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

On Wed, Jul 21, 2021 at 05:22:16PM +0300, Mike Rapoport wrote:

On Wed, Jul 21, 2021 at 03:12:03PM +0100, Matthew Wilcox wrote:

quoted

On Wed, Jul 21, 2021 at 12:58:24PM +0300, Mike Rapoport wrote:

quoted

quoted

+/**
+ * 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.

quoted

+ *
+ * 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

quoted

+ * ...
+ * 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)

This is clearer, thanks! 

Maybe just add page to Return: description:

* Return: The virtual address of page @offset.

No, it really does return the virtual address of @offset.  If you ask
for offset 0x1234 within a (sufficiently large) folio, it will map the
second page of that folio and return the address of the 0x234'th byte
within it.