mm: shmem/tmpfs hugepage defaults config choice

Allow to override defaults for shemem and tmpfs at config time.  This is
consistent with how transparent hugepages can be configured.

Same results can be achieved with the existing
'transparent_hugepage_shmem' and 'transparent_hugepage_tmpfs' settings in
the kernel command line, but it is more convenient to define basic
settings at config time instead of changing kernel command line later.

Defaults for shmem and tmpfs were not changed.  They are remained the same
as before: 'never' for both cases.  Options 'deny' and 'force' are omitted
intentionally since these are special values and supposed to be used for
emergencies or testing and are not expected to be permanent ones.

Primary motivation for adding config option is to enable policy
enforcement at build time.  In large-scale production environments (Meta's
for example), the kernel configuration is often maintained centrally close
to the kernel code itself and owned by the kernel engineers, while boot
parameters are managed independently (e.g.  by provisioning systems).  In
such setups, the kernel build defines the supported and expected behavior
in a single place, but there is no reliable or uniform control over the
kernel command line options.

A build-time default allows kernel integrators to enforce a predictable
hugepage policy for shmem/tmpfs on a base layer, ensuring reproducible
behavior and avoiding configuration drift caused by possible boot-time
differences.

In short, primary benefit is mostly operational: it provides a way to
codify preferred policy in the kernel configuration, which is versioned,
reviewed, and tested as part of the kernel build process, rather than
depending on potentially variable boot parameters.

[d@ilvokhin.com: v2]
  Link: https://lkml.kernel.org/r/aQECPpjd-fU_TC79@shell.ilvokhin.com
Link: https://lkml.kernel.org/r/aPpv8sAa2sYgNu3L@shell.ilvokhin.com
Signed-off-by: Dmitry Ilvokhin <d@ilvokhin.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Kiryl Shutsemau <kas@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 31 insertions, 2 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index e1dc2d8e939c..6580f3cd24bb 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -570,8 +570,37 @@ static int shmem_confirm_swap(struct address_space *mapping, pgoff_t index,
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 /* ifdef here to avoid bloating shmem.o when not necessary */
 
-static int shmem_huge __read_mostly = SHMEM_HUGE_NEVER;
-static int tmpfs_huge __read_mostly = SHMEM_HUGE_NEVER;
+#if defined(CONFIG_TRANSPARENT_HUGEPAGE_SHMEM_HUGE_NEVER)
+#define SHMEM_HUGE_DEFAULT SHMEM_HUGE_NEVER
+#elif defined(CONFIG_TRANSPARENT_HUGEPAGE_SHMEM_HUGE_ALWAYS)
+#define SHMEM_HUGE_DEFAULT SHMEM_HUGE_ALWAYS
+#elif defined(CONFIG_TRANSPARENT_HUGEPAGE_SHMEM_HUGE_WITHIN_SIZE)
+#define SHMEM_HUGE_DEFAULT SHMEM_HUGE_WITHIN_SIZE
+#elif defined(CONFIG_TRANSPARENT_HUGEPAGE_SHMEM_HUGE_ADVISE)
+#define SHMEM_HUGE_DEFAULT SHMEM_HUGE_ADVISE
+#else
+#define SHMEM_HUGE_DEFAULT SHMEM_HUGE_NEVER
+#endif
+
+static int shmem_huge __read_mostly = SHMEM_HUGE_DEFAULT;
+
+#undef SHMEM_HUGE_DEFAULT
+
+#if defined(CONFIG_TRANSPARENT_HUGEPAGE_TMPFS_HUGE_NEVER)
+#define TMPFS_HUGE_DEFAULT SHMEM_HUGE_NEVER
+#elif defined(CONFIG_TRANSPARENT_HUGEPAGE_TMPFS_HUGE_ALWAYS)
+#define TMPFS_HUGE_DEFAULT SHMEM_HUGE_ALWAYS
+#elif defined(CONFIG_TRANSPARENT_HUGEPAGE_TMPFS_HUGE_WITHIN_SIZE)
+#define TMPFS_HUGE_DEFAULT SHMEM_HUGE_WITHIN_SIZE
+#elif defined(CONFIG_TRANSPARENT_HUGEPAGE_TMPFS_HUGE_ADVISE)
+#define TMPFS_HUGE_DEFAULT SHMEM_HUGE_ADVISE
+#else
+#define TMPFS_HUGE_DEFAULT SHMEM_HUGE_NEVER
+#endif
+
+static int tmpfs_huge __read_mostly = TMPFS_HUGE_DEFAULT;
+
+#undef TMPFS_HUGE_DEFAULT
 
 static unsigned int shmem_get_orders_within_size(struct inode *inode,
 		unsigned long within_size_orders, pgoff_t index,