[PATCH 1/2] dma-buf: heaps: DMA_HEAP_IOCTL_ALLOC_READ_FILE framework

[Christian König]

Am 16.07.24 um 11:31 schrieb Daniel Vetter:
> On Tue, Jul 16, 2024 at 10:48:40AM +0800, Huan Yang wrote:
>> I just research the udmabuf, Please correct me if I'm wrong.
>>
>> 在 2024/7/15 20:32, Christian König 写道:
>>> Am 15.07.24 um 11:11 schrieb Daniel Vetter:
>>>> On Thu, Jul 11, 2024 at 11:00:02AM +0200, Christian König wrote:
>>>>> Am 11.07.24 um 09:42 schrieb Huan Yang:
>>>>>> Some user may need load file into dma-buf, current
>>>>>> way is:
>>>>>>      1. allocate a dma-buf, get dma-buf fd
>>>>>>      2. mmap dma-buf fd into vaddr
>>>>>>      3. read(file_fd, vaddr, fsz)
>>>>>> This is too heavy if fsz reached to GB.
>>>>> You need to describe a bit more why that is to heavy. I can only
>>>>> assume you
>>>>> need to save memory bandwidth and avoid the extra copy with the CPU.
>>>>>
>>>>>> This patch implement a feature called DMA_HEAP_IOCTL_ALLOC_READ_FILE.
>>>>>> User need to offer a file_fd which you want to load into
>>>>>> dma-buf, then,
>>>>>> it promise if you got a dma-buf fd, it will contains the file content.
>>>>> Interesting idea, that has at least more potential than trying
>>>>> to enable
>>>>> direct I/O on mmap()ed DMA-bufs.
>>>>>
>>>>> The approach with the new IOCTL might not work because it is a very
>>>>> specialized use case.
>>>>>
>>>>> But IIRC there was a copy_file_range callback in the file_operations
>>>>> structure you could use for that. I'm just not sure when and how
>>>>> that's used
>>>>> with the copy_file_range() system call.
>>>> I'm not sure any of those help, because internally they're all still
>>>> based
>>>> on struct page (or maybe in the future on folios). And that's the thing
>>>> dma-buf can't give you, at least without peaking behind the curtain.
>>>>
>>>> I think an entirely different option would be malloc+udmabuf. That
>>>> essentially handles the impendence-mismatch between direct I/O and
>>>> dma-buf
>>>> on the dma-buf side. The downside is that it'll make the permanently
>>>> pinned memory accounting and tracking issues even more apparent, but I
>>>> guess eventually we do need to sort that one out.
>>> Oh, very good idea!
>>> Just one minor correction: it's not malloc+udmabuf, but rather
>>> create_memfd()+udmabuf.
> Hm right, it's create_memfd() + mmap(memfd) + udmabuf
>
>>> And you need to complete your direct I/O before creating the udmabuf
>>> since that reference will prevent direct I/O from working.
>> udmabuf will pin all pages, so, if returned fd, can't trigger direct I/O
>> (same as dmabuf). So, must complete read before pin it.
> Why does pinning prevent direct I/O? I haven't tested, but I'd expect the
> rdma folks would be really annoyed if that's the case ...

Pinning (or rather taking another page reference) prevents writes from 
using direct I/O because writes try to find all references and make them 
read only so that nobody modifies the content while the write is done.

As far as I know the same approach is used for NUMA migration and 
replacing small pages with big ones in THP. But for the read case here 
it should still work.

>> But current way is use `memfd_pin_folios` to boost alloc and pin, so maybe
>> need suit it.
>>
>>
>> I currently doubt that the udmabuf solution is suitable for our
>> gigabyte-level read operations.
>>
>> 1. The current mmap operation uses faulting, so frequent page faults will be
>> triggered during reads, resulting in a lot of context switching overhead.
>>
>> 2. current udmabuf size limit is 64MB, even can change, maybe not good to
>> use in large size?
> Yeah that's just a figleaf so we don't have to bother about the accounting
> issue.
>
>> 3. The migration and adaptation of the driver is also a challenge, and
>> currently, we are unable to control it.
> Why does a udmabuf fd not work instead of any other dmabuf fd? That
> shouldn't matter for the consuming driver ...
>
>> Perhaps implementing `copy_file_range` would be more suitable for us.
> See my other mail, fundamentally these all rely on struct page being
> present, and dma-buf doesn't give you that. Which means you need to go
> below the dma-buf abstraction. And udmabuf is pretty much the thing for
> that, because it wraps normal struct page memory into a dmabuf.
>
> And copy_file_range on the underlying memfd might already work, I haven't
> checked though.

Yeah completely agree.

Regards,
Christian.

>
> Cheers, Sima
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