Red Hat Bugzilla – Bug 127897
Having memlock set too low can hang a system using hugetlb/tmpfs for a large Oracle SGA
Last modified: 2007-11-30 17:07:02 EST
Description of problem:
On a RHEL3 system with the hugemem kernel and running Oracle 9i
(184.108.40.206), we're attempting to set up a database with a ~6GB SGA,
comprising about 2.1GB of shared pool (allocated from hugepages) and
4GB of database cache (allocated from /dev/shm, which is mounted as
The database starts successfully and users can make connections
successfully via SQL*Net. However, if a user signs on to the system
in question and tries to run sqlplus directly and their memlock
(ulimit -l) setting is "too low", the entire system will hang as soon
as they enter a password. At this point no further logins are
possible, even on the console. I put "too low" in quotes because
it's not clear how high the value has to be; in my testing values
down to 1000000 were ok, but 500000 caused the system to hang.
Setting it to 4 (the default) hangs it every time.
We discovered this bug because of bug 113335 (apparently ignored
since it was filed), which causes a local user's memlock settings as
specified in limits.conf to be ignored when they signed on via ssh.
To trigger this bug it's also required to
set "use_indirect_data_buffers = true" in the init.ora file for the
database (which causes Oracle to use /dev/shm rather than process
memory for that portion of the SGA).
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. See above.
System operates normally, or fails gracefully.
I should add that if the shared portion of the SGA is bumped up to
about 2.5GB and hugetlb is in use, it's not even necessary to use
sqlplus to hang the system--the system will hang as soon as the
database is started. So among its other problems, hugetlb causes
erratic behavior as process memory usage approaches the 2.7GB
boundary. This seems like a distinct bug to me (though it may be
related), but I won't be filing it separately.
In addition to being a serious stability issue for Oracle 9i
installations using large SGAs, this bug also allows any local user
of such a system to hang that system, so it's potentially a serious
denial of service attack as well. However, I think it's much more
likely to happen through happenstance than it is to be used as an
The workaround so far is just to disable hugetlb. Given the issues
with hugetlb noted in bug 127896, I have to say that it appears that
the hugetlb implementation in RHEL3 is very unstable and should be
avoided like the plague.
How much physical memory does your server have ?
Martin, can try to reproduce this bug in our lab as soon as yuo get a
This bug is filed against RHEL 3, which is in maintenance phase.
During the maintenance phase, only security errata and select mission
critical bug fixes will be released for enterprise products. Since
this bug does not meet that criteria, it is now being closed.
For more information of the RHEL errata support policy, please visit:
If you feel this bug is indeed mission critical, please contact your
support representative. You may be asked to provide detailed
information on how this bug is affecting you.