On Fri, Mar 2, 2018 at 7:51 AM, David Gibson <da...@gibson.dropbear.id.au> wrote:
> On Fri, Feb 23, 2018 at 03:02:40PM +0530, Bharata B Rao wrote: > > Hi, > > > > Rebooting a hash guest after hotplugging memory to it is crashing the > > guest. This is seen only when HPT resizing is enabled. I see guest > crashing > > at multiple places, but this location is fairly commonly seen: > > > > kernel BUG at mm/slub.c:3912! > > > > Testing with latest guest kernel and ppc-for-2.12 branch of QEMU. > > Ugh. We had several bugs along these lines, but I thought I'd fixed > them. I wonder what this one is. > > > A bit of debugging shows me that when memory is added, the guest kernel > > tries to resize HPT to a htab_shift value lesser than the value with > which > > the guest has booted. For eg. a 8GB guest boots with htab_shift of 26. > When > > 1G is hot-added, > > arch/powerpc/mm/hash_utils_64.c:resize_hpt_for_hotplug() ends up > assigning > > 24 to target_hpt_shift. This looks suspicious as we are increasing the > > memory, but kernel is asking for shrinking the HPT size. > > So the shrink-HPT-on-add-memory is actually expected and should be > harmless. It occurs because qemu estimates HPT size on the > traditional HPT == RAM size / 64 formular, which was devised with 4k > pages in mind. The kernel on the other hand, knows it is using 64k > pages and so estimates a smaller HPT size. Hot plugging memory always > prompts the guest to re-estimate the required HPT size, but if the > added memory is small enough, that size can still be smaller than > qemu's initial guess. > Thanks for the clarification. > > > HPT resizing > > requests fail though, but next reboot crashes the guest. > > As noted the shrink is expected, so we need to debug the crash > separately. Do you have 9478956794c11239b7c1c3ef9ce95c883bb839a3 in > your tree? > I do now and I no longer see the crash that I was observing last week. Regards, Bharata.