Re: [PATCH v14 054/138] mm: Add kmap_local_folio()

Matthew Wilcox
2021-07-21T14:12:14Z

On Wed, Jul 21, 2021 at 12:58:24PM +0300, Mike Rapoport wrote:
> > +/**
> > + * kmap_local_folio - Map a page in this folio for temporary usage
> > + * @folio: The folio to be mapped.
> > + * @offset: The byte offset within the folio.
> > + *
> > + * Returns: The virtual address of the mapping
> > + *
> > + * Can be invoked from any context.
>
> Context: Can be invoked from any context.
>
> > + *
> > + * Requires careful handling when nesting multiple mappings because the map
> > + * management is stack based. The unmap has to be in the reverse order of
> > + * the map operation:
> > + *
> > + * addr1 = kmap_local_folio(page1, offset1);
> > + * addr2 = kmap_local_folio(page2, offset2);
>
> Please s/page/folio/g here and in the description below
>
> > + * ...
> > + * kunmap_local(addr2);
> > + * kunmap_local(addr1);
> > + *
> > + * Unmapping addr1 before addr2 is invalid and causes malfunction.
> > + *
> > + * Contrary to kmap() mappings the mapping is only valid in the context of
> > + * the caller and cannot be handed to other contexts.
> > + *
> > + * On CONFIG_HIGHMEM=n kernels and for low memory pages this returns the
> > + * virtual address of the direct mapping. Only real highmem pages are
> > + * temporarily mapped.
> > + *
> > + * While it is significantly faster than kmap() for the higmem case it
> > + * comes with restrictions about the pointer validity. Only use when really
> > + * necessary.
> > + *
> > + * On HIGHMEM enabled systems mapping a highmem page has the side effect of
> > + * disabling migration in order to keep the virtual address stable across
> > + * preemption. No caller of kmap_local_folio() can rely on this side effect.
> > + */

kmap_local_folio() only maps one page from the folio. So it's not
appropriate to s/page/folio/g. I fiddled with the description a bit to
make this clearer:

/**
* kmap_local_folio - Map a page in this folio for temporary usage
- * @folio: The folio to be mapped.
- * @offset: The byte offset within the folio.
- *
- * Returns: The virtual address of the mapping
- *
- * Can be invoked from any context.
+ * @folio: The folio containing the page.
+ * @offset: The byte offset within the folio which identifies the page.
*
* Requires careful handling when nesting multiple mappings because the map
* management is stack based. The unmap has to be in the reverse order of
* the map operation:
*
- * addr1 = kmap_local_folio(page1, offset1);
- * addr2 = kmap_local_folio(page2, offset2);
+ * addr1 = kmap_local_folio(folio1, offset1);
+ * addr2 = kmap_local_folio(folio2, offset2);
* ...
* kunmap_local(addr2);
* kunmap_local(addr1);
@@ -131,6 +127,9 @@ static inline void *kmap_local_page(struct page *page);
* On HIGHMEM enabled systems mapping a highmem page has the side effect of
* disabling migration in order to keep the virtual address stable across
* preemption. No caller of kmap_local_folio() can rely on this side effect.
+ *
+ * Context: Can be invoked from any context.
+ * Return: The virtual address of @offset.
*/
static inline void *kmap_local_folio(struct folio *folio, size_t offset);