On 05/25/2010 05:28 AM, Trond Myklebust wrote:
I encountered the following problem. We use short expiration time for
kerberos contexts created by rpc.gssd (some patches were included in mainline
nfs-utils). In particular, we use 120secs expiration time.
Now, I run application that eats 80% of available RAM. Then I run 10 parallel
dd processes that write data into NFS4 volume with sec=krb5.
As soon as the kerberos context expires (i.e., up to 120 secs), the whole
system gets stuck in do_page_fault and succesive functions. It is because
there is no free memory in kernel, all free memory is used as cache for NFS4
(due to dd traffic), kernel ask NFS to write back its pages but NFS cannot do
anything as it is missing valid context. NFS contacts rpc.gssd to provide
a renewed context, the rpc.gssd does not provide the context as it needs some memory
to scan /tmp for a ticket. I.e., it deadlocks.
Longer context expiration time is no real solution as it only makes the
deadlock less often.
Any ideas what can be done here? (Please cc me.) We could preallocate some
memory in rpc.gssd and use mlockall but not sure whether this proctects also
kernel malloc for things related to rpc.gssd and context creation (new file
descriptors and so on).
This is seen in 2.6.32 kernel but most probably this is related to all kernel
versions.
Seems like pretty fundamental problem in nfs :-(. Limiting writeback
caches for nfs, so that system has enough memory to perform rpc calls
with the rest might do the trick, but...
It's the same problem that you have for any file or storage system that
has initiators in userland. On the storage side, iSCSI in particular has
the same problem. On the filesystem side, CIFS, AFS, coda, .... do too.
The clustered filesystems can deadlock if the node that is running the
DLM runs out of memory...
Not so trivially. In ocfs2, the dlm allocates small blocks with GFP_NOFS.
Furthermore, in the time-sensitive recovery thread, it preallocates buffers,
what it can, at create time. That does not mean it is not affected by
memory pressure. It is. But that shows up as slower response and not
a deadlock.
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