[PATCH] 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 physical address
bits of the CPU (cpu_pabits, probed from CPUCFG1: 48 on
3A5000/3A6000, 47 on 3C6000, 40 on the 2K series).

The vmemmap window only covers physical space below 2^(cpu_pabits+1)
(VMEMMAP_SIZE), so on CPUs with fewer than 48 physical address bits
get_free_mem_region() may hand ZONE_DEVICE a region with no struct
page backing:

- On the 2K series the region returned at the top of the 48-bit
  physical space is outside the vmemmap window; vmemmap_populate()
  wraps the range around and maps it into low memory, silently
  corrupting the page tables.
- On 3C6000 the vmemmap range of the region's last section used to
  end exactly at 2^64 and wrap to 0, leaving vmemmap_populate() with
  nothing to map, so memmap_init_zone_device() faulted while writing
  struct page (reported with amdkfd on 6.16 [1]).  Commit 2969b42c8f99
  ("LoongArch/mm: align vmemmap to maximal folio size") moved the
  vmemmap base down by one PMD, which keeps that section in bounds on
  current 3C6000 configs, but the 2K series is still affected and the
  3C6000 case only holds for the current vmemmap layout.

Define DIRECT_MAP_PHYSMEM_END as (1ULL << cpu_pabits) - 1 so that
both searches stay within the vmemmap-covered physical space,
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>
---
 arch/loongarch/include/asm/pgtable.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/loongarch/include/asm/pgtable.h b/arch/loongarch/include/asm/pgtable.h
index 223528c04d73..244931e4bc58 100644
--- a/arch/loongarch/include/asm/pgtable.h
+++ b/arch/loongarch/include/asm/pgtable.h
@@ -125,6 +125,13 @@ struct vm_area_struct;
 
 #endif
 
+/* Needed to limit get_free_mem_region() */
+#if defined(CONFIG_FLATMEM) || defined(CONFIG_SPARSEMEM_VMEMMAP)
+#define DIRECT_MAP_PHYSMEM_END ((1ULL << cpu_pabits) - 1)
+#elif defined(CONFIG_SPARSEMEM)
+/* DIRECT_MAP_PHYSMEM_END is not limited by VA space assignment in this case */
+#endif
+
 #define ptep_get(ptep) READ_ONCE(*(ptep))
 #define pmdp_get(pmdp) READ_ONCE(*(pmdp))
 
-- 
2.47.3