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09b3b7e092
For scarce memory resources, such as hugetlb, we want to be able to prealloc such memory resources in order to not crash later on access. On simple user errors we could otherwise easily run out of memory resources an crash the VM -- pretty much undesired. For ordinary memory devices, such as DIMMs, we preallocate memory via the memory backend for such use cases; however, with virtio-mem we're dealing with sparse memory backends; preallocating the whole memory backend destroys the whole purpose of virtio-mem. Instead, we want to preallocate memory when actually exposing memory to the VM dynamically, and fail plugging memory gracefully + warn the user in case preallocation fails. A common use case for hugetlb will be using "reserve=off,prealloc=off" for the memory backend and "prealloc=on" for the virtio-mem device. This way, no huge pages will be reserved for the process, but we can recover if there are no actual huge pages when plugging memory. Libvirt is already prepared for this. Note that preallocation cannot protect from the OOM killer -- which holds true for any kind of preallocation in QEMU. It's primarily useful only for scarce memory resources such as hugetlb, or shared file-backed memory. It's of little use for ordinary anonymous memory that can be swapped, KSM merged, ... but we won't forbid it. Reviewed-by: Michal Privoznik <mprivozn@redhat.com> Signed-off-by: David Hildenbrand <david@redhat.com> Message-Id: <20211217134611.31172-9-david@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> |
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