Documentation/vm: move user guides to admin-guide/mm/

Most memory management user guide documents are in 'admin-guide/mm/',
but two of those are in 'vm/'.  This moves the two docs into
'admin-guide/mm' for easier documents finding.

Link: https://lkml.kernel.org/r/20210917123958.3819-2-sj@kernel.org
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 234 insertions, 0 deletions

diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst
index cbd19d5e625f..c21b5823f126 100644
--- a/Documentation/admin-guide/mm/index.rst
+++ b/Documentation/admin-guide/mm/index.rst
@@ -37,5 +37,7 @@ the Linux memory management.
    numaperf
    pagemap
    soft-dirty
+   swap_numa
    transhuge
    userfaultfd
+   zswap
diff --git a/Documentation/admin-guide/mm/swap_numa.rst b/Documentation/admin-guide/mm/swap_numa.rst
new file mode 100644
index 000000000000..e0466f2db8fa
--- /dev/null
+++ b/Documentation/admin-guide/mm/swap_numa.rst
@@ -0,0 +1,80 @@
+.. _swap_numa:
+
+===========================================
+Automatically bind swap device to numa node
+===========================================
+
+If the system has more than one swap device and swap device has the node
+information, we can make use of this information to decide which swap
+device to use in get_swap_pages() to get better performance.
+
+
+How to use this feature
+=======================
+
+Swap device has priority and that decides the order of it to be used. To make
+use of automatically binding, there is no need to manipulate priority settings
+for swap devices. e.g. on a 2 node machine, assume 2 swap devices swapA and
+swapB, with swapA attached to node 0 and swapB attached to node 1, are going
+to be swapped on. Simply swapping them on by doing::
+
+	# swapon /dev/swapA
+	# swapon /dev/swapB
+
+Then node 0 will use the two swap devices in the order of swapA then swapB and
+node 1 will use the two swap devices in the order of swapB then swapA. Note
+that the order of them being swapped on doesn't matter.
+
+A more complex example on a 4 node machine. Assume 6 swap devices are going to
+be swapped on: swapA and swapB are attached to node 0, swapC is attached to
+node 1, swapD and swapE are attached to node 2 and swapF is attached to node3.
+The way to swap them on is the same as above::
+
+	# swapon /dev/swapA
+	# swapon /dev/swapB
+	# swapon /dev/swapC
+	# swapon /dev/swapD
+	# swapon /dev/swapE
+	# swapon /dev/swapF
+
+Then node 0 will use them in the order of::
+
+	swapA/swapB -> swapC -> swapD -> swapE -> swapF
+
+swapA and swapB will be used in a round robin mode before any other swap device.
+
+node 1 will use them in the order of::
+
+	swapC -> swapA -> swapB -> swapD -> swapE -> swapF
+
+node 2 will use them in the order of::
+
+	swapD/swapE -> swapA -> swapB -> swapC -> swapF
+
+Similaly, swapD and swapE will be used in a round robin mode before any
+other swap devices.
+
+node 3 will use them in the order of::
+
+	swapF -> swapA -> swapB -> swapC -> swapD -> swapE
+
+
+Implementation details
+======================
+
+The current code uses a priority based list, swap_avail_list, to decide
+which swap device to use and if multiple swap devices share the same
+priority, they are used round robin. This change here replaces the single
+global swap_avail_list with a per-numa-node list, i.e. for each numa node,
+it sees its own priority based list of available swap devices. Swap
+device's priority can be promoted on its matching node's swap_avail_list.
+
+The current swap device's priority is set as: user can set a >=0 value,
+or the system will pick one starting from -1 then downwards. The priority
+value in the swap_avail_list is the negated value of the swap device's
+due to plist being sorted from low to high. The new policy doesn't change
+the semantics for priority >=0 cases, the previous starting from -1 then
+downwards now becomes starting from -2 then downwards and -1 is reserved
+as the promoted value. So if multiple swap devices are attached to the same
+node, they will all be promoted to priority -1 on that node's plist and will
+be used round robin before any other swap devices.
diff --git a/Documentation/admin-guide/mm/zswap.rst b/Documentation/admin-guide/mm/zswap.rst
new file mode 100644
index 000000000000..8edb8d578caf
--- /dev/null
+++ b/Documentation/admin-guide/mm/zswap.rst
@@ -0,0 +1,152 @@
+.. _zswap:
+
+=====
+zswap
+=====
+
+Overview
+========
+
+Zswap is a lightweight compressed cache for swap pages. It takes pages that are
+in the process of being swapped out and attempts to compress them into a
+dynamically allocated RAM-based memory pool.  zswap basically trades CPU cycles
+for potentially reduced swap I/O.  This trade-off can also result in a
+significant performance improvement if reads from the compressed cache are
+faster than reads from a swap device.
+
+.. note::
+   Zswap is a new feature as of v3.11 and interacts heavily with memory
+   reclaim.  This interaction has not been fully explored on the large set of
+   potential configurations and workloads that exist.  For this reason, zswap
+   is a work in progress and should be considered experimental.
+
+   Some potential benefits:
+
+* Desktop/laptop users with limited RAM capacities can mitigate the
+  performance impact of swapping.
+* Overcommitted guests that share a common I/O resource can
+  dramatically reduce their swap I/O pressure, avoiding heavy handed I/O
+  throttling by the hypervisor. This allows more work to get done with less
+  impact to the guest workload and guests sharing the I/O subsystem
+* Users with SSDs as swap devices can extend the life of the device by
+  drastically reducing life-shortening writes.
+
+Zswap evicts pages from compressed cache on an LRU basis to the backing swap
+device when the compressed pool reaches its size limit.  This requirement had
+been identified in prior community discussions.
+
+Whether Zswap is enabled at the boot time depends on whether
