[PATCH v2 RESEND 0/3] mm: memblock: fix debugfs flag reporting and synchronization

[PATCH v2 RESEND 0/3] mm: memblock: fix debugfs flag reporting and synchronization

[Meijing Zhao]

From: Meijing Zhao <zhaomeijing@lixiang.com>

The memblock debugfs interface has two issues when reporting region
flags. MEMBLOCK_RSRV_HUGETLB has no corresponding name, and regions
with multiple flags only show the lowest set bit.

In addition, memblock_debug_show() walks the region arrays without
synchronizing against memory hotplug. If memory hotplug grows an array,
the debugfs reader may continue accessing the old array after it has
been freed.

Add the missing HugeTLB flag name, report every set bit including those
beyond flagname[], and hold the memory hotplug read lock while walking
the arrays.

With the series applied, a HugeTLB bootmem reservation is reported as:

  RSV_KERN|RSV_HUGETLB

RESEND:
- Regenerate the series from the same clean base as v1. The previous v2
  was inadvertently generated with an unrelated MEMBLOCK_NODUMP change
  in the patch context, causing apply failures. No code changes.

Changes in v2:
- Report set bits beyond flagname[] as UNKNOWN instead of ignoring them.
- Add a separate patch to synchronize debugfs reads with memory hotplug.

v1: https://lore.kernel.org/linux-mm/20260819075422.2387980-1-zhaomeijing100@gmail.com/

Meijing Zhao (3):
  mm: memblock: add missing HugeTLB flag name
  mm: memblock: show all region flags in debugfs
  mm: memblock: synchronize debugfs reads with memory hotplug

 mm/memblock.c | 26 ++++++++++++++++++--------
 1 file changed, 18 insertions(+), 8 deletions(-)

-- 
2.25.1

[PATCH v2 RESEND 1/3] mm: memblock: add missing HugeTLB flag name

[Meijing Zhao]

From: Meijing Zhao <zhaomeijing@lixiang.com>

Commit 7d163a75f821 ("memblock: make HugeTLB bootmem allocation work
with KHO") added MEMBLOCK_RSRV_HUGETLB but did not add the corresponding
entry to flagname[]. As a result, memblock debugfs cannot report the
flag by name.

Add the missing RSV_HUGETLB entry.

Fixes: 7d163a75f821 ("memblock: make HugeTLB bootmem allocation work with KHO")
Signed-off-by: Meijing Zhao <zhaomeijing@lixiang.com>
---
 mm/memblock.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/memblock.c b/mm/memblock.c
index 9ce86349a29f..f2952d725c10 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -2886,6 +2886,7 @@ static const char * const flagname[] = {
 	[ilog2(MEMBLOCK_RSRV_NOINIT)] = "RSV_NIT",
 	[ilog2(MEMBLOCK_RSRV_KERN)] = "RSV_KERN",
 	[ilog2(MEMBLOCK_KHO_SCRATCH)] = "KHO_SCRATCH",
+	[ilog2(MEMBLOCK_RSRV_HUGETLB)] = "RSV_HUGETLB",
 };
 
 static int memblock_debug_show(struct seq_file *m, void *private)
-- 
2.25.1

[PATCH v2 RESEND 2/3] mm: memblock: show all region flags in debugfs

[Meijing Zhao]

From: Meijing Zhao <zhaomeijing@lixiang.com>

Commit 493f349e38d0 ("memblock: Add flags and nid info in memblock
debugfs") made memblock_debug_show() stop after finding the first set
flag. A memblock region can carry multiple flags, so the remaining flags
are hidden from debugfs.

In particular, memory allocated for HugeTLB pages is reserved with both
MEMBLOCK_RSRV_KERN and MEMBLOCK_RSRV_HUGETLB, but debugfs only reports
RSV_KERN.

Walk all bits in the region flags and print every set flag separated by
"|". Keep walking beyond flagname[] so that a set flag without a known
name is reported as UNKNOWN rather than silently ignored.

