Thanks for the tip, Eric.
I spent some time tracing through this in a debugger.
I believe the lingering buffer_heads are allocated by
journal_write_commit_record()->journal_get_descriptor_buffer()->__getblk().
The reference count ends up at zero, but the buffer head is never freed.
As best I can tell, this is correct as long as the associated page is
cached, so that the associated buffer_head can be looked up and reused
later.
I plan to look into this more, but perhaps the issue is just that the
kernel should be more aggressive about freeing cached journal pages?
They are highly unlikely to be used again once a committed transaction
is completely out on disk, as the journal is roughly an append-only log,
right?
Thanks again,
Don
Eric Sandeen wrote:
Don Porter wrote:
Hi,
It appears that the ext3 journal code has a slow leak of buffer_head
structs. Try this simple script:
perl -e 'while(1){ `sync`; }'
and monitor the count of allocated buffer_head structs in
/proc/slabinfo, and it seems to increase without bound. Even after this
script is killed and the machine is left idle for several minutes, the
count of buffer heads doesn't substantially decrease.
Looking around at various machines I have access to, the count of
allocated buffer_heads roughly correlates with uptime when using ext3.
This is a slow leak - one would likely have to run this script for a day
or more to drain enough lowmem to cause problems.
Other info: I have only tried this on x86 machines, but I have tried
both 2.6.22.6 and 2.6.28.8, and both have the problem. I am running
Ubuntu 7.10 on top of these kernels, but the kernels were built directly
from kernel.org tarballs.
Any advice or help with this issue is greatly appreciated.
Without investigating too far yet, I did try this, and did indeed see
the buffer_head usage go up while the script runs.
However, if I did:
# echo 3 > /proc/sys/vm/drop_caches
before the script, and noted the total nr. of buffer heads in use, and
then did it again after the script had been running a while, I got back
to the same (low) count of buffer heads in use. So I don't think this
is a leak as in "the system has lost all accounting of these buffer
heads" at least... but it'd be interesting to know what the reason for
the increase is, I'm not sure offhand.
-Eric
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