Re: [PATCH v2 1/5] virtio-mem: Probe THP size to determine default block size

[Pankaj Gupta]

> Let's allow a minimum block size of 1 MiB in all configurations. Select
> the default block size based on
> - The page size of the memory backend.
> - The THP size if the memory backend size corresponds to the real hsot

s/hsot/host
>   page size.
> - The global minimum of 1 MiB.
> and warn if something smaller is configured by the user.
>
> VIRTIO_MEM only supports Linux (depends on LINUX), so we can probe the
> THP size unconditionally.
>
> For now we only support virtio-mem on x86-64 - there isn't a user-visiable

s/visiable/visible
> change (x86-64 only supports 2 MiB THP on the PMD level) - the default
> was, and will be 2 MiB.
>
> If we ever have THP on the PUD level (e.g., 1 GiB THP on x86-64), we
> expect to have a trigger to explicitly opt-in for the new THP granularity.
>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Wei Yang <richardw.yang@linux.intel.com>
> Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Cc: Igor Mammedov <imammedo@redhat.com>
> Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  hw/virtio/virtio-mem.c | 105 +++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 101 insertions(+), 4 deletions(-)
>
> diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
> index 8fbec77ccc..9b1461cf9d 100644
> --- a/hw/virtio/virtio-mem.c
> +++ b/hw/virtio/virtio-mem.c
> @@ -33,10 +33,83 @@
>  #include "trace.h"
>
>  /*
> - * Use QEMU_VMALLOC_ALIGN, so no THP will have to be split when unplugging
> - * memory (e.g., 2MB on x86_64).
> + * Let's not allow blocks smaller than 1 MiB, for example, to keep the 
> tracking
> + * bitmap small.
>   */
> -#define VIRTIO_MEM_MIN_BLOCK_SIZE ((uint32_t)QEMU_VMALLOC_ALIGN)
> +#define VIRTIO_MEM_MIN_BLOCK_SIZE ((uint32_t)(1 * MiB))
> +
> +#if defined(__x86_64__) || defined(__arm__) || defined(__aarch64__) || \
> +    defined(__powerpc64__)
> +#define VIRTIO_MEM_DEFAULT_THP_SIZE ((uint32_t)(2 * MiB))
> +#else
> +        /* fallback to 1 MiB (e.g., the THP size on s390x) */
> +#define VIRTIO_MEM_DEFAULT_THP_SIZE VIRTIO_MEM_MIN_BLOCK_SIZE
> +#endif
> +
> +/*
> + * We want to have a reasonable default block size such that
> + * 1. We avoid splitting THPs when unplugging memory, which degrades
> + *    performance.
> + * 2. We avoid placing THPs for plugged blocks that also cover unplugged
> + *    blocks.
> + *
> + * The actual THP size might differ between Linux kernels, so we try to probe
> + * it. In the future (if we ever run into issues regarding 2.), we might want
> + * to disable THP in case we fail to properly probe the THP size, or if the
> + * block size is configured smaller than the THP size.
> + */
> +static uint32_t thp_size;
> +
> +#define HPAGE_PMD_SIZE_PATH 
> "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size"
> +static uint32_t virtio_mem_thp_size(void)
> +{
> +    gchar *content = NULL;
> +    const char *endptr;
> +    uint64_t tmp;
> +
> +    if (thp_size) {
> +        return thp_size;
> +    }
> +
> +    /*
> +     * Try to probe the actual THP size, fallback to (sane but eventually
> +     * incorrect) default sizes.
> +     */
> +    if (g_file_get_contents(HPAGE_PMD_SIZE_PATH, &content, NULL, NULL) &&
> +        !qemu_strtou64(content, &endptr, 0, &tmp) &&
> +        (!endptr || *endptr == '\n')) {
> +        /*
> +         * Sanity-check the value, if it's too big (e.g., aarch64 with 64k 
> base
> +         * pages) or weird, fallback to something smaller.
> +         */
> +        if (!tmp || !is_power_of_2(tmp) || tmp > 16 * MiB) {
> +            warn_report("Read unsupported THP size: %" PRIx64, tmp);
> +        } else {
> +            thp_size = tmp;
> +        }
> +    }
> +
> +    if (!thp_size) {
> +        thp_size = VIRTIO_MEM_DEFAULT_THP_SIZE;
> +        warn_report("Could not detect THP size, falling back to %" PRIx64
> +                    "  MiB.", thp_size / MiB);
> +    }
> +
> +    g_free(content);
> +    return thp_size;
> +}
> +
> +static uint64_t virtio_mem_default_block_size(RAMBlock *rb)
> +{
> +    const uint64_t page_size = qemu_ram_pagesize(rb);
> +
> +    /* We can have hugetlbfs with a page size smaller than the THP size. */
> +    if (page_size == qemu_real_host_page_size) {
> +        return MAX(page_size, virtio_mem_thp_size());
> +    }

