[PATCH v2] LoongArch: mm: Define DIRECT_MAP_PHYSMEM_END

[Han Gao]

get_free_mem_region() and mhp_get_pluggable_range() bound their
search to DIRECT_MAP_PHYSMEM_END.  LoongArch does not define it, so
the fallback in include/linux/mm.h applies: under
CONFIG_SPARSEMEM_VMEMMAP it is (1ULL << MAX_PHYSMEM_BITS) - 1, a
compile-time constant that does not adapt to the CPU's physical
address bits (cpu_pabits, probed from CPUCFG1).

The vmemmap window only covers physical space below 2^(cpu_pabits+1)
(VMEMMAP_SIZE), so on CPUs with fewer physical address bits than
MAX_PHYSMEM_BITS the fallback allows get_free_mem_region() to return
a ZONE_DEVICE region outside the vmemmap window;
vmemmap_populate() then wraps the memmap range around and maps it
into low memory, silently corrupting the page tables.  The same
search also picked the top-of-address-space region that crashed
memmap_init_zone_device() with amdkfd on 3C6000 in 6.16 [1];
commit 2969b42c8f99 ("LoongArch/mm: align vmemmap to maximal folio
size") keeps that region in bounds on current 3C6000 configs, but
CPUs with smaller cpu_pabits (e.g. the 2K series) are still
affected.

Define DIRECT_MAP_PHYSMEM_END as the vmemmap-covered physical
range, (1ULL << (cpu_pabits + 1)) - 1, capped at
(1ULL << MAX_PHYSMEM_BITS) - 1 under CONFIG_SPARSEMEM, mirroring
commit f3336b48cf9d ("riscv: mm: Define DIRECT_MAP_PHYSMEM_END").

[1] https://lore.kernel.org/amd-gfx/20250814032153.227285-1-jeffbai@aosc.io/

Cc: stable at vger.kernel.org # v6.13+
Signed-off-by: Han Gao <gaohan at iscas.ac.cn>
---
Changes in v2:
- Redefine DIRECT_MAP_PHYSMEM_END as the vmemmap-covered range
  (1ULL << (cpu_pabits + 1)) - 1, capped at
  (1ULL << MAX_PHYSMEM_BITS) - 1 under CONFIG_SPARSEMEM, per Huacai
  Chen's review (the previous (1ULL << cpu_pabits) - 1 wrongly
  limits CPUs with 48-bit physical addresses).
- Note that 3C6000 has 48-bit physical addresses as well.
- v1: https://lore.kernel.org/all/20260811115749.3465579-1-gaohan@iscas.ac.cn/

 arch/loongarch/include/asm/pgtable.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/loongarch/include/asm/pgtable.h b/arch/loongarch/include/asm/pgtable.h
index 223528c04d73..8395aea209b5 100644
--- a/arch/loongarch/include/asm/pgtable.h
+++ b/arch/loongarch/include/asm/pgtable.h
@@ -125,6 +125,14 @@ struct vm_area_struct;
 
 #endif
 
+/* Needed to limit get_free_mem_region() */
+#ifndef CONFIG_SPARSEMEM
+#define DIRECT_MAP_PHYSMEM_END ((1ULL << (cpu_pabits + 1)) - 1)
+#else
+#define DIRECT_MAP_PHYSMEM_END \
+	min((1ULL << (cpu_pabits + 1)) - 1, (1ULL << MAX_PHYSMEM_BITS) - 1)
+#endif
+
 #define ptep_get(ptep) READ_ONCE(*(ptep))
 #define pmdp_get(pmdp) READ_ONCE(*(pmdp))
 
-- 
2.47.3