On 10/22/22 13:07, Bjoern A. Zeeb wrote:
On Sat, 22 Oct 2022, Colin Percival wrote:
Since pages used for bouncing are typically non-consecutive, each
bounced page will typically constitute a busdma segment; as such, we
are unlikely to ever successfully use more pages than the nsegments
limit. Limit the number of pages reserved to nsegments.
This is an interesting observation. LinuxKPI desperately needs more
consecutive pages but can only ever use one segment.
We do have observed cases of bouncing failing due to the LinuxKPI
constraints and what busdma provided.
Similar issue with Firecracker -- its virtio block backend only supports a
single data segment, so for now our virtio_blk driver is limited to 4 kB
I/Os. I was considering adding code to virtio_blk to reserve a contiguous
buffer and use it when busdma failed, but theoretically this could be done
in busdma instead.
--
Colin Percival
Security Officer Emeritus, FreeBSD | The power to serve
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid
