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

[Geert Uytterhoeven]

Hi Gao,

On Wed, 12 Aug 2026 at 01:37, Gao Xiang <xiang@xxxxxxxxxx> wrote:
> On Tue, Aug 11, 2026 at 03:37:38PM -0400, Michael Bommarito wrote:
> > On Tue, Aug 11, 2026 at 3:35 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> > > > Any progress with fixing this for v7.2?
> >
> > I would like to fix this since I started the problem with the original
> > patch, but I feel a bit stuck.  My v1 and v2 patches would not have
> > triggered the build failure, but Gao made valid points about the
> > trade-offs that led us to this v3.  Maybe it would be worth your
> > opinion on the config approach in those earlier patches?
>
> Sorry about the late reply.
>
> As I said, I really dislike hardcoded range, but it is an issue that no
> NR_CPUS definition on CONFIG_SMP:
>
> I wonder if the following diff resolves the m68k issue (I'm not an
> Kconfig expert but it seems to work on x86_64 !CONFIG_SMP):
>
> diff --git a/fs/erofs/Kconfig b/fs/erofs/Kconfig
> index 8ca1767dafb6..2dfc313588d2 100644
> --- 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
>         help
>           By default EROFS allocates one LZMA decompression stream per CPU.

Thank you, that seems to work (despite still seeing the warning on m68k,
as NR_CPUS does not exist).

Test runs on ARM (NR_CPUS depends on SMP, like most architectures), m68k
(no CONFIG_NR_CPUS), and x86 (NR_CPUS is always defined) before/after:

     $ make ARCH=arm allmodconfig
     $ grep CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS .config
     CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS=4

 SMP is OK.

     $ sed -i "/CONFIG_SMP=y/d" .config
     $ yes "" | make ARCH=arm oldconfig
-
-No warning...
-
     $ grep CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS .config
-    CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS=0
+    CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS=1

-... but automatic configuration is silently broken on UP.
+UP is OK with automatic configuration.

     $ sed -i "/CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS.*/d" .config
     $ yes "" | make ARCH=arm oldconfig
-            EROFS LZMA default maximum decompression streams
(EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS) [0] (NEW)
-    [infinite loop]
+    $ grep CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS .config
+    CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS=1

-Manual configuration is broken.
+UP is OK with manual configuration.

 --------------------------------------------------------------------------------

      $ make ARCH=m68k allmodconfig
     fs/erofs/Kconfig:137:warning: range is invalid

 Warning...

     $ grep CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS .config
-    CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS=NR_CPUS
-
-... but this will actually build and work, as <linux/threads.h> has:
-
-    #ifndef CONFIG_NR_CPUS
-    /* FIXME: This should be fixed in the arch's Kconfig */
-    #define CONFIG_NR_CPUS  1
-    #endif
-
-    /* Places which use this should consider cpumask_var_t. */
-    #define NR_CPUS         CONFIG_NR_CPUS
+    CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS=1
+
+... but UP is OK with automatic configuration.

     $ sed -i "/CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS.*/d" .config
     $ yes "" | make ARCH=m68k oldconfig
-            EROFS LZMA default maximum decompression streams
(EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS) [NR_CPUS] (NEW)
-    [infinite loop]
+    fs/erofs/Kconfig:137:warning: range is invalid
+
+Warning...
+
+    $ grep CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS .config
+    CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS=1

-Manual configuration is broken.
+... but UP is OK with manual configuration.

 --------------------------------------------------------------------------------

     $ make ARCH=x86 allmodconfig
     $ grep CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS .config
     CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS=16

 SMP is OK.

     $ sed -i "/CONFIG_SMP=y/d" .config
     $ yes "" | make ARCH=x86 oldconfig
     $ grep CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS .config
     CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS=1

 UP is OK with automatic configuration.

     $ sed -i "/CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS.*/d" .config
     $ yes "" | make ARCH=x86 oldconfig
     $ grep CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS .config
     CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS=1

 UP is OK with manual configuration.

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