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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Feb 17, 2026 at 11:39:18PM +0000, Gary Guo wrote:

> >> 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
> >> 	}
> > 
> > I don;t think this is allowed. You're lifting the load over the
> > barrier(), that is invalid.
> 
> This is allowed. Compilers perform escape analysis and find out that
> "val" does not escape the function and therefore nothing can change "val".
> 
> A simple example to demonstrate this effect is that
> 
>     int x = 0;
>     x = 1;
>     barrier();
>     do_something(x);
> 
> is happily optimized into
> 
>     barrier();
>     do_something(1);
> 
> by both GCC and Clang. The fact that the local variable here is a struct and
> memcpy is used to assign the value here does not make a fundamental difference.
> 
> barrier() does nothing to local variables if pointers to them do not escape the
> local function.

So how do we stop the compiler from doing this? Because I'm thinking
there's quite a bit of code that would be broken if this were done.

Must we really go write things like:

	struct foo val, *ptr;

	ptr = kmap_local_page(page);
	memcpy(ptr, val, sizeof(val));
	kunmap_local(ptr);

	ptr = RELOC_HIDE(&val, 0);

	if (ptr->field) {
		...
	}

That seems 'unfortunate'. It basically means we must never use local
stack for copies or somesuch.