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