shmem: prepare huge= mount option and sysfs knob

This patch adds new mount option "huge=".  It can have following values:

  - "always":
	Attempt to allocate huge pages every time we need a new page;

  - "never":
	Do not allocate huge pages;

  - "within_size":
	Only allocate huge page if it will be fully within i_size.
	Also respect fadvise()/madvise() hints;

  - "advise:
	Only allocate huge pages if requested with fadvise()/madvise();

Default is "never" for now.

"mount -o remount,huge= /mountpoint" works fine after mount: remounting
huge=never will not attempt to break up huge pages at all, just stop
more from being allocated.

No new config option: put this under CONFIG_TRANSPARENT_HUGEPAGE, which
is the appropriate option to protect those who don't want the new bloat,
and with which we shall share some pmd code.

Prohibit the option when !CONFIG_TRANSPARENT_HUGEPAGE, just as mpol is
invalid without CONFIG_NUMA (was hidden in mpol_parse_str(): make it
explicit).

Allow enabling THP only if the machine has_transparent_hugepage().

But what about Shmem with no user-visible mount? SysV SHM, memfds,
shared anonymous mmaps (of /dev/zero or MAP_ANONYMOUS), GPU drivers' DRM
objects, Ashmem.  Though unlikely to suit all usages, provide sysfs knob
/sys/kernel/mm/transparent_hugepage/shmem_enabled to experiment with
huge on those.

And allow shmem_enabled two further values:

  - "deny":
	For use in emergencies, to force the huge option off from
	all mounts;
  - "force":
	Force the huge option on for all - very useful for testing;

Based on patch by Hugh Dickins.

Link: http://lkml.kernel.org/r/1466021202-61880-28-git-send-email-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 4d4780c00d34..466f18c73a49 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -28,9 +28,10 @@ struct shmem_sb_info {
 	unsigned long max_inodes;   /* How many inodes are allowed */
 	unsigned long free_inodes;  /* How many are left for allocation */
 	spinlock_t stat_lock;	    /* Serialize shmem_sb_info changes */
+	umode_t mode;		    /* Mount mode for root directory */
+	unsigned char huge;	    /* Whether to try for hugepages */
 	kuid_t uid;		    /* Mount uid for root directory */
 	kgid_t gid;		    /* Mount gid for root directory */
-	umode_t mode;		    /* Mount mode for root directory */
 	struct mempolicy *mpol;     /* default memory policy for mappings */
 };