efi_loader: Readd freed pages to memory pool

Currently each allocation creates a new mapping. Readding the mapping
as free memory (EFI_CONVENTIONAL_MEMORY) potentially allows to hand out
an existing mapping, thus limiting the number of mapping descriptors in
the memory map.

Mitigates a problem with current (4.8rc7) linux kernels when doing an
efi_get_memory map, resulting in an infinite loop. Space for the memory
map is reserved with allocate_pool (implicitly creating a new mapping) and
filled. If there is insufficient slack space (8 entries) in the map, the
space is freed and a new round is started, with space for one more entry.
As each round increases requirement and allocation by exactly one, there
is never enough slack space. (At least 32 entries are allocated, so as
long as there are less than 24 entries, there is enough slack).
Earlier kernels reserved no slack, and did less allocations, so this
problem was not visible.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>

include/efi_loader.h

@@ -117,7 +117,7 @@ void *efi_alloc(uint64_t len, int memory_type);
 /* More specific EFI memory allocator, called by EFI payloads */
 efi_status_t efi_allocate_pages(int type, int memory_type, unsigned long pages,
 				uint64_t *memory);
-/* EFI memory free function. Not implemented today */
+/* EFI memory free function. */
 efi_status_t efi_free_pages(uint64_t memory, unsigned long pages);
 /* EFI memory allocator for small allocations */
 efi_status_t efi_allocate_pool(int pool_type, unsigned long size,

lib/efi_loader/efi_memory.c

@@ -336,8 +336,15 @@ void *efi_alloc(uint64_t len, int memory_type)
 
 efi_status_t efi_free_pages(uint64_t memory, unsigned long pages)
 {
-	/* We don't free, let's cross our fingers we have plenty RAM */
+	uint64_t r = 0;
+
+	r = efi_add_memory_map(memory, pages, EFI_CONVENTIONAL_MEMORY, false);
+	/* Merging of adjacent free regions is missing */
+
+	if (r == memory)
 		return EFI_SUCCESS;
+
+	return EFI_NOT_FOUND;
 }
 
 efi_status_t efi_allocate_pool(int pool_type, unsigned long size,