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

From: Ritesh Harjani (IBM) <[email-masked]>
Date: Tue, 27 Aug 2024 10:56:12 +0530

Luis Chamberlain <[email-masked]> writes:

> On Mon, Aug 26, 2024 at 02:10:49PM -0700, Darrick J. Wong wrote:
>> On Mon, Aug 26, 2024 at 01:52:54PM -0700, Luis Chamberlain wrote:
>> > On Mon, Aug 26, 2024 at 07:43:20PM +0200, Christophe Leroy wrote:
>> > > 
>> > > 
>> > > 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
>> > 
>> > Our users expect contiguous memory [0] and so we use alloc_pages() here,
>> > so if we're architecture limitted by this I'd rather we just remove the
>> > set_memory_ro() only for PPC, I don't see why other have to skip this.

Looks like not a standard thing to do for kernel linear memory map
region then and maybe few other archs could be ignoring too?

>> 
>> Just drop it, then.
>
> OK sent a patch for that.
>

Thanks for fixing it!

-ritesh