[PATCH 0/2] Support direct I/O read and write for memory allocated by dmabuf

[PATCH 0/2] Support direct I/O read and write for memory allocated by dmabuf

[Lei Liu]

Use vm_insert_page to establish a mapping for the memory allocated
by dmabuf, thus supporting direct I/O read and write; and fix the
issue of incorrect memory statistics after mapping dmabuf memory.

Lei Liu (2):
  mm: dmabuf_direct_io: Support direct_io for memory allocated by dmabuf
  mm: dmabuf_direct_io: Fix memory statistics error for dmabuf allocated
    memory with direct_io support

 drivers/dma-buf/heaps/system_heap.c |  5 +++--
 fs/proc/task_mmu.c                  |  8 +++++++-
 include/linux/mm.h                  |  1 +
 mm/memory.c                         | 15 ++++++++++-----
 mm/rmap.c                           |  9 +++++----
 5 files changed, 26 insertions(+), 12 deletions(-)

-- 
2.34.1

[PATCH 1/2] mm: dmabuf_direct_io: Support direct_io for memory allocated by dmabuf

[Lei Liu]

1.Effects and reasons for lack of support:

Currently, memory allocated by dmabuf cannot be read from files using
direct_io. With the increasing use of AI models in mobile applications,
there is a growing need to load large model files occupying up to 3-4GB
into mobile memory. Presently, the only way to read is through
buffer_io, which limits performance. In low memory scenarios on 12GB RAM
smartphones, buffer_io requires additional memory, leading to a 3-4
times degradation in read performance with significant fluctuations.

The reason for the lack of support for direct_io reading is that the
current system establishes mappings for memory allocated by dmabuf using
remap_pfn_range, which includes the VM_PFN_MAP flag. When attempting
direct_io reads, the get_user_page process intercepts the VM_PFN_MAP
flag, preventing the page from being returned and resulting in read
failures.

2.Proposed solution:
  (1) Establish mmap mappings for memory allocated by dmabuf using the
vm_insert_page method to support direct_io read and write.

3.Advantages and benefits:
  (1) Faster and more stable reading speed.
  (2) Reduced pagecache memory usage.
  (3) Reduction in CPU data copying and unnecessary power consumption.

4.In a clean and stressapptest(a 16GB memory phone consumed 4GB of
  memory). A comparison of the time taken to read a 3.2GB large AI model
file using buffer_io and direct_io.

Read 3.21G AI large model file on mobilephone
Memstress  Rounds    DIO-Time/ms   BIO-Time/ms
             01        1432          2034
Clean        02        1406          2225
             03        1476          2097
           average     1438          2118
Memstress  Rounds    DIO-Time/ms   BIO-Time/ms
             01        1585          4821
Eat 4GB      02        1560          4957
             03        1519          4936
           average     1554          4905

Signed-off-by: Lei Liu <liulei.rjpt@vivo.com>
---
 drivers/dma-buf/heaps/system_heap.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/dma-buf/heaps/system_heap.c b/drivers/dma-buf/heaps/system_heap.c
index 9076d47ed2ef..87547791f9e1 100644
--- a/drivers/dma-buf/heaps/system_heap.c
+++ b/drivers/dma-buf/heaps/system_heap.c
@@ -203,8 +203,7 @@ static int system_heap_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma)
 	for_each_sgtable_page(table, &piter, vma->vm_pgoff) {
 		struct page *page = sg_page_iter_page(&piter);
 
-		ret = remap_pfn_range(vma, addr, page_to_pfn(page), PAGE_SIZE,
-				      vma->vm_page_prot);
+		ret = vm_insert_page(vma, addr, page);
 		if (ret)
 			return ret;
 		addr += PAGE_SIZE;
-- 
2.34.1

Re: [PATCH 0/2] Support direct I/O read and write for memory allocated by dmabuf

[Christian König]

Am 10.07.24 um 15:57 schrieb Lei Liu:
> Use vm_insert_page to establish a mapping for the memory allocated
> by dmabuf, thus supporting direct I/O read and write; and fix the
> issue of incorrect memory statistics after mapping dmabuf memory.

Well big NAK to that! Direct I/O is intentionally disabled on DMA-bufs.

We already discussed enforcing that in the DMA-buf framework and this 
patch probably means that we should really do that.

Regards,
Christian.

>
> Lei Liu (2):
>    mm: dmabuf_direct_io: Support direct_io for memory allocated by dmabuf
>    mm: dmabuf_direct_io: Fix memory statistics error for dmabuf allocated
>      memory with direct_io support
>
>   drivers/dma-buf/heaps/system_heap.c |  5 +++--
>   fs/proc/task_mmu.c                  |  8 +++++++-
>   include/linux/mm.h                  |  1 +
>   mm/memory.c                         | 15 ++++++++++-----
>   mm/rmap.c                           |  9 +++++----
>   5 files changed, 26 insertions(+), 12 deletions(-)
>

Re: [PATCH 0/2] Support direct I/O read and write for memory allocated by dmabuf

[Lei Liu]

在 2024/7/10 22:14, Christian König 写道:
> Am 10.07.24 um 15:57 schrieb Lei Liu:
>> Use vm_insert_page to establish a mapping for the memory allocated
>> by dmabuf, thus supporting direct I/O read and write; and fix the
>> issue of incorrect memory statistics after mapping dmabuf memory.
>
> Well big NAK to that! Direct I/O is intentionally disabled on DMA-bufs.

