linux-kernel - [PATCH] x86/mm: Check return value from memblock_phys_alloc

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Message-ID: <94b3e98f-96a7-3560-1f76-349eb95ccf7f@rarity.fan>
Date: Fri, 15 Nov 2024 20:36:59 +0300
From: gldrk <me@...ity.fan>
To: Dave Hansen <dave.hansen@...ux.intel.com>, 
    Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
    x86@...nel.org
cc: linux-kernel@...r.kernel.org
Subject: [PATCH] x86/mm: Check return value from
 memblock_phys_alloc_range()

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