Re: [PATCH v6 01/13] dma-buf: introduce initial file I/O infrastructure

[Pavel Begunkov <asml.silence@gmail.com>]

On 9/30/26 07:54, Matthew Brost wrote:
> On Tue, Sep 22, 2026 at 10:08:38AM +0100, Pavel Begunkov wrote:
>> On 9/21/26 14:53, sashiko-bot@kernel.org wrote:
>> ...>>   }
>>> [ ... ]
>>>> +int dma_buf_io_init_map(struct dma_buf_io_ctx *ctx, struct dma_buf_io_map *map,
>>>> +			struct sg_table *sgt)
>>>> +{
>>>> +	unsigned seg_shift = ~0U;
>>>> +	struct scatterlist *sg;
>>>> +	unsigned long tmp;
>>>> +	int ret;
>>>> +
>>>> +	for_each_sgtable_dma_sg(sgt, sg, tmp)
>>>> +		seg_shift = min(seg_shift, __ffs(sg_dma_len(sg)));
>>>> +
>>>> +	ret = percpu_ref_init(&map->refs, dma_buf_io_map_refs_release, 0,
>>>> +			      GFP_KERNEL);
>>>
>>> [Severity: High]
>>> Does this GFP_KERNEL allocation violate DMA-buf locking rules?
>>>
>>> dma_buf_io_create_map() calls ctx->dev_ops->map(ctx) while holding the
>>> dmabuf->resv ww_mutex. If the driver then calls dma_buf_io_init_map() under
>>> this lock, the GFP_KERNEL allocation could deadlock if memory reclaim
>>> attempts to acquire dma_resv locks.
>>
>> Most of these look trivial, going to fix them. This one, however,
>> doesn't sound true AFAIU, I haven't heard about any blank ban on
>> GFP_KERNEL allocation under resv lock.
>>
> 
> I can confirm this definitely not correct - see dma_resv_lockdep it
> acquires dma_resv then fs_reclaim_acquire(GFP_KERNEL). Shrinker enter
> direct reclaim and try to take dma-resv locks, but not block on the
> lock.

Just to double check, I believe by "this" you mean that the
sashiko report is incorrect. I'm going to leave it as is, but
let me know if I'm wrong.

-- 
Pavel Begunkov