Re: [PATCH] shmem: add shmem_size option for full filesystem

[Vlastimil Babka <vbabka@suse.cz>]

On 10/8/19 10:17 PM, Gary B. Genett wrote:
> 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.

Users can override the default via a mount option, no?

> 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.

Can the initramfs boot parameter specify the needed mount option
somehow? If not, could we detect that we are mounting an initramfs, and
size it accordingly over 50%? Do we know in advance how much we need?
I'd rather avoid a new config option for this, if possible.

> 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);
>