Re: [PATCH v2 11/11] mm, swap: merge zeromap into swap table

[YoungJun Park]

On Fri, Apr 17, 2026 at 02:34:41AM +0800, Kairui Song via B4 Relay wrote:

>   *
>   * Usages:
>   *
> @@ -74,17 +76,22 @@ struct swap_memcg_table {
>  #define SWP_TB_PFN_MARK_BITS	2
>  #define SWP_TB_PFN_MARK_MASK	(BIT(SWP_TB_PFN_MARK_BITS) - 1)
>  
> -/* SWAP_COUNT part for PFN or shadow, the width can be shrunk or extended */
> -#define SWP_TB_COUNT_BITS      min(4, BITS_PER_LONG - SWP_TB_PFN_BITS)
> +/* SWAP_COUNT and flags for PFN or shadow, width can be shrunk or extended */
> +#define SWP_TB_FLAGS_BITS	min(5, BITS_PER_LONG - SWP_TB_PFN_BITS)
> +#define SWP_TB_COUNT_BITS	(SWP_TB_FLAGS_BITS - 1)

Hi Kairui :)

Would this break the build on 32-bit arches with 40-bit phys
addrs (MAX_POSSIBLE_PHYSMEM_BITS = 40)?

Architectures I checked.
  - ARM LPAE   (CONFIG_ARM_LPAE=y)
  - ARC PAE40  (CONFIG_ARC_HAS_PAE40=y)
  - MIPS XPA   (CONFIG_XPA=y)

Calculations.

  SWP_TB_PFN_BITS   = 28 + 2 = 30
  SWP_TB_FLAGS_BITS = min(5, 32 - 30) = 2
  SWP_TB_COUNT_BITS = 2 - 1 = 1

The BUILD_BUG_ON looks like the real problem. it needs at
least 3 count values (free/used/overflow).

  BUILD_BUG_ON(SWP_TB_COUNT_MAX < 2 || SWP_TB_COUNT_BITS < 2);

Confirmed with a cross build (multi_v7_defconfig + lpae.config).

  error: BUILD_BUG_ON failed: SWP_TB_COUNT_MAX < 2 || SWP_TB_COUNT_BITS < 2
    at __count_to_swp_tb (mm/swap_table.h:227)

I think the right fix is widening swap_tb to 64 bits
unconditionally (atomic64_t).

(Or, uglier, these arches could always route counts through the
extend table.)

Best regards,
Youngjun Park