Re: [PATCH 0/1] cache_head leak in sunrpc_cache_lookup()

[NeilBrown <neilb@suse.com>]

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On Wed, Nov 28 2018, J. Bruce Fields wrote:

> On Wed, Nov 28, 2018 at 11:45:46AM +0300, Vasily Averin wrote:
>> Dear all, we have found memory leak on OpenVz7 node and believe it
>> affects mainline too.
>> 
>> sunrpc_cache_lookup() removes exprired cache_head from hash, however
>> if it waits for reply on submitted cache_request both of them can leak
>> forever, nobody cleans unhashed cache_heads.
>> 
>> Originally we had claim on busy loop device of stopped container, that
>> had executed nfs server inside.  Device was kept by mount that was
>> detached from already destroyed mount namespace.  By using crash
>> search we have found some structure with path struct related to our
>> mount.  Finally we have found that it was alive svc_export struct used
>> by to alive cache_request, however both of them pointed to already
>> freed cache_detail.
>> 
>> We decided that cache_detail was correctly freed during destroy of net
>> namespace, however svc_export with taken path struct, cache_request
>> and some other structures seems was leaked forever.
>> 
>> This could  happen only if cache_head of svc_export was removed from
>> hash on cache_detail before its destroy. Finally we have found that it
>> could happen when sunrpc_cache_lookup() removes expired cache_head
>> from hash.
>> 
>> Usually it works correctly and cache_put(freeme) frees expired
>> cache_head.  However in our case cache_head have an extra reference
>> counter from stalled cache_request.  Becasue of cache_head was removed
>> from hash of cache_detail it cannot be found in cache_clean() and its
>> cache_request cannot be freed in cache_dequeue(). Memory leaks
>> forever, exactly like we observed.
>> 
>> After may attempts we have reproduced this situation on OpenVz7
>> kernel, however our reproducer is quite long and complex.
>> Unfortunately we still did not reproduced this problem on mainline
>> kernel and did not validated the patch yet.
>> 
>> It would be great if someone advised us some simple way to trigger
>> described scenario.
>
> I think you should be able to produce hung upcalls by flushing the cache
> (exportfs -f), then stopping mountd, then trying to access the
> filesystem from a client.  Does that help?
>
>> We are not sure that our patch is correct, please let us know if our
>> analyze missed something.
>
> It looks OK to me, but it would be helpful to have Neil's review too.

Yes, it makes sense to me.
 Reviewed-by: NeilBrown <neilb@suse.com>

Thanks,
NeilBrown


>
> I think I'd also copy some of the above into the changelog--e.g. it
> might be useful to document that this can manifest as a stray reference
> cuont on a mount.
>
> --b.

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