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

[David Hildenbrand <david@redhat.com>]

On 18.02.25 10:19, 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).
> 
>> +
>>        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,
> 


Agreed to all, with that it LGTM, thanks!

-- 
Cheers,

David / dhildenb