[PATCH] lib: fwts_alloc: fix low 32 bit memory allocator (LP: #1295849)

[Colin King]

From: Colin Ian King <colin.king at canonical.com>

The original mmap allocator made x86 specific assumptions about
memory mappings.  Re-work the low memory allocator to try to find
a free mapping under the 2GB 32 bit threshold by always examining
the first mapping for the procss and either using this if it is below
2GB or chosing 2GB less the mapping size.  This is also more efficient
too.

Signed-off-by: Colin Ian King <colin.king at canonical.com>
---
 src/lib/src/fwts_alloc.c | 40 +++++++++++++++++++++++++++++++++++++---
 1 file changed, 37 insertions(+), 3 deletions(-)

diff --git a/src/lib/src/fwts_alloc.c b/src/lib/src/fwts_alloc.c
index 57ece72..9610159 100644
--- a/src/lib/src/fwts_alloc.c
+++ b/src/lib/src/fwts_alloc.c
@@ -46,7 +46,9 @@ typedef struct {
 	unsigned int magic;
 } fwts_mmap_header;
 
-#define LIMIT_2GB	0x80000000ULL
+#define CHUNK_SIZE	(8192)		/* page plus loads of slack */
+#define LIMIT_2GB	(0x80000000ULL)
+#define LIMIT_START	(0x00010000ULL)
 
 #ifndef MAP_32BIT
 /*
@@ -62,6 +64,7 @@ static void *fwts_low_mmap(const size_t requested_size)
 	void *addr_start;
 	void *addr_end;
 	void *last_addr_end = NULL;
+	void *first_addr_start = NULL;
 	void *ret = MAP_FAILED;
 
 	if (requested_size == 0)	/* Illegal */
@@ -74,6 +77,35 @@ static void *fwts_low_mmap(const size_t requested_size)
 		sscanf(buffer, "%p-%p %*s %*x %*s %*u %s",
 			&addr_start, &addr_end, pathname);
 
+		/*
+		 *  Try and allocate under first mmap'd address space
+		 */
+		if ((first_addr_start == NULL) &&
+		    (addr_start > (void*)LIMIT_START)) {
+			size_t sz = (requested_size + CHUNK_SIZE) & ~(CHUNK_SIZE - 1);
+			void *addr = addr_start - sz;
+
+			/*
+			 * If addr is over the 2GB limit and we know
+			 * that this is the first mapping then we should
+			 * be able to map a region below the 2GB limit as
+			 * nothing is already mapped there
+			 */
+			if (addr > (void*)LIMIT_2GB)
+				addr = (void*)LIMIT_2GB - sz;
+
+			ret = mmap(addr, requested_size, PROT_READ | PROT_WRITE,
+				MAP_PRIVATE | MAP_FIXED | MAP_ANONYMOUS, -1, 0);
+			if (ret != MAP_FAILED)
+				break;	/* Success! */
+
+			first_addr_start = addr_start;
+		}
+
+		/*
+		 *  No allocation yet, so try now to squeeze one
+		 *  in between already mapped regions
+		 */
 		if ((last_addr_end != NULL) &&
 		    (last_addr_end < (void*)LIMIT_2GB)) {
 			if ((addr_start - last_addr_end) > (ptrdiff_t)requested_size) {
@@ -85,9 +117,11 @@ static void *fwts_low_mmap(const size_t requested_size)
 			}
 		}
 
-		/* We really don't want to mmap at the end of the heap
+		/*
+		 * We really don't want to mmap at the end of the heap
 		 * and if we've reached the stack we've gone too far so
-		 * abort */
+		 * abort
+		 */
 		if ((strncmp("[heap]", pathname, 6) == 0) ||
 		    (strncmp("[stack]", pathname, 7) == 0)) {
 			ret = MAP_FAILED;
-- 
1.9.1