+the ``CONFIG_ZSWAP_DEFAULT_ON`` Kconfig option is enabled or not.
+This setting can then be overridden by providing the kernel command line
+``zswap.enabled=`` option, for example ``zswap.enabled=0``.
+Zswap can also be enabled and disabled at runtime using the sysfs interface.
+An example command to enable zswap at runtime, assuming sysfs is mounted
+at ``/sys``, is::
+
+	echo 1 > /sys/module/zswap/parameters/enabled
+
+When zswap is disabled at runtime it will stop storing pages that are
+being swapped out.  However, it will _not_ immediately write out or fault
+back into memory all of the pages stored in the compressed pool.  The
+pages stored in zswap will remain in the compressed pool until they are
+either invalidated or faulted back into memory.  In order to force all
+pages out of the compressed pool, a swapoff on the swap device(s) will
+fault back into memory all swapped out pages, including those in the
+compressed pool.
+
+Design
+======
+
+Zswap receives pages for compression through the Frontswap API and is able to
+evict pages from its own compressed pool on an LRU basis and write them back to
+the backing swap device in the case that the compressed pool is full.
+
+Zswap makes use of zpool for the managing the compressed memory pool.  Each
+allocation in zpool is not directly accessible by address.  Rather, a handle is
+returned by the allocation routine and that handle must be mapped before being
+accessed.  The compressed memory pool grows on demand and shrinks as compressed
+pages are freed.  The pool is not preallocated.  By default, a zpool
+of type selected in ``CONFIG_ZSWAP_ZPOOL_DEFAULT`` Kconfig option is created,
+but it can be overridden at boot time by setting the ``zpool`` attribute,
+e.g. ``zswap.zpool=zbud``. It can also be changed at runtime using the sysfs
+``zpool`` attribute, e.g.::
+
+	echo zbud > /sys/module/zswap/parameters/zpool
+
+The zbud type zpool allocates exactly 1 page to store 2 compressed pages, which
+means the compression ratio will always be 2:1 or worse (because of half-full
+zbud pages).  The zsmalloc type zpool has a more complex compressed page
+storage method, and it can achieve greater storage densities.  However,
+zsmalloc does not implement compressed page eviction, so once zswap fills it
+cannot evict the oldest page, it can only reject new pages.
+
+When a swap page is passed from frontswap to zswap, zswap maintains a mapping
+of the swap entry, a combination of the swap type and swap offset, to the zpool
+handle that references that compressed swap page.  This mapping is achieved
+with a red-black tree per swap type.  The swap offset is the search key for the
+tree nodes.
+
+During a page fault on a PTE that is a swap entry, frontswap calls the zswap
+load function to decompress the page into the page allocated by the page fault
+handler.
+
+Once there are no PTEs referencing a swap page stored in zswap (i.e. the count
+in the swap_map goes to 0) the swap code calls the zswap invalidate function,
+via frontswap, to free the compressed entry.
+
+Zswap seeks to be simple in its policies.  Sysfs attributes allow for one user
+controlled policy:
+
+* max_pool_percent - The maximum percentage of memory that the compressed
+  pool can occupy.
+
+The default compressor is selected in ``CONFIG_ZSWAP_COMPRESSOR_DEFAULT``
+Kconfig option, but it can be overridden at boot time by setting the
+``compressor`` attribute, e.g. ``zswap.compressor=lzo``.
+It can also be changed at runtime using the sysfs "compressor"
+attribute, e.g.::
+
+	echo lzo > /sys/module/zswap/parameters/compressor
+
+When the zpool and/or compressor parameter is changed at runtime, any existing
+compressed pages are not modified; they are left in their own zpool.  When a
+request is made for a page in an old zpool, it is uncompressed using its
+original compressor.  Once all pages are removed from an old zpool, the zpool
+and its compressor are freed.
+
+Some of the pages in zswap are same-value filled pages (i.e. contents of the
+page have same value or repetitive pattern). These pages include zero-filled
+pages and they are handled differently. During store operation, a page is
+checked if it is a same-value filled page before compressing it. If true, the
+compressed length of the page is set to zero and the pattern or same-filled
+value is stored.
+
+Same-value filled pages identification feature is enabled by default and can be
+disabled at boot time by setting the ``same_filled_pages_enabled`` attribute
+to 0, e.g. ``zswap.same_filled_pages_enabled=0``. It can also be enabled and
+disabled at runtime using the sysfs ``same_filled_pages_enabled``
+attribute, e.g.::
+
+	echo 1 > /sys/module/zswap/parameters/same_filled_pages_enabled
+
+When zswap same-filled page identification is disabled at runtime, it will stop
+checking for the same-value filled pages during store operation. However, the
+existing pages which are marked as same-value filled pages remain stored
+unchanged in zswap until they are either loaded or invalidated.
+
+To prevent zswap from shrinking pool when zswap is full and there's a high
+pressure on swap (this will result in flipping pages in and out zswap pool
+without any real benefit but with a performance drop for the system), a
+special parameter has been introduced to implement a sort of hysteresis to
+refuse taking pages into zswap pool until it has sufficient space if the limit
+has been hit. To set the threshold at which zswap would start accepting pages
+again after it became full, use the sysfs ``accept_threshold_percent``
+attribute, e. g.::
+
+	echo 80 > /sys/module/zswap/parameters/accept_threshold_percent
+
+Setting this parameter to 100 will disable the hysteresis.
+
+A debugfs interface is provided for various statistic about pool size, number
+of pages stored, same-value filled pages and various counters for the reasons
+pages are rejected.