From 09b3b7e092ca1d8c096fdda33d1d48b8c229d3c2 Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Fri, 17 Dec 2021 14:46:11 +0100 Subject: [PATCH] virtio-mem: Support "prealloc=on" option 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 Signed-off-by: David Hildenbrand Message-Id: <20211217134611.31172-9-david@redhat.com> Reviewed-by: Michael S. Tsirkin Signed-off-by: Michael S. Tsirkin --- hw/virtio/virtio-mem.c | 39 ++++++++++++++++++++++++++++++---- include/hw/virtio/virtio-mem.h | 4 ++++ 2 files changed, 39 insertions(+), 4 deletions(-) diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c index 341c3fa2c1..ab975ff566 100644 --- a/hw/virtio/virtio-mem.c +++ b/hw/virtio/virtio-mem.c @@ -429,10 +429,40 @@ static int virtio_mem_set_block_state(VirtIOMEM *vmem, uint64_t start_gpa, return -EBUSY; } virtio_mem_notify_unplug(vmem, offset, size); - } else if (virtio_mem_notify_plug(vmem, offset, size)) { - /* Could be a mapping attempt resulted in memory getting populated. */ - ram_block_discard_range(vmem->memdev->mr.ram_block, offset, size); - return -EBUSY; + } else { + int ret = 0; + + if (vmem->prealloc) { + void *area = memory_region_get_ram_ptr(&vmem->memdev->mr) + offset; + int fd = memory_region_get_fd(&vmem->memdev->mr); + Error *local_err = NULL; + + os_mem_prealloc(fd, area, size, 1, &local_err); + if (local_err) { + static bool warned; + + /* + * Warn only once, we don't want to fill the log with these + * warnings. + */ + if (!warned) { + warn_report_err(local_err); + warned = true; + } else { + error_free(local_err); + } + ret = -EBUSY; + } + } + if (!ret) { + ret = virtio_mem_notify_plug(vmem, offset, size); + } + + if (ret) { + /* Could be preallocation or a notifier populated memory. */ + ram_block_discard_range(vmem->memdev->mr.ram_block, offset, size); + return -EBUSY; + } } virtio_mem_set_bitmap(vmem, start_gpa, size, plug); return 0; @@ -1108,6 +1138,7 @@ static void virtio_mem_instance_init(Object *obj) static Property virtio_mem_properties[] = { DEFINE_PROP_UINT64(VIRTIO_MEM_ADDR_PROP, VirtIOMEM, addr, 0), DEFINE_PROP_UINT32(VIRTIO_MEM_NODE_PROP, VirtIOMEM, node, 0), + DEFINE_PROP_BOOL(VIRTIO_MEM_PREALLOC_PROP, VirtIOMEM, prealloc, false), DEFINE_PROP_LINK(VIRTIO_MEM_MEMDEV_PROP, VirtIOMEM, memdev, TYPE_MEMORY_BACKEND, HostMemoryBackend *), DEFINE_PROP_END_OF_LIST(), diff --git a/include/hw/virtio/virtio-mem.h b/include/hw/virtio/virtio-mem.h index a5dd6a493b..0ac7bcb3b6 100644 --- a/include/hw/virtio/virtio-mem.h +++ b/include/hw/virtio/virtio-mem.h @@ -30,6 +30,7 @@ OBJECT_DECLARE_TYPE(VirtIOMEM, VirtIOMEMClass, #define VIRTIO_MEM_REQUESTED_SIZE_PROP "requested-size" #define VIRTIO_MEM_BLOCK_SIZE_PROP "block-size" #define VIRTIO_MEM_ADDR_PROP "memaddr" +#define VIRTIO_MEM_PREALLOC_PROP "prealloc" struct VirtIOMEM { VirtIODevice parent_obj; @@ -62,6 +63,9 @@ struct VirtIOMEM { /* block size and alignment */ uint64_t block_size; + /* whether to prealloc memory when plugging new blocks */ + bool prealloc; + /* notifiers to notify when "size" changes */ NotifierList size_change_notifiers;