From d0f5a85442d1a0552eae681b2e8cdc86ac08aba2 Mon Sep 17 00:00:00 2001 From: Luis Chamberlain Date: Thu, 9 Mar 2023 15:05:44 -0800 Subject: shmem: update documentation Update the docs to reflect a bit better why some folks prefer tmpfs over ramfs and clarify a bit more about the difference between brd ramdisks. While at it, add THP docs for tmpfs, both the mount options and the sysfs file. Link: https://lkml.kernel.org/r/20230309230545.2930737-6-mcgrof@kernel.org Signed-off-by: Luis Chamberlain Reviewed-by: Christian Brauner Reviewed-by: David Hildenbrand Tested-by: Xin Hao Reviewed-by: Davidlohr Bueso Cc: Adam Manzanares Cc: Davidlohr Bueso Cc: Hugh Dickins Cc: Kees Cook Cc: Matthew Wilcox Cc: Pankaj Raghav Cc: Yosry Ahmed Signed-off-by: Andrew Morton --- Documentation/filesystems/tmpfs.rst | 57 +++++++++++++++++++++++++++++++------ 1 file changed, 49 insertions(+), 8 deletions(-) diff --git a/Documentation/filesystems/tmpfs.rst b/Documentation/filesystems/tmpfs.rst index 0408c245785e..1ec9a9f8196b 100644 --- a/Documentation/filesystems/tmpfs.rst +++ b/Documentation/filesystems/tmpfs.rst @@ -13,14 +13,25 @@ everything stored therein is lost. tmpfs puts everything into the kernel internal caches and grows and shrinks to accommodate the files it contains and is able to swap -unneeded pages out to swap space. It has maximum size limits which can -be adjusted on the fly via 'mount -o remount ...' - -If you compare it to ramfs (which was the template to create tmpfs) -you gain swapping and limit checking. Another similar thing is the RAM -disk (/dev/ram*), which simulates a fixed size hard disk in physical -RAM, where you have to create an ordinary filesystem on top. Ramdisks -cannot swap and you do not have the possibility to resize them. +unneeded pages out to swap space, and supports THP. + +tmpfs extends ramfs with a few userspace configurable options listed and +explained further below, some of which can be reconfigured dynamically on the +fly using a remount ('mount -o remount ...') of the filesystem. A tmpfs +filesystem can be resized but it cannot be resized to a size below its current +usage. tmpfs also supports POSIX ACLs, and extended attributes for the +trusted.* and security.* namespaces. ramfs does not use swap and you cannot +modify any parameter for a ramfs filesystem. The size limit of a ramfs +filesystem is how much memory you have available, and so care must be taken if +used so to not run out of memory. + +An alternative to tmpfs and ramfs is to use brd to create RAM disks +(/dev/ram*), which allows you to simulate a block device disk in physical RAM. +To write data you would just then need to create an regular filesystem on top +this ramdisk. As with ramfs, brd ramdisks cannot swap. brd ramdisks are also +configured in size at initialization and you cannot dynamically resize them. +Contrary to brd ramdisks, tmpfs has its own filesystem, it does not rely on the +block layer at all. Since tmpfs lives completely in the page cache and on swap, all tmpfs pages will be shown as "Shmem" in /proc/meminfo and "Shared" in @@ -85,6 +96,36 @@ mount with such options, since it allows any user with write access to use up all the memory on the machine; but enhances the scalability of that instance in a system with many CPUs making intensive use of it. +tmpfs also supports Transparent Huge Pages which requires a kernel +configured with CONFIG_TRANSPARENT_HUGEPAGE and with huge supported for +your system (has_transparent_hugepage(), which is architecture specific). +The mount options for this are: + +====== ============================================================ +huge=0 never: disables huge pages for the mount +huge=1 always: enables huge pages for the mount +huge=2 within_size: only allocate huge pages if the page will be + fully within i_size, also respect fadvise()/madvise() hints. +huge=3 advise: only allocate huge pages if requested with + fadvise()/madvise() +====== ============================================================ + +There is a sysfs file which you can also use to control system wide THP +configuration for all tmpfs mounts, the file is: + +/sys/kernel/mm/transparent_hugepage/shmem_enabled + +This sysfs file is placed on top of THP sysfs directory and so is registered +by THP code. It is however only used to control all tmpfs mounts with one +single knob. Since it controls all tmpfs mounts it should only be used either +for emergency or testing purposes. The values you can set for shmem_enabled are: + +== ============================================================ +-1 deny: disables huge on shm_mnt and all mounts, for + emergency use +-2 force: enables huge on shm_mnt and all mounts, w/o needing + option, for testing +== ============================================================ tmpfs has a mount option to set the NUMA memory allocation policy for all files in that instance (if CONFIG_NUMA is enabled) - which can be -- cgit