Re: [PATCH v2] rust: page: add byte-wise atomic memory copy methods

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Feb 17, 2026 at 01:09:20PM +0100, Peter Zijlstra wrote:
> On Tue, Feb 17, 2026 at 11:51:20AM +0000, Alice Ryhl wrote:
> 
> > In my experience with dealing with `struct page` that is mapped into a
> > vma, you need memcpy because the struct might be split across two
> > different pages in the vma. The pages are adjacent in userspace's
> > address space, but not necessarily adjacent from the kernel's POV.
> > 
> > So you might end up with something that looks like this:
> > 
> > struct foo val;
> > void *ptr1 = kmap_local_page(p1);
> > void *ptr2 = kmap_local_page(p2);
> > memcpy(ptr1 + offset, val, PAGE_SIZE - offset);
> > memcpy(ptr2, val + offset, sizeof(struct foo) - (PAGE_SIZE - offset));
> > kunmap_local(ptr2);
> > kunmap_local(ptr1);
> 
>   barrier();
> 
> > if (is_valid(&val)) {
> >     // use val
> > }
> > 
> > This exact thing happens in Binder. It has to be a memcpy.
> 
> Sure, but then stick that one barrier() in and you're good.

Are we really good? Consider this code:

	bool is_valid(struct foo *val)
	{
		// for the sake of example
		return val->my_field != 0;
	}

	struct foo val;

	void *ptr = kmap_local_page(p1);
	memcpy(ptr, val, sizeof(struct foo));
	kunmap_local(p);
	barrier();
	if (is_valid(&val)) {
	    // use val
	}

optimize it into this first:

	struct foo val;
	int my_field_copy;

	void *ptr = kmap_local_page(p1);
	memcpy(ptr, val, sizeof(struct foo));
	my_field_copy = val->my_field;
	kunmap_local(p);
	barrier();
	if (my_field_copy != 0) {
	    // use val
	}

then optimize it into:

	struct foo val;
	int my_field_copy;

	void *ptr = kmap_local_page(p1);
	memcpy(ptr, val, sizeof(struct foo));
	my_field_copy = ((struct foo *) ptr)->my_field;
	kunmap_local(p);
	barrier();
	if (my_field_copy != 0) {
	    // use val
	}

oops!