[PATCH] x86/mm: Check return value from memblock_phys_alloc_range()

[gldrk]

At least with CONFIG_PHYSICAL_START=0x100000, if there is < 4 MiB of contiguous
free memory available at this point, the kernel will crash and burn because
memblock_phys_alloc_range returns 0 on failure, which leads memblock_phys_free
to throw the first 4 MiB of physical memory to the wolves.  At a minimum it
should fail gracefully with a meaningful diagnostic, but in fact everything
seems to work fine without the weird reserve allocation.

---
 arch/x86/mm/init.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index eb503f5..3696770 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -640,8 +640,13 @@ static void __init memory_map_top_down(unsigned long 
map_start,
 	 */
 	addr = memblock_phys_alloc_range(PMD_SIZE, PMD_SIZE, map_start,
 					 map_end);
-	memblock_phys_free(addr, PMD_SIZE);
-	real_end = addr + PMD_SIZE;
+	if (unlikely(addr < map_start)) {
+		pr_warn("Failed to release memory for alloc_low_pages()");
+		real_end = ALIGN_DOWN(map_end, PMD_SIZE);
+	} else {
+		memblock_phys_free(addr, PMD_SIZE);
+		real_end = addr + PMD_SIZE;
+	}

 	/* step_size need to be small so pgt_buf from BRK could cover it 
*/
 	step_size = PMD_SIZE;
--
2.34.0