This condition is special, can think of hugetlbfs smaller in size than THP size
configured.
> +    return MAX(page_size, VIRTIO_MEM_MIN_BLOCK_SIZE);

Do we still need this? Or we can have only one return for both the cases?
Probably, I am missing something here.
> +}
> +
>  /*
>   * Size the usable region bigger than the requested size if possible. Esp.
>   * Linux guests will only add (aligned) memory blocks in case they fully
> @@ -437,10 +510,23 @@ static void virtio_mem_device_realize(DeviceState *dev, 
> Error **errp)
>      rb = vmem->memdev->mr.ram_block;
>      page_size = qemu_ram_pagesize(rb);
>
> +    /*
> +     * If the block size wasn't configured by the user, use a sane default. 
> This
> +     * allows using hugetlbfs backends of any page size without manual
> +     * intervention.
> +     */
> +    if (!vmem->block_size) {
> +        vmem->block_size = virtio_mem_default_block_size(rb);
> +    }
> +
>      if (vmem->block_size < page_size) {
>          error_setg(errp, "'%s' property has to be at least the page size 
> (0x%"
>                     PRIx64 ")", VIRTIO_MEM_BLOCK_SIZE_PROP, page_size);
>          return;
> +    } else if (vmem->block_size < virtio_mem_default_block_size(rb)) {
> +        warn_report("'%s' property is smaller than the default block size (%"
> +                    PRIx64 " MiB)", VIRTIO_MEM_BLOCK_SIZE_PROP,
> +                    virtio_mem_default_block_size(rb) / MiB);
>      } else if (!QEMU_IS_ALIGNED(vmem->requested_size, vmem->block_size)) {
>          error_setg(errp, "'%s' property has to be multiples of '%s' (0x%" 
> PRIx64
>                     ")", VIRTIO_MEM_REQUESTED_SIZE_PROP,
> @@ -731,6 +817,18 @@ static void virtio_mem_get_block_size(Object *obj, 
> Visitor *v, const char *name,
>      const VirtIOMEM *vmem = VIRTIO_MEM(obj);
>      uint64_t value = vmem->block_size;
>
> +    /*
> +     * If not configured by the user (and we're not realized yet), use the
> +     * default block size we would use with the current memory backend.
> +     */
> +    if (!value) {
> +        if (vmem->memdev && memory_region_is_ram(&vmem->memdev->mr)) {
> +            value = 
> virtio_mem_default_block_size(vmem->memdev->mr.ram_block);
> +        } else {
> +            value = virtio_mem_thp_size();
> +        }
> +    }
> +
>      visit_type_size(v, name, &value, errp);
>  }
>
> @@ -810,7 +908,6 @@ static void virtio_mem_instance_init(Object *obj)
>  {
>      VirtIOMEM *vmem = VIRTIO_MEM(obj);
>
> -    vmem->block_size = VIRTIO_MEM_MIN_BLOCK_SIZE;
>      notifier_list_init(&vmem->size_change_notifiers);
>      vmem->precopy_notifier.notify = virtio_mem_precopy_notify;