[PATCH v1 1/2] padata: downgrade padata_do_multithreaded to serial execution for non-SMP

[Gang Li <gang.li@linux.dev>]

Randy Dunlap and kernel test robot reported a warning:

```
WARNING: unmet direct dependencies detected for PADATA
  Depends on [n]: SMP [=n]
  Selected by [y]:
  - HUGETLBFS [=y] && (X86 [=y] || SPARC64 || ARCH_SUPPORTS_HUGETLBFS [=n] || BROKEN [=n]) && (SYSFS [=y] || SYSCTL [=n])
```

hugetlb parallelization depends on PADATA, and PADATA depends on SMP.

PADATA consists of two distinct functionality: One part is
padata_do_multithreaded which disregards order and simply divides
tasks into several groups for parallel execution. Hugetlb
init parallelization depends on padata_do_multithreaded.

The other part is composed of a set of APIs that, while handling data in
an out-of-order parallel manner, can eventually return the data with
ordered sequence. Currently Only `crypto/pcrypt.c` use them.

All users of PADATA of non-SMP case currently only use
padata_do_multithreaded. It is easy to implement a serial one in
include/linux/padata.h. And it is not necessary to implement another
functionality unless the only user of crypto/pcrypt.c does not depend on
SMP in the future.

Fixes: a2cefb08be66 ("hugetlb: have CONFIG_HUGETLBFS select CONFIG_PADATA")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Closes: https://lore.kernel.org/lkml/ec5dc528-2c3c-4444-9e88-d2c48395b433@infradead.org/
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202402020454.6EPkP1hi-lkp@intel.com/
Signed-off-by: Gang Li <ligang.bdlg@bytedance.com>
---
 fs/Kconfig             |  2 +-
 include/linux/padata.h | 13 +++++++++----
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/fs/Kconfig b/fs/Kconfig
index 4a51331f172e5..7963939592d70 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -261,7 +261,7 @@ menuconfig HUGETLBFS
 	depends on X86 || SPARC64 || ARCH_SUPPORTS_HUGETLBFS || BROKEN
 	depends on (SYSFS || SYSCTL)
 	select MEMFD_CREATE
-	select PADATA
+	select PADATA if SMP
 	help
 	  hugetlbfs is a filesystem backing for HugeTLB pages, based on
 	  ramfs. For architectures that support it, say Y here and read
diff --git a/include/linux/padata.h b/include/linux/padata.h
index 8f418711351bc..7b84eb7d73e7f 100644
--- a/include/linux/padata.h
+++ b/include/linux/padata.h
@@ -180,10 +180,6 @@ struct padata_instance {
 
 #ifdef CONFIG_PADATA
 extern void __init padata_init(void);
-#else
-static inline void __init padata_init(void) {}
-#endif
-
 extern struct padata_instance *padata_alloc(const char *name);
 extern void padata_free(struct padata_instance *pinst);
 extern struct padata_shell *padata_alloc_shell(struct padata_instance *pinst);
@@ -194,4 +190,13 @@ extern void padata_do_serial(struct padata_priv *padata);
 extern void __init padata_do_multithreaded(struct padata_mt_job *job);
 extern int padata_set_cpumask(struct padata_instance *pinst, int cpumask_type,
 			      cpumask_var_t cpumask);
+#else
+static inline void __init padata_init(void) {}
+static inline void __init padata_do_multithreaded(struct padata_mt_job *job)
+{
+	if (job->size)
+		job->thread_fn(job->start, job->start + job->size, job->fn_arg);
+}
+#endif
+
 #endif
-- 
2.20.1