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

[Gary Guo <gary@garyguo.net>]

On 2026-02-17 15:48, Peter Zijlstra wrote:
> On Tue, Feb 17, 2026 at 01:09:39PM +0000, Alice Ryhl wrote:
>> 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
>> 	}
> 
> 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.

>> 	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
>> 	}

Best,
Gary