tmpfs,xattr: enable limited user extended attributes

Enable "user." extended attributes on tmpfs, limiting them by tracking
the space they occupy, and deducting that space from the limited ispace
(unless tmpfs mounted with nr_inodes=0 to leave that ispace unlimited).

tmpfs inodes and simple xattrs are both unswappable, and have to be in
lowmem on a 32-bit highmem kernel: so the ispace limit is appropriate
for xattrs, without any need for a further mount option.

Add simple_xattr_space() to give approximate but deterministic estimate
of the space taken up by each xattr: with simple_xattrs_free() outputting
the space freed if required (but kernfs and even some tmpfs usages do not
require that, so don't waste time on strlen'ing if not needed).

Security and trusted xattrs were already supported: for consistency and
simplicity, account them from the same pool; though there's a small risk
that a tmpfs with enough space before would now be considered too small.

When extended attributes are used, "df -i" does show more IUsed and less
IFree than can be explained by the inodes: document that (manpage later).

xfstests tests/generic which were not run on tmpfs before but now pass:
020 037 062 070 077 097 103 117 337 377 454 486 523 533 611 618 728
with no new failures.

Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Message-Id: <2e63b26e-df46-5baa-c7d6-f9a8dd3282c5@google.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 5 insertions, 2 deletions

diff --git a/Documentation/filesystems/tmpfs.rst b/Documentation/filesystems/tmpfs.rst
index 67422ee10e03..56a26c843dbe 100644
--- a/Documentation/filesystems/tmpfs.rst
+++ b/Documentation/filesystems/tmpfs.rst
@@ -21,8 +21,8 @@ explained further below, some of which can be reconfigured dynamically on the
 fly using a remount ('mount -o remount ...') of the filesystem. A tmpfs
 filesystem can be resized but it cannot be resized to a size below its current
 usage. tmpfs also supports POSIX ACLs, and extended attributes for the
-trusted.* and security.* namespaces. ramfs does not use swap and you cannot
-modify any parameter for a ramfs filesystem. The size limit of a ramfs
+trusted.*, security.* and user.* namespaces. ramfs does not use swap and you
+cannot modify any parameter for a ramfs filesystem. The size limit of a ramfs
 filesystem is how much memory you have available, and so care must be taken if
 used so to not run out of memory.
 
@@ -97,6 +97,9 @@ mount with such options, since it allows any user with write access to
 use up all the memory on the machine; but enhances the scalability of
 that instance in a system with many CPUs making intensive use of it.
 
+If nr_inodes is not 0, that limited space for inodes is also used up by
+extended attributes: "df -i"'s IUsed and IUse% increase, IFree decreases.
+
 tmpfs blocks may be swapped out, when there is a shortage of memory.
 tmpfs has a mount option to disable its use of swap: