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 :

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.

diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index c02b266bba52..aba5cde89e14 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c

@@ -775,14 +775,22 @@ EXPORT_SYMBOL_GPL(iomap_dio_rw);
 
 static int __init iomap_dio_init(void)
 {
+	int ret;
+
 	zero_page = alloc_pages(GFP_KERNEL | __GFP_ZERO,
 				IOMAP_ZERO_PAGE_ORDER);
 
 	if (!zero_page)
 		return -ENOMEM;
 
-	set_memory_ro((unsigned long)page_address(zero_page),
-		      1U << IOMAP_ZERO_PAGE_ORDER);
-	return 0;
+	if (IS_ENABLED(CONFIG_PPC))
+		return 0;
+
+	ret = set_memory_ro((unsigned long)page_address(zero_page),
+			    1U << IOMAP_ZERO_PAGE_ORDER);
+	if (ret)
+		free_pages((unsigned long) zero_page, IOMAP_ZERO_PAGE_ORDER);
+
+	return ret;
 }
 fs_initcall(iomap_dio_init);

Thoughts?

[0] https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git/commit/?h=vfs.blocksize&id=d940b3b7b76b409b0550fdf2de6dc2183f01526f

  Luis