Re: [PATCH 2/7] dma-buf/udmabuf: account the page array metadata to memcg

[Tao Cui]

Hello，Christian，T.J.

在 2026/11/7 01:25, T.J. Mercier 写道:
> On Fri, Nov 6, 2026 at 5:23 AM Christian König <christian.koenig@xxxxxxx> wrote:
>>
>> On 11/6/26 14:18, Tao Cui wrote:
>>> From: Tao Cui <cuitao@xxxxxxxxxx>
>>>
>>> udmabuf pins the pages of a memfd and hands them out as a dma-buf.  The
>>> data pages keep the accounting of the memfd they were instantiated in,
>>> but the bookkeeping around them does not: the page pointer array and
>>> the pinned_folios array, both sized by the buffer page count, plus the
>>> transient folios array sized by the memfd folio count, are plain
>>> GFP_KERNEL.  A 1GB buffer costs about 4MB of uncharged SUnreclaim
>>> memory, and size_limit_mb defaults to INT_MAX.
>>>
>>> Charge the three kvmalloc allocations so the creator pays for the
>>> metadata it induces.  The system heap in the same subsystem already
>>> accounts its buffer allocation;
> 
> Only if the mem_accounting param is set to true. We (Android)
> currently depend on it being disabled because system heap buffers
> don't have a single owner for their entire lifetime (ownership often
> changes immediately after they are allocated), and the kernel doesn't
> have the ability to switch who is charged for the buffer memory quite
> yet. Albert is working hard on it at the moment.
> 
> Could you update the message to note the mem_accounting param pls?
> 
>>> the data pages here stay with the
>>> memfd.
> 
> memfd / udmabuf is a little special because it uses shmem for the
> backing store, which is already tracked by memcg under all conditions
> (except hugetlb). We don't currently use udmabuf, so the lack of memcg
> charge transfer for udmabuf is not an issue. But once we get charge
> transfer working for the system heap it'd be good to follow up with
> udmabuf.
> 
>> T.J. needs to take a look here, but I fear that this might break uAPI.
> 
> Thanks for looping me in Christian. The buffer allocations cost
> gigabytes, but these metadata arrays are only 4MB per 1GB of buffer,
> so I don't expect charging them to break anyone. Leaving them
> uncharged is an accounting gap though, since someone could spam many
> udmabufs from the same memfd and only pay for the memfd pages once. So
> this change is ok with me.
> 
>> A lot of the DMA-buf resources are intentionally not charged against memcg.
>>
>> Regards,
>> Christian.
>>
>>>
>>> Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
> 
> With the commit message updated to mention the mem_accounting param:
> Reviewed-by: T.J. Mercier <tjmercier@xxxxxxxxxx>
Thanks, Christian，T.J.

I'll update the commit message to clarify the mem_accounting parameter
and pick up your Reviewed-by tag in v2.

Tao