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

[Amir Goldstein]

On Wed, Jul 8, 2026 at 3:11 PM Luis Henriques <luis@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx>
> ---
>  .../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
> index 000000000000..52b2558b5a68
> --- /dev/null
> +++ b/Documentation/filesystems/fuse/fuse-caches.rst
> @@ -0,0 +1,86 @@
> +.. 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.

It looks a bit misleading as if the list is complete.
Maybe list all the caches: attribute, dentry, data, readdir, symlink, ACL
(anything else?) and then say that documenting them is TBD.

However, I must say that mentioning attribute cache in the context of ACL
cache without a minimal description of attribute cache is a bit confusing.

Thanks,
Amir.

> +
> +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.
>