130 lines
6.4 KiB
Plaintext
130 lines
6.4 KiB
Plaintext
NFS Attribute Caching OS Problems and Amd
|
|
Last updated September 18, 2005
|
|
|
|
* Summary:
|
|
|
|
Some OSs don't seem to have a way to turn off the NFS attribute cache, which
|
|
breaks the Amd automounter so badly that it is not recommend using Amd on
|
|
such OS for heavy use, not until this is fixed.
|
|
|
|
|
|
* Details:
|
|
|
|
Amd is a user-level NFSv2 server that manages automounts of all other file
|
|
systems. The kernel contacts Amd via RPCs, and Amd in turn performs the
|
|
actual mounts, and then responds back to the kernel's RPCs. Every kernel
|
|
caches attributes of files, in a cache called the Directory Name Lookup
|
|
Cache (DNLC), or a Directory Cache (dcache).
|
|
|
|
Amd manages its namespace in the user level, but the kernel caches names
|
|
itself. So the two must coordinate to ensure that both namespaces are in
|
|
sync. If the kernel uses a cached entry from the DNLC, without consulting
|
|
Amd, users may see corruption of the automounter namespace (symlinks
|
|
pointing to the wrong places, ESTALE errors, and more). For example,
|
|
suppose Amd timed out an entry and removed the entry from Amd's namespace.
|
|
Amd has to tell the kernel to purge its corresponding DNLC entry too. The
|
|
way Amd often does that is by incrementing the last modification time
|
|
(mtime) of the parent directory. This is the most common method for kernels
|
|
to check if their DNLC entries are stale: if the parent directory mtime is
|
|
newer, the kernel will discard all cached entries for that directory, and
|
|
will re-issue lookup methods. Those lookups will result in
|
|
NFS_GETATTR/NFS_LOOKUP calls sent from the kernel down to Amd, and Amd can
|
|
then properly inform the kernel of the new state of automounted entries.
|
|
|
|
In order to ensure that Amd is "in charge" of its namespace without
|
|
interference from the kernel, Amd will try to turn off the NFS attribute
|
|
cache. It does so by using the NFSMNT_NOAC flag, if it exists, or by
|
|
setting various "cache timeout" fields in struct nfs_args to 0 (acregmin,
|
|
acregmax, acdirmin, or acdirmax).
|
|
|
|
We have released a major new version of am-utils, version 6.1, in June 2005.
|
|
Since then, a lot of people have experimented with Amd, in anticipation of
|
|
migrating from the very old am-utils 6.0 to the new 6.1. For a couple of
|
|
months since the release of 6.1, we have received reports of problems with
|
|
Amd, especially under heavy use. Users reported getting ESTALE errors from
|
|
time to time, or seeing automounted entries whose symlinks don't point to
|
|
where it should be. After much debugging, we traced it to a few places in
|
|
Amd where it wasn't updating the parent directory mtime as it should have;
|
|
in some places where Amd was indeed updating the mtime, it was using a
|
|
resolution of only 1 second, which was not fine enough under heavy load. We
|
|
fixed this problem and switched to using a microsecond resolution mtime.
|
|
|
|
After fixing this in Amd, we went on to verify that things work for other
|
|
OSs. When we got to test certain BSDs, we found out that they always cache
|
|
directory entries, and there is no way to turn it off completely.
|
|
Specifically, if we set the ac{reg,dir}{min,max} fields in struct nfs_args
|
|
all to zero, the kernel seems to cache the entries for a default number of
|
|
seconds (something like 5-30 seconds). On some OSs, setting these four
|
|
fields to 0 turns off the attribute cache, but not on some BSDs. We were
|
|
able to verify this using Amd and a script that exercises the interaction of
|
|
the kernel's attrcache and Amd. (If you're interested, the script can be
|
|
made available.)
|
|
|
|
We then experimented by setting the ac{reg,dir}{min,max} fields in struct
|
|
nfs_args all to 1, the smallest non-zero value we could. When we ran the
|
|
Amd exercising script, we found that the value of 1 reduced the race between
|
|
the DNLC and Amd, and the script took a little longer to run before it
|
|
detected an incoherency. That makes sense: the smaller the DNLC cache
|
|
interval is, the shorter the window of vulnerability is. (BTW, the man
|
|
pages on some OSs say that the ac{reg,dir}{min,max} fields use a 1 second
|
|
resolution, but experimentation indicated it was in 0.1 second units.)
|
|
|
|
Clearly, setting the ac{reg,dir}{min,max} fields to 0 is worse than setting
|
|
it to 1 on those OSs that don't have a way to turn off the attribute cache.
|
|
So the current workaround I've implemented in am-utils is to create a
|
|
configuration parameter called "broken_attrcache" which, if turned on, will
|
|
set these nfs_args fields to 1 instead of 0. I wish I didn't have to create
|
|
such ugly workaround features in Amd, but I've got no choice.
|
|
|
|
The near term solution is for every OS to support a true 'noac' flag, which
|
|
can be added fairly easily. This'd make Amd work reliably.
|
|
|
|
The long term solution is to implement Autofs support for all OSs and to
|
|
support it in Amd. Currently, Amd supports autofs on Solaris and Linux;
|
|
FreeBSD is next. Still, we found that even with autofs support, many
|
|
sysadmins still prefer to use the good 'ol non-autofs mode.
|
|
|
|
|
|
* Confirmed Status
|
|
|
|
This is the confirmed status of various OSs' vulnerability to this attribute
|
|
cache bug. We are slowly checking the status of other OSs. The status of
|
|
any OS not listed is unknown as of the date at the top of this file.
|
|
|
|
** Not Vulnerable (support a proper "noac" flag):
|
|
|
|
Sun Solaris 8 and 9 (10 probably works fine)
|
|
Linux: 2.6.11 kernel (2.4.latest probably works fine)
|
|
FreeBSD 5.4 and 6.0-SNAP001 (older versions probably work fine)
|
|
OpenBSD 3.7 (older versions probably work fine)
|
|
|
|
** Vulnerable (don't support a proper "noac" flag natively):
|
|
|
|
NetBSD 2.0.2 (older versions are also probably affected)
|
|
|
|
Note: NetBSD has promised to support a noac flag hopefully after 2.1.0 is
|
|
released (maybe in 3.0 or 2.2). In the mean time, you can apply one of
|
|
these two kernel patchs to support a 'noac' flag in NetBSD 2.x or 3.x:
|
|
ftp://ftp.netbsd.org/pub/NetBSD/misc/christos/2x.nfs.noac.diff
|
|
ftp://ftp.netbsd.org/pub/NetBSD/misc/christos/3x.nfs.noac.diff
|
|
After applying this patch and rebuilding your kernel, reboot with the new
|
|
kernel. Then copy the new nfs.h and nfsmount.h from /sys/nfs/ to
|
|
/usr/include/nfs/, and finally rebuild am-utils from scratch.
|
|
|
|
** Testing
|
|
|
|
When you build am-utils, a script named scripts/test-attrcache is built,
|
|
which can be used to test the NFS attribute cache behavior of the current
|
|
OS. You can run this script as root as follows:
|
|
|
|
# make install
|
|
# cd scripts
|
|
# sh test-attrcache
|
|
|
|
If you run this script on an OS whose status is known (and not listed
|
|
above), please report it to us via Bugzilla or the am-utils mailing list
|
|
(see www.am-utils.org), so we can record it in this file.
|
|
|
|
Sincerely,
|
|
Erez.
|