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authorMatthew Wilcox (Oracle)2020-12-30 10:21:39 -0500
committerMatthew Wilcox (Oracle)2021-10-18 07:49:38 -0400
commit53c36de0701f2fdda6b1d209dc89e2bad4a9c7b8 (patch)
treebb2ad9e09ae5c30285e16ea6914fd30b52464c25 /include/linux/highmem.h
parent08b0b0059bf1c2e8637f724cc7cc4d29b1e808de (diff)
mm: Add kmap_local_folio()
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> Acked-by: Vlastimil Babka <vbabka@suse.cz>
Diffstat (limited to 'include/linux/highmem.h')
-rw-r--r--include/linux/highmem.h37
1 files changed, 37 insertions, 0 deletions
diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index b4c49f9cc379..27cdd715c5f9 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -97,6 +97,43 @@ 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 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(folio1, offset1);
+ * addr2 = kmap_local_folio(folio2, 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.
+ *
+ * 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);
+
+/**
* kmap_atomic - Atomically map a page for temporary usage - Deprecated!
* @page: Pointer to the page to be mapped
*