Re: [PATCH v2 2/6] memory: Change memory_region_set_ram_discard_manager() to return the result

[Alexey Kardashevskiy <aik@xxxxxxx>]

On 17/2/25 19:18, Chenyi Qiang wrote:
> Modify memory_region_set_ram_discard_manager() to return false if a
> RamDiscardManager is already set in the MemoryRegion. The caller must
> handle this failure, such as having virtio-mem undo its actions and fail
> the realize() process. Opportunistically move the call earlier to avoid
> complex error handling.
>
> This change is beneficial when introducing a new RamDiscardManager
> instance besides virtio-mem. After
> ram_block_coordinated_discard_require(true) unlocks all
> RamDiscardManager instances, only one instance is allowed to be set for
> a MemoryRegion at present.
>
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Chenyi Qiang <chenyi.qiang@xxxxxxxxx>
> ---
> Changes in v2:
>      - newly added.
> ---
>   hw/virtio/virtio-mem.c | 30 +++++++++++++++++-------------
>   include/exec/memory.h  |  6 +++---
>   system/memory.c        | 11 ++++++++---
>   3 files changed, 28 insertions(+), 19 deletions(-)
>
> diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
> index 21f16e4912..ef818a2cdf 100644
> --- a/hw/virtio/virtio-mem.c
> +++ b/hw/virtio/virtio-mem.c
> @@ -1074,6 +1074,18 @@ static void virtio_mem_device_realize(DeviceState *dev, Error **errp)
>                           vmem->block_size;
>       vmem->bitmap = bitmap_new(vmem->bitmap_size);
>   
> +    /*
> +     * Set ourselves as RamDiscardManager before the plug handler maps the
> +     * memory region and exposes it via an address space.
> +     */
> +    if (memory_region_set_ram_discard_manager(&vmem->memdev->mr,
> +                                              RAM_DISCARD_MANAGER(vmem))) {
> +        error_setg(errp, "Failed to set RamDiscardManager");
> +        g_free(vmem->bitmap);
> +        ram_block_coordinated_discard_require(false);
> +        return;
> +    }

Looks like this can move before vmem->bitmap is allocated (or even 
before ram_block_coordinated_discard_require(true)?). Then you can drop 
g_free() and avoid having a stale pointer in vmem->bitmap (not that it 
matters here though).

> +
>       virtio_init(vdev, VIRTIO_ID_MEM, sizeof(struct virtio_mem_config));
>       vmem->vq = virtio_add_queue(vdev, 128, virtio_mem_handle_request);
>   vmem->bitmap
> @@ -1124,13 +1136,6 @@ static void virtio_mem_device_realize(DeviceState *dev, Error **errp)
>       vmem->system_reset = VIRTIO_MEM_SYSTEM_RESET(obj);
>       vmem->system_reset->vmem = vmem;
>       qemu_register_resettable(obj);
> -
> -    /*
> -     * Set ourselves as RamDiscardManager before the plug handler maps the
> -     * memory region and exposes it via an address space.
> -     */
> -    memory_region_set_ram_discard_manager(&vmem->memdev->mr,
> -                                          RAM_DISCARD_MANAGER(vmem));
>   }
>   
>   static void virtio_mem_device_unrealize(DeviceState *dev)
> @@ -1138,12 +1143,6 @@ static void virtio_mem_device_unrealize(DeviceState *dev)
>       VirtIODevice *vdev = VIRTIO_DEVICE(dev);
>       VirtIOMEM *vmem = VIRTIO_MEM(dev);
>   
> -    /*
> -     * The unplug handler unmapped the memory region, it cannot be
> -     * found via an address space anymore. Unset ourselves.
> -     */
> -    memory_region_set_ram_discard_manager(&vmem->memdev->mr, NULL);
> -
>       qemu_unregister_resettable(OBJECT(vmem->system_reset));
>       object_unref(OBJECT(vmem->system_reset));
>   
> @@ -1155,6 +1154,11 @@ static void virtio_mem_device_unrealize(DeviceState *dev)
>       host_memory_backend_set_mapped(vmem->memdev, false);
>       virtio_del_queue(vdev, 0);
>       virtio_cleanup(vdev);
> +    /*
> +     * The unplug handler unmapped the memory region, it cannot be
> +     * found via an address space anymore. Unset ourselves.
> +     */
> +    memory_region_set_ram_discard_manager(&vmem->memdev->mr, NULL);
>       g_free(vmem->bitmap);
>       ram_block_coordinated_discard_require(false);
>   }
> diff --git a/include/exec/memory.h b/include/exec/memory.h
> index 3bebc43d59..390477b588 100644
> --- a/include/exec/memory.h
> +++ b/include/exec/memory.h
> @@ -2487,13 +2487,13 @@ static inline bool memory_region_has_ram_discard_manager(MemoryRegion *mr)
>    *
>    * This function must not be called for a mapped #MemoryRegion, a #MemoryRegion
>    * that does not cover RAM, or a #MemoryRegion that already has a
> - * #RamDiscardManager assigned.
> + * #RamDiscardManager assigned. Return 0 if the rdm is set successfully.
>    *
>    * @mr: the #MemoryRegion
>    * @rdm: #RamDiscardManager to set
>    */
> -void memory_region_set_ram_discard_manager(MemoryRegion *mr,
> -                                           RamDiscardManager *rdm);
> +int memory_region_set_ram_discard_manager(MemoryRegion *mr,
> +                                          RamDiscardManager *rdm);
>   
>   /**
>    * memory_region_find: translate an address/size relative to a
> diff --git a/system/memory.c b/system/memory.c
> index b17b5538ff..297a3dbcd4 100644
> --- a/system/memory.c
> +++ b/system/memory.c
> @@ -2115,12 +2115,17 @@ RamDiscardManager *memory_region_get_ram_discard_manager(MemoryRegion *mr)
>       return mr->rdm;
>   }
>   
> -void memory_region_set_ram_discard_manager(MemoryRegion *mr,
> -                                           RamDiscardManager *rdm)
> +int memory_region_set_ram_discard_manager(MemoryRegion *mr,
> +                                          RamDiscardManager *rdm)
>   {
>       g_assert(memory_region_is_ram(mr));
> -    g_assert(!rdm || !mr->rdm);
> +    if (mr->rdm && rdm != NULL) {

Drop "!= NULL".

> +        return -1;

-EBUSY?

> +    }
> +
> +    /* !rdm || !mr->rdm */

See, like here - no "!= NULL" :) (and the comment is useless). Thanks,


>       mr->rdm = rdm;
> +    return 0;
>   }
>   
>   uint64_t ram_discard_manager_get_min_granularity(const RamDiscardManager *rdm,

--
Alexey