Luis Chamberlain [off-list ref] writes:

On Mon, Aug 26, 2024 at 02:10:49PM -0700, Darrick J. Wong wrote:

quoted

On Mon, Aug 26, 2024 at 01:52:54PM -0700, Luis Chamberlain wrote:

quoted

On Mon, Aug 26, 2024 at 07:43:20PM +0200, Christophe Leroy wrote:

quoted

Le 26/08/2024 à 17:48, Pankaj Raghav (Samsung) a écrit :

quoted

On Mon, Aug 26, 2024 at 05:59:31PM +1000, Stephen Rothwell wrote:

quoted

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?

quoted

Just drop it, then.

OK sent a patch for that.

Thanks for fixing it!

-ritesh