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

[Chenyi Qiang <chenyi.qiang@intel.com>]

On 2/18/2025 5:19 PM, Alexey Kardashevskiy wrote:
> 
> 
> 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@redhat.com>
>> Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
>> ---
>> 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).

I'm not sure if moving up the memory_region_set_ram_discard_manager()
will have bring any side effect (seems no requirement for the operation
order here). But Maybe it's better to put after
ram_block_coordinated_discard_require(true) as
ram_block_coordinated_discard_require(true) unlocks RDM users.

> 
>> +
>>       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,

LGTM, will change it. Thanks!

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