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

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Feb 18, 2026 at 09:31:13AM +0000, Alice Ryhl wrote:

> > 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.
> 
> No I don't think RELOC_HIDE is what you want to be using here.
> 
> The way to stop the compiler from doing this is to ensure that, in
> LLVM's eyes, the memcpy is either a volatile memcpy, an atomic memcpy,
> or an opaque function call. According to Gary's reply to my email on V3,
> it sounds like an explicit call to memcpy like this apparently falls into
> opaque function call, so it should be okay.

Ah, so we should write:

 	struct foo val, *ptr;
 
 	ptr = kmap_local_page(page);
 	__memcpy(ptr, val, sizeof(val));
 	kunmap_local(ptr);
 
 	if (val.field) {
 		...
 	}

That forces things to be a function call since it elides the C memcpy
intrinsic for not having the right name.

But that also elides KASAN and such that want to hijack the various mem
functions for instrumentation purposes.