Re: [FYI] DAMON and virtio-balloon in DeepSeek's DSec paper

["David Hildenbrand (Arm)" <david@kernel.org>]

On 9/25/26 10:15, Lance Yang wrote:
> +Cc DeepSeek and Muchun
> 
> On Fri, Sep 25, 2026 at 09:04:41AM +0200, David Hildenbrand (Arm) wrote:
>> On 9/25/26 07:44, Lance Yang wrote:
>>> Hi all,
>>>
>>> I was reading DeepSeek's new DSec paper[1] and found a nice use of DAMON
>>> and virtio-balloon:
>>>
>>> With this kind of workload, an agent may read a file once and never touch
>>> it again, while those pages remain in the guest page cache. Without memory
>>> pressure in the guest, they can stay cached even though the host would
>>> like that memory back ...
>>>
>>> The trick is DAMON + virtio-balloon free-page reporting :) DAMON reclaims
>>
>> Heh, I read "virtio-balloon" and thought "balloon inflation/deflation, what year
>> is it?!". Free-page reporting makes much more sense.
>>
>>> cold file pages from the guest page cache; buddy gets a chance to coalesce
>>> them into reportable blocks, and virtio-balloon passes those blocks to
>>> Firecracker. Firecracker can then drop the host backing with MADV_DONTNEED.
>>
>> When I was at RH we were looking at this issue as well. virtio-pmem was one way
>> of avoiding the page cache in VM entirely. But it has its own limitations.
> 
> YES, they enable virtio-pmem with DAX for the read-only EROFS base-image
> and toolkit layers, while using DAMON with balloon free-page reporting to
> reclaim cold file pages from the guest page cache on larger writable disks.
> 
> They also point out that the guest must allocate struct page metadata for
> the entire pmem-backed address range. So virtio-pmem is not free either :)
> 
> BTW, Muchun recently posted a pretty cool series for exactly that:
> 
> https://lore.kernel.org/linux-mm/20260903122128.12264-1-songmuchun@bytedance.com/

Yes, that will be helpful in that regard.

> 
> (It shares vmemmap backing until a DAX fault needs private metadata,
> avoiding the full per-PFN cost up front.)
> 
> I have a feeling the DeepSeek team will be watching this one closely :P
;)

-- 
Cheers,

David