On Mon, Aug 25, 2025 at 06:23:48PM +0200, David Hildenbrand wrote:

quoted

I don't quite understand the interaction with PG_Reserved and why anybody
using this function should care.

So maybe you can rephrase in a way that is easier to digest, and rather
focuses on what callers of this function are supposed to do vs. have the
liberty of not doing?

How about
  

diff --git a/include/linux/memblock.h b/include/linux/memblock.h
index b96746376e17..fcda8481de9a 100644
--- a/include/linux/memblock.h
+++ b/include/linux/memblock.h

@@ -40,8 +40,9 @@ extern unsigned long long max_possible_pfn;
   * via a driver, and never indicated in the firmware-provided memory map as
   * system RAM. This corresponds to IORESOURCE_SYSRAM_DRIVER_MANAGED in the
   * kernel resource tree.
- * @MEMBLOCK_RSRV_NOINIT: memory region for which struct pages are
- * not initialized (only for reserved regions).
+ * @MEMBLOCK_RSRV_NOINIT: reserved memory region for which struct pages are not
+ * fully initialized. Users of this flag are responsible to properly initialize
+ * struct pages of this region
   * @MEMBLOCK_RSRV_KERN: memory region that is reserved for kernel use,
   * either explictitly with memblock_reserve_kern() or via memblock
   * allocation APIs. All memblock allocations set this flag.

diff --git a/mm/memblock.c b/mm/memblock.c
index 154f1d73b61f..46b411fb3630 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c

@@ -1091,13 +1091,20 @@ int __init_memblock memblock_clear_nomap(phys_addr_t base, phys_addr_t size)
  
  /**
   * memblock_reserved_mark_noinit - Mark a reserved memory region with flag
- * MEMBLOCK_RSRV_NOINIT which results in the struct pages not being initialized
- * for this region.
+ * MEMBLOCK_RSRV_NOINIT
+ *
   * @base: the base phys addr of the region
   * @size: the size of the region
   *
- * struct pages will not be initialized for reserved memory regions marked with
- * %MEMBLOCK_RSRV_NOINIT.
+ * The struct pages for the reserved regions marked %MEMBLOCK_RSRV_NOINIT will
+ * not be fully initialized to allow the caller optimize their initialization.
+ *
+ * When %CONFIG_DEFERRED_STRUCT_PAGE_INIT is enabled, setting this flag
+ * completely bypasses the initialization of struct pages for such region.
+ *
+ * When %CONFIG_DEFERRED_STRUCT_PAGE_INIT is disabled, struct pages in this
+ * region will be initialized with default values but won't be marked as
+ * reserved.