Re: [PATCH v7 00/31] Introduce discontiguous frames management

On 18/07/2025 20:45, Stefano Brivio wrote:

On Mon, 23 Jun 2025 13:06:04 +0200 Laurent Vivier wrote:

...

This series introduces iov_tail to convey frame information between functions.

This is only an API change, for the moment the memory pool is only able to store contiguous buffer, so, except for vhost-user in a special case, we only play with iovec array with only one entry.

v7: - Add a patch to fix comment style of 'Return:' - Fix ignore_arp()/accept_arp() - Fix coverity error - Fix several comments

I was about to apply this without 1/31 (I applied the v2 of it you sent outside of this series instead, which is actually up to date) and with the minor comment fix to 31/31... but the test perf/passt_vu_tcp fails rather consistently now (and I triple checked without this series):

- "TCP throughput over IPv6: guest to host" with MTU 1500 and 9000 bytes now reports between 0 and 0.6 Gbps. The guest kernel prints a series of two messages with ~1-10 µs interval:

[ 21.159827] TCP: out of memory -- consider tuning tcp_mem [ 21.159831] TCP: out of memory -- consider tuning tcp_mem

- "TCP throughput over IPv4: guest to host" never reports 0 Gbps, but the throughput figure for large MTU (65520 bytes) is very low (5.4 Gbps in the last run). Here I'm getting four messages:

[ 40.807818] TCP: out of memory -- consider tuning tcp_mem [ 40.807829] TCP: out of memory -- consider tuning tcp_mem [ 40.807829] TCP: out of memory -- consider tuning tcp_mem [ 40.807830] TCP: out of memory -- consider tuning tcp_mem

- on the reverse direction, "TCP throughput over IPv4: host to guest" (but not with IPv6), the iperf3 client gets SIGSEGV, but not consistently, it happened once out of five times.

To me it smells a bit like we're leaking virtqueue slots but I looked again at the whole series and I couldn't find anything obvious... at least not yet.

UDP tests never fail and the throughput is the same as before.

I think the problem is the way we use the iovec array. In tap4_handler() we have a packet_get() that provides a pointer to the iovec array from pool. Idx is 0, iovec idx is 0. Then we have a pool_flush(), so first available idx is now 0 again. And then we have packet_add() with the iovec idx (in "data") of the previous packet_get() that we try to add at the same index (as pool is empty again, and first available idx is 0). When I wrote this patch I guessed that when we release packet (pool_flush()) we don't use anymore the iovec array of the packet, it appears to be not true. Thanks, Laurent