

So the function is declared unconditionally...

>   extern void early_iounmap(void __iomem *addr, unsigned long size);
>   extern void early_memunmap(void *addr, unsigned long size);
>
> diff --git a/include/asm-generic/fixmap.h b/include/asm-generic/fixmap.h
> index f23174f..1cbb833 100644
> --- a/include/asm-generic/fixmap.h
> +++ b/include/asm-generic/fixmap.h
> @@ -46,6 +46,9 @@ static inline unsigned long virt_to_fix(const unsigned long vaddr)
>   #ifndef FIXMAP_PAGE_NORMAL
>   #define FIXMAP_PAGE_NORMAL PAGE_KERNEL
>   #endif
> +#if !defined(FIXMAP_PAGE_RO) && defined(PAGE_KERNEL_RO)
> +#define FIXMAP_PAGE_RO PAGE_KERNEL_RO
> +#endif
>   #ifndef FIXMAP_PAGE_NOCACHE
>   #define FIXMAP_PAGE_NOCACHE PAGE_KERNEL_NOCACHE
>   #endif
> diff --git a/mm/early_ioremap.c b/mm/early_ioremap.c
> index e10ccd2..0cfadaf 100644
> --- a/mm/early_ioremap.c
> +++ b/mm/early_ioremap.c
> @@ -217,6 +217,13 @@ early_memremap(resource_size_t phys_addr, unsigned long size)
>   	return (__force void *)__early_ioremap(phys_addr, size,
>   					       FIXMAP_PAGE_NORMAL);
>   }
> +#ifdef FIXMAP_PAGE_RO
> +void __init *
> +early_memremap_ro(resource_size_t phys_addr, unsigned long size)
> +{
> +	return (__force void *)__early_ioremap(phys_addr, size, FIXMAP_PAGE_RO);
> +}
> +#endif

... here we provide a implementation when both CONFIG_MMU and 
FIXMAP_PAGE_RO are defined...

>   #else /* CONFIG_MMU */
>
>   void __init __iomem *
> @@ -231,6 +238,11 @@ early_memremap(resource_size_t phys_addr, unsigned long size)
>   {
>   	return (void *)phys_addr;
>   }
> +void __init *
> +early_memremap_ro(resource_size_t phys_addr, unsigned long size)
> +{
> +	return (void *)phys_addr;
> +}

... and here for !CONFIG_MMU.

So, what about CONFIG_MMU && !FIXMAP_PAGE_RO combinations? Which 
translates to CONFIG_MMU && !PAGE_KERNEL_RO. Maybe they don't exist, but 
then it's still awkward to see the combination in the code left 
unimplemented.

Would it be perhaps simpler to assume the same thing as in
drivers/base/firmware_class.c ?

/* Some architectures don't have PAGE_KERNEL_RO */
#ifndef PAGE_KERNEL_RO
#define PAGE_KERNEL_RO PAGE_KERNEL
#endif

Or would it be dangerous here to silently lose the read-only protection?

>   void __init early_iounmap(void __iomem *addr, unsigned long size)
>   {

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