shmem-add-shmem_size-option-for-full-filesystem.v5.4-rc2.patch

From 04d8722cad919fc62bc0df11e219ba3ff59bd44f Mon Sep 17 00:00:00 2001
From: "Gary B. Genett" <me@garybgenett.net>
Date: Tue, 8 Oct 2019 12:12:46 -0700
Subject: [PATCH] shmem: add shmem_size option for full filesystem

Adds a kernel configuration option to specify the size of the shmem
filesystem.  It is currently hard-coded to 50% of memory.  Users should
have the option to set this value as they see fit.

A specific case where this would be necessary is if the initramfs were
larger than half of the memory, such as a 2.5GB "live" filesystem on
a system with 4GB of memory.  Without this option, this causes a kernel
panic.  With this option, the user may specify the number of pages of
memory they need for their root filesystem.

This patch creates the SHMEM_SIZE configuration option, which is
specified as the number of memory pages to use for the shmem
filesystem.  The default remains unchanged.  This patch has no impact
unless the values are changed.

The option is marked as expert, and the help text is clear that it
should only be set if the user knows what they are doing.

Signed-off-by: Gary B. Genett <me@garybgenett.net>
---
 init/Kconfig | 11 +++++++++++
 mm/shmem.c   | 20 ++++++++++++++++++--
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index b4daad2bac23..fcff0f1ea3e8 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1537,6 +1537,17 @@ config SHMEM
 	  option replaces shmem and tmpfs with the much simpler ramfs code,
 	  which may be appropriate on small systems without swap.
 
+config SHMEM_SIZE
+	int "Set the full shmem filesystem size" if EXPERT
+	default 0
+	depends on SHMEM
+	help
+	   Size of full shmem filesystem.  Defaults to 50% of memory,
+	   dynamically.  Must be defined as the number of 4096 byte memory
+	   pages, which will be static regardless of the amount of actual
+	   memory.  Unless you have a special use case, this value should not
+	   be changed.
+
 config AIO
 	bool "Enable AIO support" if EXPERT
 	default y
diff --git a/mm/shmem.c b/mm/shmem.c
index cd570cc79c76..d5baaf7ac9e8 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -95,6 +95,13 @@ static struct vfsmount *shm_mnt;
 /* Symlink up to this size is kmalloc'ed instead of using a swappable page */
 #define SHORT_SYMLINK_LEN 128
 
+/* Default size of shmem filesystem (defaults to 50% of memory) */
+#if defined(CONFIG_SHMEM_SIZE) && (CONFIG_SHMEM_SIZE > 0)
+#define SHMEM_SIZE_DEFAULT ((unsigned long)CONFIG_SHMEM_SIZE)
+#else
+#define SHMEM_SIZE_DEFAULT (totalram_pages() / 2)
+#endif
+
 /*
  * shmem_fallocate communicates with shmem_fault or shmem_writepage via
  * inode->i_private (with i_mutex making sure that it has only one user at
@@ -123,16 +130,22 @@ struct shmem_options {
 };
 
 #ifdef CONFIG_TMPFS
+static void shmem_size_info(void)
+{
+	pr_info("shmem: setting default size: %lu pages, %lu bytes\n",
+		SHMEM_SIZE_DEFAULT, (SHMEM_SIZE_DEFAULT * PAGE_SIZE));
+}
+
 static unsigned long shmem_default_max_blocks(void)
 {
-	return totalram_pages() / 2;
+	return SHMEM_SIZE_DEFAULT;
 }
 
 static unsigned long shmem_default_max_inodes(void)
 {
 	unsigned long nr_pages = totalram_pages();
 
-	return min(nr_pages - totalhigh_pages(), nr_pages / 2);
+	return min(nr_pages - totalhigh_pages(), SHMEM_SIZE_DEFAULT);
 }
 #endif
 
@@ -3887,6 +3900,9 @@ int __init shmem_init(void)
 {
 	int error;
 
+#ifdef CONFIG_TMPFS
+	shmem_size_info();
+#endif
 	shmem_init_inodecache();
 
 	error = register_filesystem(&shmem_fs_type);
-- 
2.15.2