On 02.03.26 16:11, Marek Marczykowski-Górecki wrote:
On Mon, Mar 02, 2026 at 03:54:12PM +0100, David Hildenbrand (Arm) wrote:Whatever leaves the buddy shall be zeroed out. If there is a double-zeroing happen, the latter could get optimized out by checking something like user_alloc_needs_zeroing(). See mm/huge_memory.c:vma_alloc_anon_folio_pmd() as an example where we avoid double-zeroing.It isn't just reducing double-zeroing to single zeroing. It's about avoiding zeroing such pages at all. If a domU is started with populate-on-demand, many (sometimes most) of its pages are populated in EPT. The idea of PoD is to start guest with high static memory size, but low actual allocation and fake it until balloon driver kicks in and make the domU really not use more pages than it has. When balloon driver try to return those pages to the hypervisor, normally it would just take unallocated page one by one and made Linux not use them. But if _any_ zeroing is happening, each page first needs to be mapped to the guest by the hypervisor (one trip through EPT), just to be removed from them a moment later...The same is true for most balloon drivers, including virtio-balloon. So far nobody really cared about that, though, as init_on_free usually comes with such a high performance price tag that people in cheap VMs (where you overcommit etc) don't enable it. __GFP_BALLOON_OUT is just nasty. We could probably have a special allocation interface (not exposed to arbitrary kernel modules) and have things like mm/balloon.c consume that. IIUC, xen balloon does not use the memory balloon infrastructure, though.Is there some fundamental reason for that? By looking at the code, the migration to use mm/balloon.c shouldn't be that hard (famous last words...).
I wanted to do that for years, but -ENOTIME. Patches welcome. :-) Juergen
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