cxgbe(4): Save proper zone index on low memory in refill_fl().

[mav]

Summary

When refill_fl() fails to allocate large (9/16KB) mbuf cluster, it falls back to safe (4KB) ones. But it still saved into sd->zidx the original fl->zidx instead of fl->safe_zidx. It caused number of problems with the later use of that cluster, including possibility of memory and/or data corruption.

While there, make refill_fl() to fallback to the safe zone for all the following clusters of the call.

Test Plan

Fragment memory into 12KB chunks and try to write to iSCSI target from another host. Without the patch the iSCSI connection fails on timeout and/or incorrect values in received iSCSI headers. With the patch the write is slow, but it is working.

[mav]

I've slightly modified the patch to remove lowmem variable.

[jhb]

So lowmem is just an optimization to keep using safe_zidx for the rest of the loop once a zidx has failed once? I could see perhaps doing this as two small commits if so, one to fix the bug (sd->zidx being wrong), and the second to add the lowmem optimization.

[mav]

In D28716#642898, @jhb wrote:

So lowmem is just an optimization to keep using safe_zidx for the rest of the loop once a zidx has failed once?

Right.

I could see perhaps doing this as two small commits if so, one to fix the bug (sd->zidx being wrong), and the second to add the lowmem optimization.

In the new version I've just uploaded I've done that without extra variable, so now it is even smaller change to split between two commits.