Hi Amir, On Sat, Sep 05 2026, Amir Goldstein wrote:
> On Fri, Sep 4, 2026 at 12:38 PM Luis Henriques <[email protected]> wrote: >> >> 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``. >> + > > OK from here... > >> +- 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. > > ... till here - to me it feels like TMI > I don't think it helps to keep a description of the code in a text file > There is very little hope in getting them to stay in sync and very little > point to begin with. This list is not complete even now (e.g. fallocate) > nor should it be complete IMO. > If one wants to know what operations invalidate which attrs one can grep > for fuse_invalidate_attr_mask(). > I think it is more important for the doc to explain the principles behind the > attribute mask and give some examples. Something like: OK, I see what you mean -- keeping this document in lockstep with any code changes would likely be impossible. I appreciate the suggestions below (thanks a lot!). I integrate them in the next revision. Cheers, -- Luís > > FUSE uses fine-grained invalidation masks rather than invalidating all > attributes at once. > The principle is that each operation only invalidates the specific > attributes that the operation > could have changed on the server. The masks used are: > > STATX_ATIME - after reads and readlink, since the server may update access > time > STATX_CTIME - after xattr changes (including ACL set/remove) and rename > STATX_BLOCKS - after a successful flush with writeback cache, since > the server's block count may differ from the local one > FUSE_STATX_MODIFY (STATX_MTIME | STATX_CTIME | STATX_BLOCKS) - after > writeback completion (without writeback cache), since the server may > have updated modification metadata > FUSE_STATX_MODSIZE (FUSE_STATX_MODIFY | STATX_SIZE) - after writes, > truncate-on-open, and fallocate, since the server's size and > modification metadata may have changed > FUSE_STATX_MODDIR (FUSE_STATX_MODSIZE | STATX_NLINK) - after directory > modifications (create, unlink, mkdir, rmdir, rename), since the server > may have updated the directory's size, timestamps, and link count > STATX_BASIC_STATS - as a full invalidation, used for server-initiated > invalidation (FUSE_NOTIFY_INVAL_INODE), interrupted setattr, and > interrupted link > The full set of invalidation points can be found by searching for > fuse_invalidate_attr_mask() in the FUSE source. > > >> + >> +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 > > Isn't this a private case which is already covered by the last item? > >> + 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. > > I think that both ->d_revalidate() and readdirplus could be generalized > to something like "Whenever attributes are refreshed from the server" > > Because those specific cases are not related to ACL specifically, > they are related to attribute cache and to the unwritten dentry cache. > > Again, the idea is to write a doc that explains the principles and is less > likely to get out of sync with the code. > > It's fine to list d_revalidate and READDIRPLUS as examples to the > category, but as a comprehensive list of use caes, I think this is wrong. > >> +- 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. > > AFAIK this is not what KEEP_CACHE means. > It means keeping the cache from the past opens for this open. > IIRC I gave the exact same comment in v2. > > Thanks, > Amir.
