This new file aims at documenting the caches that are used by FUSE. At the moment only symlink, attributes, ACLs and readdir caches are described.
Signed-off-by: Luis Henriques <[email protected]> --- .../filesystems/fuse/fuse-caches.rst | 158 ++++++++++++++++++ 1 file changed, 158 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 index 000000000000..e9f60355d7ff --- /dev/null +++ b/Documentation/filesystems/fuse/fuse-caches.rst @@ -0,0 +1,158 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=========== +FUSE Caches +=========== + +Introduction +============ + +This document summarises the different types of caches used in FUSE. For each +cache type, it documents the rules to insert data into it. It also documents the +rules for validating and invalidating data in the cache. + +symlink caching +=============== + +Whenever there's a link resolution request for a FUSE filesystem, the VFS will +call into ``fuse_get_link()``, the ``->get_link()`` inode operation. This +function will then send a ``FUSE_READLINK`` request to the user-space FUSE +server. + +The server can ask the kernel to cache all link resolutions by setting the +``FUSE_CACHE_SYMLINKS`` flag during the ``FUSE_INIT`` negotiation. If this flag +is set, when the VFS calls into the ``->get_link()`` operation, FUSE will +immediately call ``__page_get_link()``. The first time this is done for a +specific inode, it will result in sending the ``FUSE_READLINK`` request to +user-space. But the result returned from this request will then be added into +the page-cache. The next time this link needs to be resolved, it will use the +link resolution already cached, and will only fallback to user-space if the +folio isn't up-to-date. + +Attributes caching +================== + +Inode attributes may be obtained from user-space by different FUSE operations. +For example, ``FUSE_LOOKUP``, ``FUSE_GETATTR``, and also several other +operations that create file system objects (e.g. ``FUSE_MKDIR``). These +attributes obtained from user-space are cached by the kernel. They have, +however, a timeout associated and once it expires, they are invalidated. The +next time the attributes are needed, a request (``FUSE_GETATTR``) will be sent +to the FUSE server. + +The ``FUSE_GETATTR`` request can be sent to user-space in three different +scenarios: + +#. if the attributes for the inode aren't yet available in the kernel; +#. if they are not valid any more (timed-out, or have been invalidated), or +#. if there is an explicit request for forcing the request to be sent (for + example, by using the ``AT_STATX_FORCE_SYNC`` flag in ``statx``). + +Regarding the attributes invalidation, they may happen in several occasions: + +- Upon user-space request for invalidation: + + - A ``FUSE_NOTIFY_INVAL_INODE`` will invalidate ``STATX_BASIC_STATS``; + - ``FUSE_NOTIFY_DELETE`` and ``FUSE_NOTIFY_INVAL_ENTRY`` invalidate + ``FUSE_STATX_MODDIR``. + +- When setting (or removing) an ACL on an inode ``STATX_CTIME`` is invalidated; + if the ``FUSE_POSIX_ACL`` flag was set by the FUSE server, + ``STATX_BASIC_STATS`` will also be invalidated. +- On a ``->rename()`` operation, both the old and the new entities will have + it's ctime invalidated (``STATX_CTIME``). Also, the directories for both the + old and the new entities will also have their attributes invalidated + (``FUSE_STATX_MODDIR``) +- When creating or deleting a new file system object (``->link()/->unlink()``, + ``->symlink()``, ``->mkdir()/->rmdir()``, ``->tmpfile()``, or + ``->atomic_open()``), the directory where the object is created/deleted will + have it's attributes invalidated (``FUSE_STATX_MODDIR``). +- If a ``->link()`` operation is interrupted by a signal (``EINTR``) the inode + being linked will have it's attributes invalidated (``STATX_BASIC_STATS``). +- When doing a readdir (``->iterate_shared()`` operation) and the directory + contents is not cached, ``STATX_ATIME`` attributes will be invalidated. +- When doing a symlink resolution (by sending a ``FUSE_READLINK`` request) + ``STATX_ATIME`` will be invalidated. +- When doing a ``->flush()`` (i.e. sending a ``FUSE_FLUSH`` request) and + writeback cache is enabled, ``STATX_BLOCKS`` will be invalidated. +- When truncating a file on open using ``O_TRUNC`` open flag (and the FUSE + server has set ``FUSE_ATOMIC_O_TRUNC`` during ``FUSE_INIT``), then + ``FUSE_STATX_MODSIZE`` will be invalidated. +- When setting attributes in an inode (``->setattr()``) and there's a signal + that interrupts the operation (``EINTR``), then ``STATX_BASIC_STATS`` will be + invalidated. +- When data is read from a file, ``STATX_ATIME`` will be invalidated (unless the + file system is read-only). +- When data is written info a file, ``FUSE_STATX_MODSIZE`` is invalidated. + +ACL caching +=========== + +FUSE has allowed the usage of POSIX Access Control Lists (ACLs) for a long time, +as they can be set and accessed simply as extended attributes. However, it was +only with the introduction of the ``FUSE_POSIX_ACL`` flag that ACLs started to +be fully supported. Without this flag being set during the ``FUSE_INIT`` +negotiation, 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 the ``FUSE_POSIX_ACL`` flag is set then, when an inode ACL +is accessed, VFS will first check if it's already cached. If it is not, FUSE +``->get_acl()`` operation (``fuse_get_acl()``) is called, which will eventually +send a user-space request. Future accesses to this inode ACL will use the cached +data. + +Setting an ACL in an inode will also result in sending a request to the FUSE +server for setting it. But this operation won't immediately cache the ACL -- it +will only be cached after it is accessed again and requested from user-space. + +On the other hand, ACLs will be removed from the cache in the following +situations: + +- When setting an ACL in an inode (and the ``FUSE_POSIX_ACL`` flag is set), + 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. Thus, any + cached ACLs for this inode are also invalidated. +- While processing ``FUSE_READDIRPLUS`` and an already existing dentry needs to + be updated. +- In general, when there is the need to send a ``FUSE_STATX`` or + ``FUSE_GETATTR`` to user-space (e.g. when attributes expired). + +readdir caching +=============== + +When opening a directory a ``FUSE_OPENDIR`` will be sent to the FUSE server, and +server will be responsible for setting the open flags related with caching, +namely ``FOPEN_KEEP_CACHE`` and ``FOPEN_CACHE_DIR``. + +If neither flags are set by the user-space FUSE server, then every ``readdir`` +will result in a ``FUSE_READDIR`` (or ``FUSE_READDIRPLUS``) request being sent. +If ``FOPEN_CACHE_DIR`` is set by the server, then the result of a ``readdir`` +will be cached by the kernel and reused for the current open. If +``FOPEN_KEEP_CACHE`` is also set, the cache will be kept and reused in the +future, when the directory is open again for reading. + +The readdir cache will also expire and reset if the inode's ``mtime`` or +``iversion`` don't match the cached values, or if the FUSE connection ``epoch`` +doesn't match the cache ``epoch``. + +dentry caching +============== + +TBD + +data caching +============ + +TBD +
