[RFC PATCH v1 1/5] Documentation: fuse: add document on caches being used by FUSE

[Luis Henriques <luis@igalia.com>]

This new file aims at documenting the caches that are used by FUSE.  At
the moment only the symlink and the ACLs caches are described.

Signed-off-by: Luis Henriques <luis@igalia.com>
---
 .../filesystems/fuse/fuse-caches.rst          | 86 +++++++++++++++++++
 1 file changed, 86 insertions(+)
 create mode 100644 Documentation/filesystems/fuse/fuse-caches.rst

diff --git a/Documentation/filesystems/fuse/fuse-caches.rst b/Documentation/filesystems/fuse/fuse-caches.rst
new file mode 100644
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+.. SPDX-License-Identifier: GPL-2.0
+
+===========
+FUSE Caches
+===========
+
+Introduction
+============
+
+This document summarises the different types of caches that are used in FUSE.
+For each cache type, it attempts to document the rules that are followed to
+insert, validate and invalidate data into the cache.
+
+symlink caching
+===============
+
+Whenever there's a link resolution request, the VFS will call into
+``fuse_get_link()`` which will then send a ``FUSE_READLINK`` request to the
+user-space FUSE server. However, the server can ask the kernel to cache all
+links resolutions by setting the ``FUSE_CACHE_SYMLINKS`` flag during the
+``FUSE_INIT`` negotiation.
+
+If this flag is set, FUSE will immediately call into the VFS
+``__page_get_link()`` from the ``->get_link()`` inode operation. The first time
+this is done for a specific link, it will end-up sending the ``FUSE_READLINK``
+to user-space but the link contents will then be added into page-cache. The next
+time the link needs to be resolved, it will use the link content that is already
+cached, and will only fallback into sending the request to use-space if the
+folio isn't up-to-date.
+
+ACL caching
+===========
+
+FUSE has allowed the usage of POSIX ACLs for a long time as they could be set
+and accessed simply as extended attributes. However, it was only with the
+addition of the ``FUSE_POSIX_ACL`` flag that ACLs started to be fully supported.
+Without this flag, ACLs can still be set, but the VFS won't use them for
+performing permission checks - that would be the user-space server's
+responsibility.
+
+Also, without setting ``FUSE_POSIX_ACL``, ACLs will not be cached by the kernel.
+In this case, new inodes ``i_acl`` and ``i_default_acl`` fields will be set to
+``ACL_DONT_CACHE``.
+
+On the other hand, if ``FUSE_POSIX_ACL`` is set during ``FUSE_INIT``, when an
+ACL is accessed the VFS layer will first check if it's already cached. If it is
+not, FUSE ``->get_acl`` operation is called, which will eventually send a
+user-space request. Future accesses to this inode ACL will then use the cached
+data.
+
+Setting an ACL in an inode, however, won't cache it immediately. It will send
+user-space a request with the new ACL, and the FUSE server may perform some
+modifications before storing it.
+
+On the other hand, ACLs will be removed for the cache in the following
+situations:
+
+-  When setting an ACL in an inode and the user-space server has set the
+   ``FUSE_POSIX_ACL`` flag, all previously cached ACLs for this inode will be
+   invalidated.
+-  When invalidating an inode through the ``FUSE_NOTIFY_INVAL_INODE`` operation.
+-  When ``->d_revalidate()`` is called for a dentry that requires a lookup (e.g.
+   it has expired) and that lookup operation is successful.
+-  When the VFS needs to check access rights for an inode (by calling
+   ``->permission()``), attributes may need to be refreshed. If that happens,
+   any cached ACLs for that inode will be invalidated.
+-  After setting an inode attribute (i.e. operation ``FUSE_SETATTR`` is sent to
+   user-space), the user-space server may have also updated the ACLs, so any
+   cached ACLs for this inode are also invalidated.
+-  While processing ``FUSE_READDIRPLUS`` and a new dentry is added (unless this
+   dentry is already being looked up (``DCACHE_PAR_LOOKUP``))
+-  In general, when there is the need to sent a ``FUSE_STATX`` or
+   ``FUSE_GETATTR`` to user-space (e.g. because the attributes have expired).
+   This may happen in the following cases:
+
+   -  When doing an ``->llseek()`` on a file with ``SEEK_END``, ``SEEK_HOLE`` or
+      ``SEEK_DATA``.
+   -  When the ``FUSE_AUTO_INVAL_DATA`` flag is set at ``INIT`` time (to
+      automatically invalidate cached pages), and a buffered read
+      (``->read_iter()``) past EOF is done on a non-passthrough file.
+   -  When the ``FUSE_WRITEBACK_CACHE`` flag is set at ``INIT`` time, and a
+      buffered write (``->write_iter()``) past EOF is done on a non-passthrough
+      file.
+   -  When the ``FUSE_AUTO_INVAL_DATA`` flag is set at ``INIT`` time and the VFS
+      needs to read a directory contents (``->iterate_shared()``) for a
+      directory that is allowed to be cached.