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>

1 files changed, 0 insertions, 152 deletions

diff --git a/Documentation/vm/zswap.rst b/Documentation/vm/zswap.rst
deleted file mode 100644
index 8edb8d578caf..000000000000
--- a/Documentation/vm/zswap.rst
+++ /dev/null
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-.. _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.