Re: [PATCH v6 3/3] x86/mm: add set_direct_map_ro_noflush()

[Dave Hansen]

On 7/30/26 02:06, Xueyuan Chen wrote:
> +int set_direct_map_ro_noflush(const void *addr, unsigned long nr_pages)
> +{
> +	unsigned long tempaddr = (unsigned long)addr;
> +	struct cpa_data cpa = {
> +		.vaddr = &tempaddr,
> +		.pgd = NULL,
> +		.numpages = nr_pages,
> +		.mask_set = __pgprot(0),
> +		.mask_clr = __pgprot(_PAGE_RW | _PAGE_DIRTY),
> +		.flags = CPA_NO_CHECK_ALIAS,
> +	};
> +
> +	return __change_page_attr_set_clr(&cpa, 1);
> +}

A couple of concerns here.

First, why the "_noflush"? Sure, the "this is a best effort hardening"
function argument can be made, so it doesn't need to be correct. But the
result is a function that's called once and also has some sharp corners
and relatively high potential for misuse. Let's just do the flush.

Second, I see that the other set_direct_map*() callers use
CPA_NO_CHECK_ALIAS. The reasoning behind it dates back to 2008 and I'm
not 100% sure what it is referring to. On one hand, it would be nice to
have all the set_direct_map*() callers be consistent. On the other hand,
there shouldn't *be* any aliases of a 2M page that came out of the page
allocator. We almost want a CPA_ASSERT_NO_ALIASES that goes out and
checks for aliases more than we want to ignore them. (Note: I don't
expect you to fix this, but a simple comment saying that no aliases are
expected would be nice)

Third, what's with the 'tempaddr'? Are you working around the 'const'?
Honestly, I'd rather have no const than have it and subvert it with
casting trickery.

Last:

int set_direct_map_invalid_noflush(struct page *page)
int set_direct_map_default_noflush(struct page *page)
int set_direct_map_valid_noflush(struct page *page, unsigned nr, ...
int set_direct_map_ro_noflush(const void *addr, unsigned long nr_pages)

Which one of these things is not like the other, despite being named
just like them?

Imagine the fun if someone did:

	set_direct_map_ro_noflush(page, 1);
and
	set_direct_map_default_noflush(page)