mm: shmem: add a kernel command line to change the default huge policy for tmpfs

Now the tmpfs can allow to allocate any sized large folios, and the default
huge policy is still preferred to be 'never'. Due to tmpfs not behaving like
other file systems in some cases as previously explained by David[1]:

: I think I raised this in the past, but tmpfs/shmem is just like any
: other file system .. except it sometimes really isn't and behaves much
: more like (swappable) anonymous memory. (or mlocked files)
: 
: There are many systems out there that run without swap enabled, or with
: extremely minimal swap (IIRC until recently kubernetes was completely
: incompatible with swapping). Swap can even be disabled today for shmem
: using a mount option.
: 
: That's a big difference to all other file systems where you are
: guaranteed to have backend storage where you can simply evict under
: memory pressure (might temporarily fail, of course).
: 
: I *think* that's the reason why we have the "huge=" parameter that also
: controls the THP allocations during page faults (IOW possible memory
: over-allocation). Maybe also because it was a new feature, and we only
: had a single THP size.

Thus adding a new command line to change the default huge policy will be
helpful to use the large folios for tmpfs, which is similar to the
'transparent_hugepage_shmem' cmdline for shmem.

[1] https://lore.kernel.org/all/cbadd5fe-69d5-4c21-8eb8-3344ed36c721@redhat.com/

Link: https://lkml.kernel.org/r/ff390b2656f0d39649547f8f2cbb30fcb7e7be2d.1732779148.git.baolin.wang@linux.alibaba.com
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Daniel Gomez <da.gomez@samsung.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -6992,6 +6992,13 @@
 			See Documentation/admin-guide/mm/transhuge.rst
 			for more details.
 
+	transparent_hugepage_tmpfs= [KNL]
+			Format: [always|within_size|advise|never]
+			Can be used to control the default hugepage allocation policy
+			for the tmpfs mount.
+			See Documentation/admin-guide/mm/transhuge.rst
+			for more details.
+
 	trusted.source=	[KEYS]
 			Format: <string>
 			This parameter identifies the trust source as a backend

Documentation/admin-guide/mm/transhuge.rst

@@ -332,6 +332,12 @@ allocation policy for the internal shmem mount by using the kernel parameter
 seven valid policies for shmem (``always``, ``within_size``, ``advise``,
 ``never``, ``deny``, and ``force``).
 
+Similarly to ``transparent_hugepage_shmem``, you can control the default
+hugepage allocation policy for the tmpfs mount by using the kernel parameter
+``transparent_hugepage_tmpfs=<policy>``, where ``<policy>`` is one of the
+four valid policies for tmpfs (``always``, ``within_size``, ``advise``,
+``never``). The tmpfs mount default policy is ``never``.
+
 In the same manner as ``thp_anon`` controls each supported anonymous THP
 size, ``thp_shmem`` controls each supported shmem THP size. ``thp_shmem``
 has the same format as ``thp_anon``, but also supports the policy

mm/shmem.c

@@ -553,6 +553,7 @@ static bool shmem_confirm_swap(struct address_space *mapping,
 /* 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;
 
 /**
  * shmem_mapping_size_orders - Get allowable folio orders for the given file size.
@@ -4953,7 +4954,12 @@ static int shmem_fill_super(struct super_block *sb, struct fs_context *fc)
 	sbinfo->gid = ctx->gid;
 	sbinfo->full_inums = ctx->full_inums;
 	sbinfo->mode = ctx->mode;
-	sbinfo->huge = ctx->huge;
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	if (ctx->seen & SHMEM_SEEN_HUGE)
+		sbinfo->huge = ctx->huge;
+	else
+		sbinfo->huge = tmpfs_huge;
+#endif
 	sbinfo->mpol = ctx->mpol;
 	ctx->mpol = NULL;
 
@@ -5504,6 +5510,21 @@ static int __init setup_transparent_hugepage_shmem(char *str)
 }
 __setup("transparent_hugepage_shmem=", setup_transparent_hugepage_shmem);
 
+static int __init setup_transparent_hugepage_tmpfs(char *str)
+{
+	int huge;
+
+	huge = shmem_parse_huge(str);
+	if (huge < 0) {
+		pr_warn("transparent_hugepage_tmpfs= cannot parse, ignored\n");
+		return huge;
+	}
+
+	tmpfs_huge = huge;
+	return 1;
+}
+__setup("transparent_hugepage_tmpfs=", setup_transparent_hugepage_tmpfs);
+
 static char str_dup[PAGE_SIZE] __initdata;
 static int __init setup_thp_shmem(char *str)
 {