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

Matthew Wilcox (Oracle)
2021-07-15T04:21:49Z

This allows us to map a portion of a folio. Callers can only expect
to access up to the next page boundary.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
include/linux/highmem-internal.h | 11 +++++++++
include/linux/highmem.h | 38 ++++++++++++++++++++++++++++++++
2 files changed, 49 insertions(+)

diff --git a/include/linux/highmem-internal.h b/include/linux/highmem-internal.h
index 7902c7d8b55f..d5d6f930ae1d 100644
--- a/include/linux/highmem-internal.h
+++ b/include/linux/highmem-internal.h
@@ -73,6 +73,12 @@ static inline void *kmap_local_page(struct page *page)
return __kmap_local_page_prot(page, kmap_prot);
}

+static inline void *kmap_local_folio(struct folio *folio, size_t offset)
+{
+ struct page *page = folio_page(folio, offset / PAGE_SIZE);
+ return __kmap_local_page_prot(page, kmap_prot) + offset % PAGE_SIZE;
+}
+
static inline void *kmap_local_page_prot(struct page *page, pgprot_t prot)
{
return __kmap_local_page_prot(page, prot);
@@ -160,6 +166,11 @@ static inline void *kmap_local_page(struct page *page)
return page_address(page);
}

+static inline void *kmap_local_folio(struct folio *folio, size_t offset)
+{
+ return page_address(&folio->page) + offset;
+}
+
static inline void *kmap_local_page_prot(struct page *page, pgprot_t prot)
{
return kmap_local_page(page);
diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 8c6e8e996c87..85de3bd0b47d 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -96,6 +96,44 @@ static inline void kmap_flush_unused(void);
*/
static inline void *kmap_local_page(struct page *page);

+/**
+ * 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.
+ *
+ * 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);
+ * ...
+ * 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.
+ */
+static inline void *kmap_local_folio(struct folio *folio, size_t offset);
+
/**
* kmap_atomic - Atomically map a page for temporary usage - Deprecated!
* @page: Pointer to the page to be mapped
--
2.30.2