On Fri, May 05, 2017 at 10:49:05AM +0000, Gonglei (Arei) wrote:
Currently my workmate encountered an issues in the testing environment:
A letter from him:
In order to boot a BIG vm (with 4T mem, 255 vCPUs, 60 virtio-scsi disk...), i have to increase the BUILD_MAX_HIGHTABLE to 512KB. But, then i found i can not boot a specific VM anymore (oracle linux 6.7 64bits with kernel 3.8.13, 2G mem). It seems be related with the unused high ram given VM back by SeaBios (maybe also related to the memory align). I've tested if setting giveback memory 64KB alignment other than 4KB, SeaBios with 512KB BUILD_MAX_HIGHTABLE can boot the specific VM. But , i don't know the reason...
As far as i know, if seabios does not give back the unused ZoneHigh memory seems will not trigger any problems but just waste only a little memory ,right?
Can you tell me what's the worst effect if i apply the following patch?
Does it influence live migration?
The only downside I'm aware of is that it would waste a small amount of ram.
I'm surprised that "oracle linux" has this issue. I can only see a few reasons for it to fail there - a "use after free" bug in seabios, oracle linux expects the ram to be zero'd, or oracle linux is expecting the e820 map to have an alignment larger than page size. The first seems unlikely as "permanent high ram" is rarely ever free'd in seabios. The second also seems unlikely.