Commit 53c36de0 authored by Matthew Wilcox (Oracle)'s avatar Matthew Wilcox (Oracle)
Browse files

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: default avatarMatthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
parent 08b0b005

include/linux/highmem-internal.h

+11 −0
Original line number Diff line number Diff line
@@ -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);
@@ -171,6 +177,11 @@ 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);

include/linux/highmem.h

+37 −0
Original line number Diff line number Diff line
@@ -96,6 +96,43 @@ 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