[PATCH v2 2/2] mm: Add folio_map_local()

["Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>]

Some filesystems benefit from being able to map the entire folio.
On 32-bit platforms with HIGHMEM, we fall back to using vmap, which
will be slow.  If it proves to be a performance problem, we can look at
optimising it in a number of ways.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
 include/linux/highmem.h | 40 ++++++++++++++++++++++++++++++++++++++++
 include/linux/vmalloc.h |  6 ++++--
 mm/vmalloc.c            | 32 ++++++++++++++++++++++++++++++++
 3 files changed, 76 insertions(+), 2 deletions(-)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index e9912da5441b..d56ae62db252 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -10,6 +10,7 @@
 #include <linux/mm.h>
 #include <linux/uaccess.h>
 #include <linux/hardirq.h>
+#include <linux/vmalloc.h>
 
 #include "highmem-internal.h"
 
@@ -132,6 +133,45 @@ static inline void *kmap_local_page(struct page *page);
  */
 static inline void *kmap_local_folio(struct folio *folio, size_t offset);
 
+/**
+ * folio_map_local - Map an entire folio.
+ * @folio: The folio to map.
+ *
+ * Unlike kmap_local_folio(), map an entire folio.  This should be undone
+ * with folio_unmap_local().  The address returned should be treated as
+ * stack-based, and local to this CPU, like kmap_local_folio().
+ *
+ * Context: May allocate memory using GFP_KERNEL if it takes the vmap path.
+ * Return: A kernel virtual address which can be used to access the folio,
+ * or NULL if the mapping fails.
+ */
+static inline __must_check void *folio_map_local(struct folio *folio)
+{
+	might_alloc(GFP_KERNEL);
+
+	if (!IS_ENABLED(CONFIG_HIGHMEM))
+		return folio_address(folio);
+	if (folio_test_large(folio))
+		return vm_map_folio(folio);
+	return kmap_local_page(&folio->page);
+}
+
+/**
+ * folio_unmap_local - Unmap an entire folio.
+ * @addr: Address returned from folio_map_local()
+ *
+ * Undo the result of a previous call to folio_map_local().
+ */
+static inline void folio_unmap_local(const void *addr, unsigned long nr_pages)
+{
+	if (!IS_ENABLED(CONFIG_HIGHMEM))
+		return;
+	if (is_vmalloc_addr(addr))
+		vm_unmap_ram(addr, nr_pages);
+	else
+		kunmap_local(addr);
+}
+
 /**
  * kmap_atomic - Atomically map a page for temporary usage - Deprecated!
  * @page:	Pointer to the page to be mapped
diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index 096d48aa3437..4bb34c939c01 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -13,6 +13,7 @@
 #include <asm/vmalloc.h>
 
 struct vm_area_struct;		/* vma defining user mapping in mm_types.h */
+struct folio;			/* also mm_types.h */
 struct notifier_block;		/* in notifier.h */
 
 /* bits in flags of vmalloc's vm_struct below */
@@ -163,8 +164,9 @@ extern void *vcalloc(size_t n, size_t size) __alloc_size(1, 2);
 extern void vfree(const void *addr);
 extern void vfree_atomic(const void *addr);
 
-extern void *vmap(struct page **pages, unsigned int count,
-			unsigned long flags, pgprot_t prot);
+void *vmap(struct page **pages, unsigned int count, unsigned long flags,
+		pgprot_t prot);
+void *vm_map_folio(struct folio *folio);
 void *vmap_pfn(unsigned long *pfns, unsigned int count, pgprot_t prot);
 extern void vunmap(const void *addr);
 
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index dcab1d3cf185..c101b09d15d3 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2288,6 +2288,38 @@ void *vm_map_ram(struct page **pages, unsigned int count, int node)
 }
 EXPORT_SYMBOL(vm_map_ram);
 
+#ifdef CONFIG_HIGHMEM
+/**
+ * vm_map_folio() - Map an entire folio into virtually contiguous space.
+ * @folio: The folio to map.
+ *
+ * Maps all pages in @folio into contiguous kernel virtual space.  This
+ * function is only available in HIGHMEM builds; for !HIGHMEM, use
+ * folio_address().  The pages are mapped with PAGE_KERNEL permissions.
+ *
+ * Return: The address of the area or %NULL on failure
+ */
+void *vm_map_folio(struct folio *folio)
+{
+	size_t size = folio_size(folio);
+	void *mem = vmap_alloc(size, NUMA_NO_NODE);
+	unsigned long addr = (unsigned long)mem;
+
+	if (vmap_range_noflush(addr, addr + size,
+				folio_pfn(folio) << PAGE_SHIFT,
+				PAGE_KERNEL, folio_shift(folio))) {
+		vm_unmap_ram(mem, folio_nr_pages(folio));
+		return NULL;
+	}
+	flush_cache_vmap(addr, addr + size);
+
+	mem = kasan_unpoison_vmalloc(mem, size, KASAN_VMALLOC_PROT_NORMAL);
+
+	return mem;
+}
+EXPORT_SYMBOL(vm_map_folio);
+#endif
+
 static struct vm_struct *vmlist __initdata;
 
 static inline unsigned int vm_area_page_order(struct vm_struct *vm)
-- 
2.35.1