linux-kernel - Re: [RFC v2 05/11] block: add infra to handle dmabuf tokens

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Message-ID: <d3389449-f344-48ae-ab13-697e01d1cc46@gmail.com>
Date: Thu, 22 Jan 2026 11:46:59 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: Nitesh Shetty <nj.shetty@...sung.com>
Cc: linux-block@...r.kernel.org, io-uring@...r.kernel.org,
 Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...nel.dk>,
 Christoph Hellwig <hch@....de>, Sagi Grimberg <sagi@...mberg.me>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, linux-kernel@...r.kernel.org,
 linux-nvme@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
 linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linaro-mm-sig@...ts.linaro.org
Subject: Re: [RFC v2 05/11] block: add infra to handle dmabuf tokens

On 1/21/26 07:37, Nitesh Shetty wrote:
> On 23/11/25 10:51PM, Pavel Begunkov wrote:
>> Add blk-mq infrastructure to handle dmabuf tokens. There are two main
>> objects. The first is struct blk_mq_dma_token, which is an extension of
>> struct dma_token and passed in an iterator. The second is struct
>> blk_mq_dma_map, which keeps the actual mapping and unlike the token, can
>> be ejected (e.g. by move_notify) and recreated.
>>
>> The token keeps an rcu protected pointer to the mapping, so when it
>> resolves a token into a mapping to pass it to a request, it'll do an rcu
>> protected lookup and get a percpu reference to the mapping.
>>
>> If there is no current mapping attached to a token, it'll need to be
>> created by calling the driver (e.g. nvme) via a new callback. It
>> requires waiting, thefore can't be done for nowait requests and couldn't
>> happen deeper in the stack, e.g. during nvme request submission.
>>
>> The structure split is needed because move_notify can request to
>> invalidate the dma mapping at any moment, and we need a way to
>> concurrently remove it and wait for the inflight requests using the
>> previous mapping to complete.
>>
>> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
>> ---
>> block/Makefile                   |   1 +
>> block/bdev.c                     |  14 ++
>> block/blk-mq-dma-token.c         | 236 +++++++++++++++++++++++++++++++
>> block/blk-mq.c                   |  20 +++
>> block/fops.c                     |   1 +
>> include/linux/blk-mq-dma-token.h |  60 ++++++++
>> include/linux/blk-mq.h           |  21 +++
>> include/linux/blkdev.h           |   3 +
>> 8 files changed, 356 insertions(+)
>> create mode 100644 block/blk-mq-dma-token.c
>> create mode 100644 include/linux/blk-mq-dma-token.h
>>
>> diff --git a/block/Makefile b/block/Makefile
>> index c65f4da93702..0190e5aa9f00 100644
...
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index f2650c97a75e..1ff3a7e3191b 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -29,6 +29,7 @@
>> #include <linux/blk-crypto.h>
>> #include <linux/part_stat.h>
>> #include <linux/sched/isolation.h>
>> +#include <linux/blk-mq-dma-token.h>
>>
>> #include <trace/events/block.h>
>>
>> @@ -439,6 +440,7 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
>>     rq->nr_integrity_segments = 0;
>>     rq->end_io = NULL;
>>     rq->end_io_data = NULL;
>> +    rq->dma_map = NULL;
>>
>>     blk_crypto_rq_set_defaults(rq);
>>     INIT_LIST_HEAD(&rq->queuelist);
>> @@ -794,6 +796,7 @@ static void __blk_mq_free_request(struct request *rq)
>>     blk_pm_mark_last_busy(rq);
>>     rq->mq_hctx = NULL;
>>
>> +    blk_rq_drop_dma_map(rq);
> blk_rq_drop_dma_map(rq), needs to be added in blk_mq_end_request_batch
> as well[1], otherwise I am seeing we leave with increased reference
> count in dma-buf exporter side.
> 
> Thanks,
> Nitesh
> 
> [1]
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1214,6 +1214,7 @@ void blk_mq_end_request_batch(struct io_comp_batch *iob)
> 
>                   blk_crypto_free_request(rq);
>                   blk_pm_mark_last_busy(rq);
> +               blk_rq_drop_dma_map(rq);

Ah yes, thanks Nitesh

-- 
Pavel Begunkov