"Pankaj Raghav (Samsung)" [off-list ref] writes:

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 should probably just turn that into a printk(), WARN is kind of heavy handed.

We call set_memory_ro() on the zero page as a extra security measure.

 
Or a data integrity measure. But either way it makes sense.

On architectures that do implement set_memory_ro() it potentially breaks
the linear mapping into small pages, which could have a performance impact.

cheers