UDP Fragmentation Offload bug in 2.6.32.x

There’s a nasty bug in 2.6.32.x: when UFO (UDP Fragmentation Offload) is enabled on an interface and datagrams follow the software fallback path, NFS sessions get corrupted. It’s easy to reproduce – just copy a large (few hundred MB) file to a UDP NFS mount. The transfer will stall within 1-5 minutes and your copy process on the client will go into D state – stuck on nfs_wait_bit_uninterruptible.

This patch: Herbert Xu: udp: Fix bogus UFO packet generation fixes the problem in 2.6.35-rc4.

Sadly, it’s not fixed in, though I reported it back in early October.

The workaround is obvious: disable UFO.

BTW: this will also hit you if you use qdev on KVM, as it enables several offload features in the guests, including UFO.

UPDATE: still not fixed in See bug #33972.

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