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

[Peter Zijlstra <peterz@infradead.org>]

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.

Anyway, I don't think something like the below is an unreasonable patch.

It ensures all accesses to the ptr obtained from kmap_local_*() and
released by kunmap_local() stays inside those two.

---
diff --git a/include/linux/highmem-internal.h b/include/linux/highmem-internal.h
index 0574c21ca45d..2fe71b715a46 100644
--- a/include/linux/highmem-internal.h
+++ b/include/linux/highmem-internal.h
@@ -185,31 +185,42 @@ static inline void kunmap(const struct page *page)
 
 static inline void *kmap_local_page(const struct page *page)
 {
-	return page_address(page);
+	void *addr = page_address(page);
+	barrier();
+	return addr;
 }
 
 static inline void *kmap_local_page_try_from_panic(const struct page *page)
 {
-	return page_address(page);
+	void *addr = page_address(page);
+	barrier();
+	return addr;
 }
 
 static inline void *kmap_local_folio(const struct folio *folio, size_t offset)
 {
-	return folio_address(folio) + offset;
+	void *addr = folio_address(folio) + offset;
+	barrier();
+	return addr;
 }
 
 static inline void *kmap_local_page_prot(const struct page *page, pgprot_t prot)
 {
-	return kmap_local_page(page);
+	void *addr = kmap_local_page(page);
+	barrier();
+	return addr;
 }
 
 static inline void *kmap_local_pfn(unsigned long pfn)
 {
-	return kmap_local_page(pfn_to_page(pfn));
+	void *addr = kmap_local_page(pfn_to_page(pfn));
+	barrier();
+	return addr;
 }
 
 static inline void __kunmap_local(const void *addr)
 {
+	barrier();
 #ifdef ARCH_HAS_FLUSH_ON_KUNMAP
 	kunmap_flush_on_unmap(PTR_ALIGN_DOWN(addr, PAGE_SIZE));
 #endif