[PATCH 6/8] add cleancache documentation

[Suren Baghdasaryan <surenb@google.com>]

Document cleancache, it's APIs and sysfs interface.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 Documentation/mm/cleancache.rst | 112 ++++++++++++++++++++++++++++++++
 MAINTAINERS                     |   1 +
 2 files changed, 113 insertions(+)
 create mode 100644 Documentation/mm/cleancache.rst

diff --git a/Documentation/mm/cleancache.rst b/Documentation/mm/cleancache.rst
new file mode 100644
index 000000000000..deaf7de51829
--- /dev/null
+++ b/Documentation/mm/cleancache.rst
@@ -0,0 +1,112 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==========
+Cleancache
+==========
+
+Motivation
+==========
+
+Cleancache is a feature to utilize unused reserved memory for extending
+page cache.
+
+Cleancache can be thought of as a folio-granularity victim cache for clean
+file-backed pages that the kernel's pageframe replacement algorithm (PFRA)
+would like to keep around, but can't since there isn't enough memory. So
+when the PFRA "evicts" a folio, it stores the data contained in the folio
+into cleancache memory which is not directly accessible or addressable by
+the kernel (transcendent memory) and is of unknown and possibly
+time-varying size.
+
+Later, when a filesystem wishes to access a folio in a file on disk, it
+first checks cleancache to see if it already contains required data; if it
+does, the folio data is copied into the kernel and a disk access is
+avoided.
+
+The memory cleancache uses is donated by other system components, which
+reserve memory not directly addressable by the kernel. By donating this
+memory to cleancache, the memory owner enables its utilization while it
+is not used. Memory donation is done using cleancache backend API and any
+donated memory can be taken back at any time by its donor without no delay
+and with guarantees success. Since cleancache uses this memory only to
+store clean file-backed data, it can be dropped at any time and therefore
+the donor's request to take back the memory can be always satisfied.
+
+Implementation Overview
+=======================
+
+Cleancache "backend" (donor that provides transcendent memory), registers
+itself with cleancache "frontend" and received a unique pool_id which it
+can use in all later API calls to identify the pool of folios it donates.
+Once registered, backend can call cleancache_backend_put_folio() or
+cleancache_backend_put_folios() to donate memory to cleancache. Note that
+cleancache currently supports only 0-order folios and will not accept
+larger-order ones. Once the backend needs that memory back, it can get it
+by calling cleancache_backend_get_folio(). Only the original backend can
+take the folio it donated from the cleancache.
+
+Kernel uses cleancache by first calling cleancache_add_fs() to register
+each file system and then using a combination of cleancache_store_folio(),
+cleancache_restore_folio(), cleancache_invalidate_{folio|inode} to store,
+restore and invalidate folio content.
+cleancache_{start|end}_inode_walk() are used to walk over folios inside
+an inode and cleancache_restore_from_inode() is used to restore folios
+during such walks.
+
+From kernel's point of view folios which are copied into cleancache have
+an indefinite lifetime which is completely unknowable by the kernel and so
+may or may not still be in cleancache at any later time. Thus, as its name
+implies, cleancache is not suitable for dirty folios. Cleancache has
+complete discretion over what folios to preserve and what folios to discard
+and when.
+
+Cleancache Performance Metrics
+==============================
+
+If CONFIG_CLEANCACHE_SYSFS is enabled, monitoring of cleancache performance
+can be done via sysfs in the `/sys/kernel/mm/cleancache` directory.
+The effectiveness of cleancache can be measured (across all filesystems)
+with provided stats.
+Global stats are published directly under `/sys/kernel/mm/cleancache` and
+include:
+
+``stored``
+	number of successful cleancache folio stores.
+
+``skipped``
+	number of folios skipped during cleancache store operation.
+
+``restored``
+	number of successful cleancache folio restore operations.
+
+``missed``
+	number of failed cleancache folio restore operations.
+
+``reclaimed``
+	number of folios reclaimed from the cleancache due to insufficient
+	memory.
+
+``recalled``
+	number of times cleancache folio content was discarded as a result
+	of the cleancache backend taking the folio back.
+
+``invalidated``
+	number of times cleancache folio content was discarded as a result
+	of invalidation.
+
+``cached``
+	number of folios currently cached in the cleancache.
+
+Per-pool stats are published under `/sys/kernel/mm/cleancache/<pool name>`
+where "pool name" is the name pool was registered under. These stats
+include:
+
+``size``
+	number of folios donated to this pool.
+
+``cached``
+	number of folios currently cached in the pool.
+
+``recalled``
+	number of times cleancache folio content was discarded as a result
+	of the cleancache backend taking the folio back from the pool.
diff --git a/MAINTAINERS b/MAINTAINERS
index 1c97227e7ffa..441e68c94177 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -6053,6 +6053,7 @@ CLEANCACHE
 M:	Suren Baghdasaryan <surenb@google.com>
 L:	linux-mm@kvack.org
 S:	Maintained
+F:	Documentation/mm/cleancache.rst
 F:	include/linux/cleancache.h
 F:	mm/cleancache.c
 F:	mm/cleancache_sysfs.c
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