Re: [PATCH] hw/virtio/virtio-mem: Prohibit unplugging when size <= requested_size

[David Hildenbrand]

On 30.11.24 13:48, Wei Chen wrote:
>  > That doesn't explain the full story about MIGRATE_UNMOVABLE. I assume
>  > one precondition is missing in your explanation.
>
> I have double-checked the source code. My initial description of the
> process seems somewhat imprecise. vIOMMU does not convert pages to
> UNMOVABLE during pinning, it is that pinning causes page faults, and the
> fault handler allocates UNMOVABLE pages. (vaddr_get_pfns() calls
> __gup_longterm_locked(), who then calls memalloc_pin_save(), and it
> implicitly removes the __GFP_MOVABLE flag.)

Ah, that makes sense! I forgot about memalloc_pin_save(), which we 
primarily added to avoid allocation+immediate migration during longterm 
pinning IIRC.

> Therefore, there is no requirement of ZONE_MOVABLE and MIGRATE_CMA.

Indeed. On systems without that, one workaround would be driving 
virtio-mem in "prealloc" mode (prealloc=on in QEMU on the device), 
whereby we first preallocate the memory using MADV_POPULATW_WRITE, to 
then longterm pin it.

--
Cheers,

David / dhildenb