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. > 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? -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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