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NFSD/sunrpc: avoid deadlock on TCP connection due to memory pressure.
Since we enabled auto-tuning for sunrpc TCP connections we do not guarantee that there is enough write-space on each connection to queue a reply. If memory pressure causes the window to shrink too small, the request throttling in sunrpc/svc will not accept any requests so no more requests will be handled. Even when pressure decreases the window will not grow again until data is sent on the connection. This means we get a deadlock: no requests will be handled until there is more space, and no space will be allocated until a request is handled. This can be simulated by modifying svc_tcp_has_wspace to inflate the number of byte required and removing the 'svc_sock_setbufsize' calls in svc_setup_socket. I found that multiplying by 16 was enough to make the requirement exceed the default allocation. With this modification in place: mount -o vers=3,proto=tcp 127.0.0.1:/home /mnt would block and eventually time out because the nfs server could not accept any requests. This patch relaxes the request throttling to always allow at least one request through per connection. It does this by checking both sk_stream_min_wspace() and xprt->xpt_reserved are zero. The first is zero when the TCP transmit queue is empty. The second is zero when there are no RPC requests being processed. When both of these are zero the socket is idle and so one more request can safely be allowed through. Applying this patch allows the above mount command to succeed cleanly. Tracing shows that the allocated write buffer space quickly grows and after a few requests are handled, the extra tests are no longer needed to permit further requests to be processed. The main purpose of request throttling is to handle the case when one client is slow at collecting replies and the send queue gets full of replies that the client hasn't acknowledged (at the TCP level) yet. As we only change behaviour when the send queue is empty this main purpose is still preserved. Reported-by: Ben Myers <email@example.com> Signed-off-by: NeilBrown <firstname.lastname@example.org> Signed-off-by: J. Bruce Fields <email@example.com>
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