[PATCH v3 1/2] mm/kmap: Add kmap_local_page_try_from_panic()

[Jocelyn Falempe]

kmap_local_page() can be unsafe to call from a panic handler, if
CONFIG_HIGHMEM is set, and the page is in the highmem zone.
So add kmap_local_page_try_from_panic() to handle this case.

Suggested-by: Simona Vetter <simona.vetter@xxxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Jocelyn Falempe <jfalempe@xxxxxxxxxx>
---

v3:
 * Add a comment in kmap_local_page_try_from_panic() (Thomas Gleixner)
 
 include/linux/highmem-internal.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/include/linux/highmem-internal.h b/include/linux/highmem-internal.h
index dd100e849f5e0..9a7683d79a4b1 100644
--- a/include/linux/highmem-internal.h
+++ b/include/linux/highmem-internal.h
@@ -73,6 +73,14 @@ static inline void *kmap_local_page(struct page *page)
 	return __kmap_local_page_prot(page, kmap_prot);
 }
 
+static inline void *kmap_local_page_try_from_panic(struct page *page)
+{
+	if (!PageHighMem(page))
+		return page_address(page);
+	/* If the page is in HighMem, it's not safe to kmap it.*/
+	return NULL;
+}
+
 static inline void *kmap_local_folio(struct folio *folio, size_t offset)
 {
 	struct page *page = folio_page(folio, offset / PAGE_SIZE);
@@ -180,6 +188,11 @@ static inline void *kmap_local_page(struct page *page)
 	return page_address(page);
 }
 
+static inline void *kmap_local_page_try_from_panic(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;
-- 
2.49.0