A HugeTLB reservation is now shown as:

  RSV_KERN|RSV_HUGETLB

instead of:

  RSV_KERN

Fixes: 493f349e38d0 ("memblock: Add flags and nid info in memblock debugfs")
Signed-off-by: Meijing Zhao <zhaomeijing@lixiang.com>
---
 mm/memblock.c | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/mm/memblock.c b/mm/memblock.c
index f2952d725c10..36a8d2a9378d 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -2895,7 +2895,9 @@ static int memblock_debug_show(struct seq_file *m, void *private)
 	struct memblock_region *reg;
 	int i, j, nid;
 	unsigned int count = ARRAY_SIZE(flagname);
+	unsigned int flags;
 	phys_addr_t end;
+	bool first;
 
 	for (i = 0; i < type->cnt; i++) {
 		reg = &type->regions[i];
@@ -2909,16 +2911,20 @@ static int memblock_debug_show(struct seq_file *m, void *private)
 		else
 			seq_printf(m, "%4c ", 'x');
 		if (reg->flags) {
-			for (j = 0; j < count; j++) {
-				if (reg->flags & (1U << j)) {
-					seq_printf(m, "%s\n", flagname[j]);
-					break;
-				}
+			flags = reg->flags;
+			first = true;
+			for (j = 0; flags; j++, flags >>= 1) {
+				if (!(flags & 1))
+					continue;
+				if (!first)
+					seq_putc(m, '|');
+				seq_puts(m, j < count && flagname[j] ?
+					 flagname[j] : "UNKNOWN");
+				first = false;
 			}
-			if (j == count)
-				seq_printf(m, "%s\n", "UNKNOWN");
+			seq_putc(m, '\n');
 		} else {
-			seq_printf(m, "%s\n", "NONE");
+			seq_puts(m, "NONE\n");
 		}
 	}
 	return 0;
-- 
2.25.1

Re: [PATCH v2 RESEND 2/3] mm: memblock: show all region flags in debugfs

[Mike Rapoport]

On Fri, Aug 21, 2026 at 10:09:09AM +0800, Meijing Zhao wrote:
> From: Meijing Zhao <zhaomeijing@lixiang.com>
> 
> Commit 493f349e38d0 ("memblock: Add flags and nid info in memblock
> debugfs") made memblock_debug_show() stop after finding the first set
> flag. A memblock region can carry multiple flags, so the remaining flags
> are hidden from debugfs.
> 
> In particular, memory allocated for HugeTLB pages is reserved with both
> MEMBLOCK_RSRV_KERN and MEMBLOCK_RSRV_HUGETLB, but debugfs only reports
> RSV_KERN.
> 
> Walk all bits in the region flags and print every set flag separated by
> "|". Keep walking beyond flagname[] so that a set flag without a known
> name is reported as UNKNOWN rather than silently ignored.
> 
> A HugeTLB reservation is now shown as:
> 
>   RSV_KERN|RSV_HUGETLB
> 
> instead of:
> 
>   RSV_KERN
> 
> Fixes: 493f349e38d0 ("memblock: Add flags and nid info in memblock debugfs")
> Signed-off-by: Meijing Zhao <zhaomeijing@lixiang.com>
> ---
>  mm/memblock.c | 22 ++++++++++++++--------
>  1 file changed, 14 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/memblock.c b/mm/memblock.c
> index f2952d725c10..36a8d2a9378d 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -2895,7 +2895,9 @@ static int memblock_debug_show(struct seq_file *m, void *private)
>  	struct memblock_region *reg;
>  	int i, j, nid;
>  	unsigned int count = ARRAY_SIZE(flagname);
> +	unsigned int flags;
>  	phys_addr_t end;
> +	bool first;
>  
>  	for (i = 0; i < type->cnt; i++) {
>  		reg = &type->regions[i];
> @@ -2909,16 +2911,20 @@ static int memblock_debug_show(struct seq_file *m, void *private)
>  		else
>  			seq_printf(m, "%4c ", 'x');
>  		if (reg->flags) {
> -			for (j = 0; j < count; j++) {
> -				if (reg->flags & (1U << j)) {
> -					seq_printf(m, "%s\n", flagname[j]);
> -					break;
> -				}
> +			flags = reg->flags;
> +			first = true;

first and flags can be declared here.

> +			for (j = 0; flags; j++, flags >>= 1) {
> +				if (!(flags & 1))
> +					continue;
> +				if (!first)
> +					seq_putc(m, '|');
> +				seq_puts(m, j < count && flagname[j] ?
> +					 flagname[j] : "UNKNOWN");

Can we really have both j < count and !flagname[j]?

> +				first = false;
>  			}
> -			if (j == count)
> -				seq_printf(m, "%s\n", "UNKNOWN");
> +			seq_putc(m, '\n');
>  		} else {
> -			seq_printf(m, "%s\n", "NONE");
> +			seq_puts(m, "NONE\n");

No need to change this

>  		}
>  	}
>  	return 0;
> -- 
> 2.25.1
> 

-- 
Sincerely yours,
Mike.

Re: [PATCH v2 RESEND 2/3] mm: memblock: show all region flags in debugfs

[Meijing Zhao]

On Sun, Aug 23, 2026, Mike Rapoport wrote:
> first and flags can be declared here.

