| Message ID | 20230117112249.244096-9-david@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | virtio-mem: Handle preallocation with migration | expand |
David Hildenbrand <david@redhat.com> wrote: > Ordinary memory preallocation runs when QEMU starts up and creates the > memory backends, before processing the incoming migration stream. With > virtio-mem, we don't know which memory blocks to preallocate before > migration started. Now that we migrate the virtio-mem bitmap early, before > migrating any RAM content, we can safely preallocate memory for all plugged > memory blocks before migrating any RAM content. > > This is especially relevant for the following cases: > > (1) User errors > > With hugetlb/files, if we don't have sufficient backend memory available on > the migration destination, we'll crash QEMU (SIGBUS) during RAM migration > when running out of backend memory. Preallocating memory before actual > RAM migration allows for failing gracefully and informing the user about > the setup problem. > > (2) Excluded memory ranges during migration > > For example, virtio-balloon free page hinting will exclude some pages > from getting migrated. In that case, we won't crash during RAM > migration, but later, when running the VM on the destination, which is > bad. > > To fix this for new QEMU machines that migrate the bitmap early, > preallocate the memory early, before any RAM migration. Warn with old > QEMU machines. > > Getting postcopy right is a bit tricky, but we essentially now implement > the same (problematic) preallocation logic as ordinary preallocation: > preallocate memory early and discard it again before precopy starts. During > ordinary preallocation, discarding of RAM happens when postcopy is advised. > As the state (bitmap) is loaded after postcopy was advised but before > postcopy starts listening, we have to discard memory we preallocated > immediately again ourselves. > > Note that nothing (not even hugetlb reservations) guarantees for postcopy > that backend memory (especially, hugetlb pages) are still free after they > were freed ones while discarding RAM. Still, allocating that memory at > least once helps catching some basic setup problems. > > Before this change, trying to restore a VM when insufficient hugetlb > pages are around results in the process crashing to to a "Bus error" > (SIGBUS). With this change, QEMU fails gracefully: > > qemu-system-x86_64: qemu_prealloc_mem: preallocating memory failed: Bad address > qemu-system-x86_64: error while loading state for instance 0x0 of device '0000:00:03.0/virtio-mem-device-early' > qemu-system-x86_64: load of migration failed: Cannot allocate memory > > And we can even introspect the early migration data, including the > bitmap: > $ ./scripts/analyze-migration.py -f STATEFILE > { > "ram (2)": { > "section sizes": { > "0000:00:03.0/mem0": "0x0000000780000000", > "0000:00:04.0/mem1": "0x0000000780000000", > "pc.ram": "0x0000000100000000", > "/rom@etc/acpi/tables": "0x0000000000020000", > "pc.bios": "0x0000000000040000", > "0000:00:02.0/e1000.rom": "0x0000000000040000", > "pc.rom": "0x0000000000020000", > "/rom@etc/table-loader": "0x0000000000001000", > "/rom@etc/acpi/rsdp": "0x0000000000001000" > } > }, > "0000:00:03.0/virtio-mem-device-early (51)": { > "tmp": "00 00 00 01 40 00 00 00 00 00 00 07 80 00 00 00 00 00 00 00 00 20 00 00 00 00 00 00", > "size": "0x0000000040000000", > "bitmap": "ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff [...] > }, > "0000:00:04.0/virtio-mem-device-early (53)": { > "tmp": "00 00 00 08 c0 00 00 00 00 00 00 07 80 00 00 00 00 00 00 00 00 20 00 00 00 00 00 00", > "size": "0x00000001fa400000", > "bitmap": "ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff [...] > }, > [...] > > Reported-by: Jing Qi <jinqi@redhat.com> > Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > Signed-off-by: David Hildenbrand <david@redhat.com> Reviewed-by: Juan Quintela <quintela@redhat.com>
diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c index ca37949df8..957fe77dc0 100644 --- a/hw/virtio/virtio-mem.c +++ b/hw/virtio/virtio-mem.c @@ -204,6 +204,30 @@ static int virtio_mem_for_each_unplugged_range(const VirtIOMEM *vmem, void *arg, return ret; } +static int virtio_mem_for_each_plugged_range(const VirtIOMEM *vmem, void *arg, + virtio_mem_range_cb cb) +{ + unsigned long first_bit, last_bit; + uint64_t offset, size; + int ret = 0; + + first_bit = find_first_bit(vmem->bitmap, vmem->bitmap_size); + while (first_bit < vmem->bitmap_size) { + offset = first_bit * vmem->block_size; + last_bit = find_next_zero_bit(vmem->bitmap, vmem->bitmap_size, + first_bit + 1) - 1; + size = (last_bit - first_bit + 1) * vmem->block_size; + + ret = cb(vmem, arg, offset, size); + if (ret) { + break; + } + first_bit = find_next_bit(vmem->bitmap, vmem->bitmap_size, + last_bit + 2); + } + return ret; +} + /* * Adjust the memory section to cover the intersection with the given range. * @@ -938,6 +962,10 @@ static int virtio_mem_post_load(void *opaque, int version_id) RamDiscardListener *rdl; int ret; + if (vmem->prealloc && !vmem->early_migration) { + warn_report("Proper preallocation with migration requires a newer QEMU machine"); + } + /* * We started out with all memory discarded and our memory region is mapped * into an address space. Replay, now that we updated the bitmap. @@ -957,6 +985,64 @@ static int virtio_mem_post_load(void *opaque, int version_id) return virtio_mem_restore_unplugged(vmem); } +static int virtio_mem_prealloc_range_cb(const VirtIOMEM *vmem, void *arg, + uint64_t offset, uint64_t size) +{ + void *area = memory_region_get_ram_ptr(&vmem->memdev->mr) + offset; + int fd = memory_region_get_fd(&vmem->memdev->mr); + Error *local_err = NULL; + + qemu_prealloc_mem(fd, area, size, 1, NULL, &local_err); + if (local_err) { + error_report_err(local_err); + return -ENOMEM; + } + return 0; +} + +static int virtio_mem_post_load_early(void *opaque, int version_id) +{ + VirtIOMEM *vmem = VIRTIO_MEM(opaque); + RAMBlock *rb = vmem->memdev->mr.ram_block; + int ret; + + if (!vmem->prealloc) { + return 0; + } + + /* + * We restored the bitmap and verified that the basic properties + * match on source and destination, so we can go ahead and preallocate + * memory for all plugged memory blocks, before actual RAM migration starts + * touching this memory. + */ + ret = virtio_mem_for_each_plugged_range(vmem, NULL, + virtio_mem_prealloc_range_cb); + if (ret) { + return ret; + } + + /* + * This is tricky: postcopy wants to start with a clean slate. On + * POSTCOPY_INCOMING_ADVISE, postcopy code discards all (ordinarily + * preallocated) RAM such that postcopy will work as expected later. + * + * However, we run after POSTCOPY_INCOMING_ADVISE -- but before actual + * RAM migration. So let's discard all memory again. This looks like an + * expensive NOP, but actually serves a purpose: we made sure that we + * were able to allocate all required backend memory once. We cannot + * guarantee that the backend memory we will free will remain free + * until we need it during postcopy, but at least we can catch the + * obvious setup issues this way. + */ + if (migration_incoming_postcopy_advised()) { + if (ram_block_discard_range(rb, 0, qemu_ram_get_used_length(rb))) { + return -EBUSY; + } + } + return 0; +} + typedef struct VirtIOMEMMigSanityChecks { VirtIOMEM *parent; uint64_t addr; @@ -1068,6 +1154,7 @@ static const VMStateDescription vmstate_virtio_mem_device_early = { .minimum_version_id = 1, .version_id = 1, .early_setup = true, + .post_load = virtio_mem_post_load_early, .fields = (VMStateField[]) { VMSTATE_WITH_TMP(VirtIOMEM, VirtIOMEMMigSanityChecks, vmstate_virtio_mem_sanity_checks),
Ordinary memory preallocation runs when QEMU starts up and creates the memory backends, before processing the incoming migration stream. With virtio-mem, we don't know which memory blocks to preallocate before migration started. Now that we migrate the virtio-mem bitmap early, before migrating any RAM content, we can safely preallocate memory for all plugged memory blocks before migrating any RAM content. This is especially relevant for the following cases: (1) User errors With hugetlb/files, if we don't have sufficient backend memory available on the migration destination, we'll crash QEMU (SIGBUS) during RAM migration when running out of backend memory. Preallocating memory before actual RAM migration allows for failing gracefully and informing the user about the setup problem. (2) Excluded memory ranges during migration For example, virtio-balloon free page hinting will exclude some pages from getting migrated. In that case, we won't crash during RAM migration, but later, when running the VM on the destination, which is bad. To fix this for new QEMU machines that migrate the bitmap early, preallocate the memory early, before any RAM migration. Warn with old QEMU machines. Getting postcopy right is a bit tricky, but we essentially now implement the same (problematic) preallocation logic as ordinary preallocation: preallocate memory early and discard it again before precopy starts. During ordinary preallocation, discarding of RAM happens when postcopy is advised. As the state (bitmap) is loaded after postcopy was advised but before postcopy starts listening, we have to discard memory we preallocated immediately again ourselves. Note that nothing (not even hugetlb reservations) guarantees for postcopy that backend memory (especially, hugetlb pages) are still free after they were freed ones while discarding RAM. Still, allocating that memory at least once helps catching some basic setup problems. Before this change, trying to restore a VM when insufficient hugetlb pages are around results in the process crashing to to a "Bus error" (SIGBUS). With this change, QEMU fails gracefully: qemu-system-x86_64: qemu_prealloc_mem: preallocating memory failed: Bad address qemu-system-x86_64: error while loading state for instance 0x0 of device '0000:00:03.0/virtio-mem-device-early' qemu-system-x86_64: load of migration failed: Cannot allocate memory And we can even introspect the early migration data, including the bitmap: $ ./scripts/analyze-migration.py -f STATEFILE { "ram (2)": { "section sizes": { "0000:00:03.0/mem0": "0x0000000780000000", "0000:00:04.0/mem1": "0x0000000780000000", "pc.ram": "0x0000000100000000", "/rom@etc/acpi/tables": "0x0000000000020000", "pc.bios": "0x0000000000040000", "0000:00:02.0/e1000.rom": "0x0000000000040000", "pc.rom": "0x0000000000020000", "/rom@etc/table-loader": "0x0000000000001000", "/rom@etc/acpi/rsdp": "0x0000000000001000" } }, "0000:00:03.0/virtio-mem-device-early (51)": { "tmp": "00 00 00 01 40 00 00 00 00 00 00 07 80 00 00 00 00 00 00 00 00 20 00 00 00 00 00 00", "size": "0x0000000040000000", "bitmap": "ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff [...] }, "0000:00:04.0/virtio-mem-device-early (53)": { "tmp": "00 00 00 08 c0 00 00 00 00 00 00 07 80 00 00 00 00 00 00 00 00 20 00 00 00 00 00 00", "size": "0x00000001fa400000", "bitmap": "ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff [...] }, [...] Reported-by: Jing Qi <jinqi@redhat.com> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com> Signed-off-by: David Hildenbrand <david@redhat.com> --- hw/virtio/virtio-mem.c | 87 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 87 insertions(+)