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

[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

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

[Ingo Molnar]

* gldrk <me@rarity.fan> wrote:

> 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;
> +	}

Makes sense to fix this bug I suppose, but the usual error check 
pattern for memblock_phys_alloc_range() failure would not be 'addr < 
map_start' but !addr.

( If memblock_phys_alloc_range() succeeds but returns an address below 
  'map_start', that would be a different failure I guess. )

Also, no need to add the 'unlikely()' I suspect - this is early boot 
code, micro-performance of branching is immaterial.

Just curious: what type of system has < 4 MiB of contiguous free memory 
available in early boot? Or was it something intentionally constrained 
via qemu?

Thanks,

	Ingo

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

[gldrk]

> Just curious: what type of system has < 4 MiB of contiguous free memory
> available in early boot? Or was it something intentionally constrained
> via qemu?

Yes, I was able to boot a basic Alpine system off virtiofs with a few MiB
total and a stripped-down config.  I happen to have a memory-starved 486
machine that is technically "supported", but it's not running Linux just
yet.

Here's an updated patch.

Signed-off-by: Philip Redkin <me@rarity.fan>
---
  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..6c4ec4f 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 (!addr) {
+		pr_warn("Failed to release memory for alloc_low_pages()");
+		real_end = max(map_start, 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

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

[tip-bot2 for Philip Redkin]

The following commit has been merged into the x86/mm branch of tip:

Commit-ID:     fd5935f9c20431eeadd6993fd4d2672e3e17a6b8
Gitweb:        https://git.kernel.org/tip/fd5935f9c20431eeadd6993fd4d2672e3e17a6b8
Author:        Philip Redkin <me@rarity.fan>
AuthorDate:    Fri, 15 Nov 2024 20:36:59 +03:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Fri, 28 Feb 2025 18:02:10 +01:00

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.

Signed-off-by: Philip Redkin <me@rarity.fan>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/94b3e98f-96a7-3560-1f76-349eb95ccf7f@rarity.fan
---
 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 62aa4d6..bfa444a 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -645,8 +645,13 @@ 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 (!addr) {
+		pr_warn("Failed to release memory for alloc_low_pages()");
+		real_end = max(map_start, 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;

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

[tip-bot2 for Philip Redkin]

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     631ca8909fd5c62b9fda9edda93924311a78a9c4
Gitweb:        https://git.kernel.org/tip/631ca8909fd5c62b9fda9edda93924311a78a9c4
Author:        Philip Redkin <me@rarity.fan>
AuthorDate:    Fri, 15 Nov 2024 20:36:59 +03:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Wed, 19 Mar 2025 11:05:22 +01:00

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.

Signed-off-by: Philip Redkin <me@rarity.fan>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/94b3e98f-96a7-3560-1f76-349eb95ccf7f@rarity.fan
---
 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 62aa4d6..bfa444a 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -645,8 +645,13 @@ 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 (!addr) {
+		pr_warn("Failed to release memory for alloc_low_pages()");
+		real_end = max(map_start, 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;