Re: [PATCH v3] erofs: cap LZMA stream pool size

[Geert Uytterhoeven]

On Mon, 3 Aug 2026 at 11:12, Uwe Kleine-König
<u.kleine-koenig@xxxxxxxxxxxx> wrote:
> On Mon, Aug 03, 2026 at 10:09:06AM +0200, Geert Uytterhoeven wrote:
> > On Tue, 28 Jul 2026 at 08:55, SJ Park <sj@xxxxxxxxxx> wrote:
> > > I don't have a m68k testfarm, either.  I'm doing crossbuild [1].  Hopefully you
> > > could also reproduce it.
>
> FTR, I don't have m68k hardware either, but as this is a build time
> failure installing the Debian package gcc-m68k-linux-gnu and then doing
> builds using
>
>         make ARCH=m68k CROSS_COMPILE=m68k-linux-gnu-
>
> is good enough to reproduce the failure.
>
> > I think you can reproduce some of the issues on most architectures
> > with a non-SMP kernel, cfr. my reply[1] to Uwe's proposed fix.
> > Unfortunately the Kconfig warning only shows up on architectures that
> > don't define NR_CPUS (because they do not support SMP) at all,
> > while ending up with zero streams is probably a runtime failure.
> >
> > [1] https://lore.kernel.org/CAMuHMdVGKX=3cfOrLWp1uSHX76xf1-=C0+GVCXkA8JTqNPg-ew@xxxxxxxxxxxxxx
>
> I guess EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS should depend on SMP and
> it's use should be conditionalized using something like
>
> diff --git a/fs/erofs/decompressor_lzma.c b/fs/erofs/decompressor_lzma.c
> index 6b0cdb446c6a..c00e94f8206f 100644
> --- a/fs/erofs/decompressor_lzma.c
> +++ b/fs/erofs/decompressor_lzma.c
> @@ -50,9 +50,13 @@ static int __init z_erofs_lzma_init(void)
>         unsigned int i;
>
>         /* by default, use # of possible CPUs instead */
> -       if (!z_erofs_lzma_nstrms)
> -               z_erofs_lzma_nstrms = min_t(unsigned int, num_possible_cpus(),
> -                               CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS);
> +       if (!z_erofs_lzma_nstrms) {
> +               if (IS_ENABLED(CONFIG_SMP))
> +                       z_erofs_lzma_nstrms = min_t(unsigned int, num_possible_cpus(),
> +                                       CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS);
> +               else
> +                       z_erofs_lzma_nstrms = 1;
> +       }
>
>         for (i = 0; i < z_erofs_lzma_nstrms; ++i) {
>                 struct z_erofs_lzma *strm = kzalloc_obj(*strm);

LGTM.

>
> I played a bit with the definition of
> EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS in fs/erofs/Kconfig, but didn't
> find a way to define it that both uses NR_CPUS and doesn't generate a
> warning with ARCH=m68k.

Any progress with fixing this for v7.2?

NR_CPUS depends on SMP on most architectures, so you get e.g.
with ARCH=arm allmodconfig + CONFIG_SMP=n:

    CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS=0

X86 defines NR_CPUS unconditionally, so there it works as expected.
Thanks!

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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