linux-kernel - Re: 答复: Using ion memory for direct-io

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Date:   Tue, 18 Apr 2017 08:56:09 -0700
From:   Laura Abbott <labbott@...hat.com>
To:     "Zengtao (B)" <prime.zeng@...ilicon.com>,
        "sumit.semwal@...aro.org" <sumit.semwal@...aro.org>
Cc:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "arve@...roid.com" <arve@...roid.com>,
        "riandrews@...roid.com" <riandrews@...roid.com>,
        "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: 答复: Using ion memory for direct-io

On 04/17/2017 07:05 PM, Zengtao (B) wrote:
> Hi Laura:
> 
>> -----邮件原件-----
>> 发件人: Laura Abbott [mailto:labbott@...hat.com]
>> 发送时间: 2017年4月18日 0:14
>> 收件人: Zengtao (B) <prime.zeng@...ilicon.com>; sumit.semwal@...aro.org
>> 抄送: gregkh@...uxfoundation.org; arve@...roid.com;
>> riandrews@...roid.com; devel@...verdev.osuosl.org;
>> linux-kernel@...r.kernel.org
>> 主题: Re: Using ion memory for direct-io
>>
>> On 04/14/2017 02:18 AM, Zengtao (B) wrote:
>>> Hi
>>>
>>> Currently, the ion mapped to userspace will be forced with VM_IO and
>> VM_PFNMAP flags.
>>> When I use the ion memory to do the direct-io, it will fail when
>>> reaching the get_user_pages,
>>>
>>> Back to the VM_IO and VM_PFNMAP flag, there two flags are introduced
>>> by the remap_pfn_range called by the ion_heap_mmap_user.
>>>
>>> From my point of view, all ion memory(cma/vmalloc/system heap) are
>>> managed by linux vm, it is not reasonable to have the VM_IO and
>>> VM_PFNMAP flag, but I don't any suitable function to replace the
>> remap_pfn_range, any suggestions?
>>>
>>> Thanks && Regards
>>>
>>> Zengtao
>>>
>>
>> The carveout heap is omitted from your list of 'all ion memory'. At one
>> time, carveout memory was not backed by struct pages so I suspect
>> this is a holdover from then. This would probably be better served
> Yes, you are right, I missed the carveout heap which needs the VM_IO and VM_PFNMAP,
> and I think the carveout heap can implement its own map_user rather then using the common
> ion_heap_map_user.
> 

The carveout heap only uses memory with struct pages these
days. My point was that the VM_IO and VFM_PFNMAP shouldn't
need to be used at all anymore. Sorry for the confusion.

>> by using vm_insert_page and handling higher order pages properly.
> Your latest patch has remove the the page faulting support, I didn't deep into the reason,
> but I think this conflicts with the vm_insert_page.
> 

vm_insert_page should be able to be used outside the fault
handler.

>>
>> Thanks,
>> Laura
> 
> I tried to use the similar way as the dma framework do(split the page and map_vm_area), but
> the split will break the ion sg design, maybe we need a new lowlevel map function instead of directly 
> using the remap_pfn_range.

Yes, I don't think we can directly copy what the
dma framework does. I think we should be okay
with allocating pages with __GFP_COMP and then
just using vm_insert_page but I'm not 100% sure
this would work with CMA.

For completeness sake, I know the request is for direct-io
but can you share a few more details about the
driver/use case where you want to use direct-io with Ion
buffers?

> 
> Thanks
> Zengtao 
> 

Thanks,
Laura