Re: [PATCH] erofs: fix EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS on some UP platforms

Hi Gao,

On Wed, 12 Aug 2026 at 15:12, Gao Xiang <[email protected]> wrote:
> CONFIG_NR_CPUS doesn't define on some UP platforms (e.g. arm), so this
> can cause make oldconfig to loop indefinitely when CONFIG_SMP=n:
>
>  $ make ARCH=arm allmodconfig
>  $ sed -i "/CONFIG_SMP=y/d" .config
>  $ sed -i "/CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS.*/d" .config
>
> EROFS LZMA default maximum decompression streams (EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS) [0] (NEW)
> EROFS LZMA default maximum decompression streams (EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS) [0] (NEW)
> ...
>
> Let's guard NR_CPUS with SMP instead of using a hardcoded arbitrary CPU
> uplimit here, similar to commit a3344078101c ("mm: make SPLIT_PTE_PTLOCKS
> depend on SMP").
>
> The initial report from SJ Park was for m68k [1] (m68k is the only arch
> without NR_CPUS in Kconfig), and it will be changed in another patch [2].
>
> [1] https://lore.kernel.org/all/[email protected]/T/#u
> [2] https://lore.kernel.org/r/[email protected]
>
> Reported-by: SJ Park <[email protected]>
> Closes: https://lore.kernel.org/r/[email protected]
> Reported-by: Guenter Roeck <[email protected]>
> Closes: https://lore.kernel.org/r/[email protected]
> Fixes: c9b47e6b2311 ("erofs: cap LZMA stream pool size")
> Cc: Geert Uytterhoeven <[email protected]>
> Signed-off-by: Gao Xiang <[email protected]>

Thanks for your patch, which is now commit a64d500b0078e16e ("erofs: fix
EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS on some UP platforms") in v7.2.

> --- a/fs/erofs/Kconfig
> +++ b/fs/erofs/Kconfig
> @@ -134,7 +134,8 @@ config EROFS_FS_ZIP_LZMA
>  config EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS
>         int "EROFS LZMA default maximum decompression streams"
>         depends on EROFS_FS_ZIP_LZMA
> -       range 1 NR_CPUS
> +       range 1 NR_CPUS if SMP
> +       range 1 1 if !SMP
>         default 16

I have just discovered another oddity: when you have less than 16
CPUS (including SMP=n), this default value is invalid, and "make
savedefconfig" will always write a line like

    CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS=1

to the defconfig file, even if that is the default suggested value.

Needlessly polluting the defconfig files could be fixed using:

    -       default 16
    +       default 16 if SMP && NR_CPUS >= 16
    +       default NR_CPUS if SMP
    +       default 1 if !SMP

Thoughts?

>         help
>           By default EROFS allocates one LZMA decompression stream per CPU.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds