Re: linux-next: boot warning after merge of the vfs-brauner tree

[Christophe Leroy]

Le 26/08/2024 à 17:48, Pankaj Raghav (Samsung) a écrit :
> On Mon, Aug 26, 2024 at 05:59:31PM +1000, Stephen Rothwell wrote:
> > Hi all,
> >
> > After merging the vfs-brauner tree, today's linux-next boot test (powerpc
> > pseries_le_defconfig) produced this warning:
>
> iomap dio calls set_memory_ro() on the page that is used for sub block
> zeroing.
>
> But looking at powerpc code, they don't support set_memory_ro() for
> memory region that belongs to the kernel(LINEAR_MAP_REGION_ID).
>
> /*
>   * On hash, the linear mapping is not in the Linux page table so
>   * apply_to_existing_page_range() will have no effect. If in the future
>   * the set_memory_* functions are used on the linear map this will need
>   * to be updated.
>   */
> if (!radix_enabled()) {
>          int region = get_region_id(addr);
>
>          if (WARN_ON_ONCE(region != VMALLOC_REGION_ID && region != IO_REGION_ID))
>                  return -EINVAL;
> }
>
> We call set_memory_ro() on the zero page as a extra security measure.
> I don't know much about powerpc, but looking at the comment, is it just
> adding the following to support it in powerpc:
>
> diff --git a/arch/powerpc/mm/pageattr.c b/arch/powerpc/mm/pageattr.c
> index ac22bf28086fa..e6e0b40ba6db4 100644
> --- a/arch/powerpc/mm/pageattr.c
> +++ b/arch/powerpc/mm/pageattr.c
> @@ -94,7 +94,9 @@ int change_memory_attr(unsigned long addr, int numpages, long action)
>          if (!radix_enabled()) {
>                  int region = get_region_id(addr);
>   
> -               if (WARN_ON_ONCE(region != VMALLOC_REGION_ID && region != IO_REGION_ID))
> +               if (WARN_ON_ONCE(region != VMALLOC_REGION_ID &&
> +                                region != IO_REGION_ID &&
> +                                region != LINEAR_MAP_REGION_ID))
>                          return -EINVAL;
>          }
>   #endif

By doing this you will just hide the fact that it didn't work.

See commit 1f9ad21c3b38 ("powerpc/mm: Implement set_memory() routines") 
for details. The linear memory region is not mapped using page tables so 
set_memory_ro() will have no effect on it.

You can either use vmalloc'ed pages, or do a const static allocation at 
buildtime so that it will be allocated in the kernel static rodata area.

By the way, your code should check the value returned by 
set_memory_ro(), there is some work in progress to make it mandatory, 
see https://github.com/KSPP/linux/issues/7

Christophe

>   If it involves changing more things and this feature will be added to
>   powerpc in the future, we could drop the set_memory_ro() call from
>   iomap.
>
>   CC: Darrick(as he suggested set_memory_ro() on zero page), Leroy,
>   Ritesh, ppc list