make m_getm2() resilient to zone_jumbop exhaustion

[gallatin]

Summary

Currently, m_getm2() will sleep or fail if we're out of page sized mbufs. This leads to most things using sosend* (like sshd) eventually hanging waiting for memory in the page-size mbuf zone, and makes it impossible to communicate with a box which is under attack and has had its page-sized zone exhausted.

Rather than depending on the page size zone, also try cluster allocations to satisfy the request. This allows me to ssh to, and serve 100Gb/s of traffic from a server which is under attack and has had its page-sized zone exhausted.

Test Plan

Boot with the loader tunable kern.ipc.nmbjumbop="4" and verify that ssh and networking in general still works.

Note via vmstat -z | egrep mbuf\|ITEM that the zone is limited in size, and has failed memory allocations.

[markj]

sys/kern/kern_mbuf.c

1442 ↗

(On Diff #76071)

I think this check can be lifted into the previous block.

1443 ↗

(On Diff #76071)

This test for nm != NULL is redundant.

[rmacklem]

Looks reasonable to me. I once modified the NFS code to use page_size clusters
for larger RPC messages and it could get hung when 4K clusters were exhausted.
--> I never though to do the 4K clusters with M_NOWAIT and then fallback to

regular clusters.

This patch looks fine to me. (I'll let you decide if markj@'s suggested changes
make the code more readable?)

[gallatin]

Address markj's feedback:

• Pull mb == NULL test into the previous block
• Simplify by removing (existing) nm == NULL check and letting that happen in m_freem()

[rlibby]

Is there a possible side effect where if we now never do an M_WAITOK
alloc from zone_jumbop then we may not increase the zone size up to the
limit? As in, should there be a condition in here where we only convert
M_WAITOK to M_NOWAIT if we are actually near the limit? For example
should mb_reclaim arm this?

[gallatin]

In D26150#584330, @rlibby wrote:

Is there a possible side effect where if we now never do an M_WAITOK
alloc from zone_jumbop then we may not increase the zone size up to the
limit? As in, should there be a condition in here where we only convert
M_WAITOK to M_NOWAIT if we are actually near the limit? For example
should mb_reclaim arm this?

I'm woefully ignorant of how UMA manages things, and had not realized that this would cause the zone to be more limited.

Mark, what do you think?

[markj]

In D26150#584330, @rlibby wrote:

Is there a possible side effect where if we now never do an M_WAITOK
alloc from zone_jumbop then we may not increase the zone size up to the
limit?

You mean, because of a memory shortage? In a scenario where we are persistently failing to allocate slabs for UMA I'm not sure that it's very important to prioritize allocation from a specific zone. Maybe I'm misunderstanding?

As in, should there be a condition in here where we only convert
M_WAITOK to M_NOWAIT if we are actually near the limit? For example
should mb_reclaim arm this?

It sounds like you are suggesting a new semantic: sleep during memory shortages, but bail without sleeping if the zone limit is reached. That seems easiest to accomplish with a new malloc flag.