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

["J. Bruce Fields" <bfields@fieldses.org>]

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.

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.