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

Luis Chamberlain
2024-08-26T20:52:58Z

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.

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