Agreed. I'll move them into the reg->flags block.

> Can we really have both j < count and !flagname[j]?

No. All currently defined memblock flag bits are contiguous and every
entry in flagname[] is initialized. I'll remove the redundant NULL
check.

> No need to change this

Agreed. I'll keep the existing code unchanged.

Thanks for the review.

[PATCH v2 RESEND 3/3] mm: memblock: synchronize debugfs reads with memory hotplug

[Meijing Zhao]

From: Meijing Zhao <zhaomeijing@lixiang.com>

memblock_debug_show() walks a memblock region array without
synchronization. With CONFIG_ARCH_KEEP_MEMBLOCK, memory hotplug can
concurrently add a region. If the array has to grow,
memblock_double_array() replaces type->regions and frees the old
allocation while the debugfs reader may still be using it.

Hold mem_hotplug_lock in read mode while producing the debugfs output.
Memory hotplug updates already hold the write side of this lock, so the
region array remains stable throughout the walk. The helpers are no-ops
when CONFIG_MEMORY_HOTPLUG is disabled.

Fixes: f9126ab9241f ("memory-hotplug: fix wrong edge when hot add a new node")
Signed-off-by: Meijing Zhao <zhaomeijing@lixiang.com>
---
 mm/memblock.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/memblock.c b/mm/memblock.c
index 36a8d2a9378d..f1ddbcce47ac 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -14,6 +14,7 @@
 #include <linux/pfn.h>
 #include <linux/debugfs.h>
 #include <linux/kmemleak.h>
+#include <linux/memory_hotplug.h>
 #include <linux/seq_file.h>
 #include <linux/memblock.h>
 #include <linux/mutex.h>
@@ -2899,6 +2900,7 @@ static int memblock_debug_show(struct seq_file *m, void *private)
 	phys_addr_t end;
 	bool first;
 
+	get_online_mems();
 	for (i = 0; i < type->cnt; i++) {
 		reg = &type->regions[i];
 		end = reg->base + reg->size - 1;
@@ -2927,6 +2929,7 @@ static int memblock_debug_show(struct seq_file *m, void *private)
 			seq_puts(m, "NONE\n");
 		}
 	}
+	put_online_mems();
 	return 0;
 }
 DEFINE_SHOW_ATTRIBUTE(memblock_debug);
-- 
2.25.1

Re: [PATCH v2 RESEND 0/3] mm: memblock: fix debugfs flag reporting and synchronization

[Mike Rapoport]

Hi,

On Fri, Aug 21, 2026 at 10:09:07AM +0800, Meijing Zhao wrote:
> From: Meijing Zhao <zhaomeijing@lixiang.com>
> 
> The memblock debugfs interface has two issues when reporting region
> flags. MEMBLOCK_RSRV_HUGETLB has no corresponding name, and regions
> with multiple flags only show the lowest set bit.
> 
> In addition, memblock_debug_show() walks the region arrays without
> synchronizing against memory hotplug. If memory hotplug grows an array,
> the debugfs reader may continue accessing the old array after it has
> been freed.
> 
> Add the missing HugeTLB flag name, report every set bit including those
> beyond flagname[], and hold the memory hotplug read lock while walking
> the arrays.
> 
> With the series applied, a HugeTLB bootmem reservation is reported as:
> 
>   RSV_KERN|RSV_HUGETLB
> 
> RESEND:
> - Regenerate the series from the same clean base as v1. The previous v2
>   was inadvertently generated with an unrelated MEMBLOCK_NODUMP change
>   in the patch context, causing apply failures. No code changes.

I presume the changes were on top of linux-next and it's not clear how to
apply them before the end of the merge window.

Can you please resend the patches after v7.3-rc1 is out and base them on
that?
 
> Changes in v2:
> - Report set bits beyond flagname[] as UNKNOWN instead of ignoring them.
> - Add a separate patch to synchronize debugfs reads with memory hotplug.
> 
> v1: https://lore.kernel.org/linux-mm/20260819075422.2387980-1-zhaomeijing100@gmail.com/
> 
> Meijing Zhao (3):
>   mm: memblock: add missing HugeTLB flag name
>   mm: memblock: show all region flags in debugfs
>   mm: memblock: synchronize debugfs reads with memory hotplug
> 
>  mm/memblock.c | 26 ++++++++++++++++++--------
>  1 file changed, 18 insertions(+), 8 deletions(-)
> 
> -- 
> 2.25.1

-- 
Sincerely yours,
Mike.