Re: [PATCH v9 01/10] memblock: Introduce MEMBLOCK_LLMAP

On Sun, Sep 06, 2026 at 10:33:11PM +0300, Mike Rapoport wrote:

On Wed, Sep 02, 2026 at 11:47:03AM +0100, Vincent Donnefort wrote:

quoted

Keeping last-level mappings is interesting on some architectures as it
allows mapping/unmapping pages from the kernel direct map without the
risk of splitting blocks which, under the break-before-make rule, may
trigger page-faults the kernel can't handle.

However, mapping the entire direct map at PTE-level is costly. So
instead, create a new memblock flag MEMBLOCK_LLMAP to enable the system

I believe MEMBLOCK_PTE_MAP sounds more descriptive.

The idea was to have something close from NOMAP, to emphasis it is one or the
other. But PTE_MAP sounds good too.

quoted

to decide which region must be covered by mappings up to the last-level.

Signed-off-by: Vincent Donnefort <redacted>

diff --git a/include/linux/memblock.h b/include/linux/memblock.h
index d62db9e776cf..d40a5ded188d 100644
--- a/include/linux/memblock.h
+++ b/include/linux/memblock.h

@@ -52,6 +52,7 @@ extern unsigned long long max_possible_pfn;
  * kernel that we know is good to use. It is the only memory that
  * allocations may happen from in this phase.
  * @MEMBLOCK_RSRV_HUGETLB: memory is reserved for hugetlb pages
+ * @MEMBLOCK_LLMAP: memory region to be mapped using last-level mapping
  */
 enum memblock_flags {
 	MEMBLOCK_NONE		= 0x0,	/* No special request */

@@ -63,6 +64,7 @@ enum memblock_flags {
 	MEMBLOCK_RSRV_KERN	= 0x20,	/* memory reserved for kernel use */
 	MEMBLOCK_KHO_SCRATCH	= 0x40,	/* scratch memory for kexec handover */
 	MEMBLOCK_RSRV_HUGETLB	= 0x80, /* memory reserved for hugetlb pages */
+	MEMBLOCK_LLMAP		= 0x100,/* last-level mapping */
 };
 
 /**

@@ -160,6 +162,8 @@ int memblock_reserved_mark_noinit(phys_addr_t base, phys_addr_t size);
 int memblock_reserved_mark_kern(phys_addr_t base, phys_addr_t size);
 int memblock_mark_kho_scratch(phys_addr_t base, phys_addr_t size);
 int memblock_clear_kho_scratch(phys_addr_t base, phys_addr_t size);
+int memblock_mark_llmap(phys_addr_t base, phys_addr_t size);
+int memblock_clear_llmap(phys_addr_t base, phys_addr_t size);
 
 void memblock_free(void *ptr, size_t size);
 void reset_all_zones_managed_pages(void);

@@ -306,6 +310,11 @@ static inline bool memblock_is_kho_scratch(struct memblock_region *m)
 	return m->flags & MEMBLOCK_KHO_SCRATCH;
 }
 
+static inline bool memblock_is_llmap(struct memblock_region *m)
+{
+	return m->flags & MEMBLOCK_LLMAP;
+}
+
 int memblock_search_pfn_nid(unsigned long pfn, unsigned long *start_pfn,
 			    unsigned long  *end_pfn);
 void __next_mem_pfn_range(int *idx, int nid, unsigned long *out_start_pfn,

diff --git a/mm/memblock.c b/mm/memblock.c
index 9ce86349a29f..1591b50503ed 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c

@@ -1119,6 +1119,16 @@ int __init_memblock memblock_mark_mirror(phys_addr_t base, phys_addr_t size)
  */
 int __init_memblock memblock_mark_nomap(phys_addr_t base, phys_addr_t size)
 {
+	struct memblock_region *r;
+
+	memblock_cap_size(base, &size);
+
+	for_each_mem_region(r) {
+		if (memblock_is_llmap(r) &&
+		    memblock_addrs_overlap(base, size, r->base, r->size))
+			return -EINVAL;
+	}

I'm not very fond of implicit skips here. memblock has no idea what's the
caller intention, maybe it actually wants to change the memory from nomap
to pte-mapped.

I'd rather warn in memblock_is_nomap() and memblock_is_llmap() if they both
are set and let the caller deal with making sure they are not.

ack.

quoted

+
 	return memblock_setclr_flag(&memblock.memory, base, size, 1, MEMBLOCK_NOMAP);
 }
 

@@ -1204,6 +1214,45 @@ __init int memblock_clear_kho_scratch(phys_addr_t base, phys_addr_t size)
 				    MEMBLOCK_KHO_SCRATCH);
 }
 
+/**
+ * memblock_mark_llmap - Mark a memory region with flag MEMBLOCK_LLMAP.
+ * @base: the base phys addr of the region
+ * @size: the size of the region
+ *
+ * If supported by the architecture, such region is mapped at the last-level in
+ * the kernel direct map.
+ *
+ * Return: 0 on success, -errno on failure.
+ */
+int __init_memblock memblock_mark_llmap(phys_addr_t base, phys_addr_t size)
+{
+	struct memblock_region *r;
+
+	memblock_cap_size(base, &size);
+
+	for_each_mem_region(r) {
+		if (memblock_is_nomap(r) &&
+		    memblock_addrs_overlap(base, size, r->base, r->size))
+			return -EINVAL;
+	}

same here

ack

quoted

+
+	return memblock_setclr_flag(&memblock.memory, base, size, 1,
+				    MEMBLOCK_LLMAP);
+}
+
+/**
+ * memblock_clear_llmap - Clear flag MEMBLOCK_LLMAP for a specified region.
+ * @base: the base phys addr of the region
+ * @size: the size of the region
+ *
+ * Return: 0 on success, -errno on failure.
+ */
+int __init_memblock memblock_clear_llmap(phys_addr_t base, phys_addr_t size)
+{
+	return memblock_setclr_flag(&memblock.memory, base, size, 0,
+				    MEMBLOCK_LLMAP);
+}
+
 static bool should_skip_region(struct memblock_type *type,
 			       struct memblock_region *m,
 			       int nid, int flags)

@@ -2886,6 +2935,7 @@ 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_LLMAP)] = "LLMAP",
 };
 
 static int memblock_debug_show(struct seq_file *m, void *private)

-- 
2.55.0.970.g62bdec98f9-goog

-- 
Sincerely yours,
Mike.

-- 
Vincent