Hello! Could you explain why direct_io is disabled on DMABUF? Is there 
any historical reason for this?

>
> We already discussed enforcing that in the DMA-buf framework and this 
> patch probably means that we should really do that.
>
> Regards,
> Christian.

Thank you for your response. With the application of AI large model 
edgeification, we urgently need support for direct_io on DMABUF to read 
some very large files. Do you have any new solutions or plans for this?

Regards,
Lei Liu.

>
>>
>> Lei Liu (2):
>>    mm: dmabuf_direct_io: Support direct_io for memory allocated by 
>> dmabuf
>>    mm: dmabuf_direct_io: Fix memory statistics error for dmabuf 
>> allocated
>>      memory with direct_io support
>>
>>   drivers/dma-buf/heaps/system_heap.c |  5 +++--
>>   fs/proc/task_mmu.c                  |  8 +++++++-
>>   include/linux/mm.h                  |  1 +
>>   mm/memory.c                         | 15 ++++++++++-----
>>   mm/rmap.c                           |  9 +++++----
>>   5 files changed, 26 insertions(+), 12 deletions(-)
>>
>

Re: [PATCH 0/2] Support direct I/O read and write for memory allocated by dmabuf

[Christian König]

Am 10.07.24 um 16:35 schrieb Lei Liu:
>
> 在 2024/7/10 22:14, Christian König 写道:
>> Am 10.07.24 um 15:57 schrieb Lei Liu:
>>> Use vm_insert_page to establish a mapping for the memory allocated
>>> by dmabuf, thus supporting direct I/O read and write; and fix the
>>> issue of incorrect memory statistics after mapping dmabuf memory.
>>
>> Well big NAK to that! Direct I/O is intentionally disabled on DMA-bufs.
>
> Hello! Could you explain why direct_io is disabled on DMABUF? Is there 
> any historical reason for this?

It's basically one of the most fundamental design decision of DMA-Buf. 
The attachment/map/fence model DMA-buf uses is not really compatible 
with direct I/O on the underlying pages.

>>
>> We already discussed enforcing that in the DMA-buf framework and this 
>> patch probably means that we should really do that.
>>
>> Regards,
>> Christian.
>
> Thank you for your response. With the application of AI large model 
> edgeification, we urgently need support for direct_io on DMABUF to 
> read some very large files. Do you have any new solutions or plans for 
> this?

We have seen similar projects over the years and all of those turned out 
to be complete shipwrecks.

There is currently a patch set under discussion to give the network 
subsystem DMA-buf support. If you are interest in network direct I/O 
that could help.

Additional to that a lot of GPU drivers support userptr usages, e.g. to 
import malloced memory into the GPU driver. You can then also do direct 
I/O on that malloced memory and the kernel will enforce correct handling 
with the GPU driver through MMU notifiers.

But as far as I know a general DMA-buf based solution isn't possible.

Regards,
Christian.

>
> Regards,
> Lei Liu.
>
>>
>>>
>>> Lei Liu (2):
>>>    mm: dmabuf_direct_io: Support direct_io for memory allocated by 
>>> dmabuf
>>>    mm: dmabuf_direct_io: Fix memory statistics error for dmabuf 
>>> allocated
>>>      memory with direct_io support
>>>
>>>   drivers/dma-buf/heaps/system_heap.c |  5 +++--
>>>   fs/proc/task_mmu.c                  |  8 +++++++-
>>>   include/linux/mm.h                  |  1 +
>>>   mm/memory.c                         | 15 ++++++++++-----
>>>   mm/rmap.c                           |  9 +++++----
>>>   5 files changed, 26 insertions(+), 12 deletions(-)
>>>
>>

Re: [PATCH 0/2] Support direct I/O read and write for memory allocated by dmabuf

[Lei Liu]

on 2024/7/10 22:48, Christian König wrote:
> Am 10.07.24 um 16:35 schrieb Lei Liu:
>>
>> on 2024/7/10 22:14, Christian König wrote:
>>> Am 10.07.24 um 15:57 schrieb Lei Liu:
>>>> Use vm_insert_page to establish a mapping for the memory allocated
>>>> by dmabuf, thus supporting direct I/O read and write; and fix the
>>>> issue of incorrect memory statistics after mapping dmabuf memory.
>>>
>>> Well big NAK to that! Direct I/O is intentionally disabled on DMA-bufs.
>>
>> Hello! Could you explain why direct_io is disabled on DMABUF? Is 
>> there any historical reason for this?
>
> It's basically one of the most fundamental design decision of DMA-Buf. 
> The attachment/map/fence model DMA-buf uses is not really compatible 
> with direct I/O on the underlying pages.

Thank you! Is there any related documentation on this? I would like to 
understand and learn more about the fundamental reasons for the lack of 
support.
>
>>>
>>> We already discussed enforcing that in the DMA-buf framework and 
>>> this patch probably means that we should really do that.
>>>
>>> Regards,
>>> Christian.
>>
>> Thank you for your response. With the application of AI large model 
>> edgeification, we urgently need support for direct_io on DMABUF to 
>> read some very large files. Do you have any new solutions or plans 
>> for this?
>
> We have seen similar projects over the years and all of those turned 
> out to be complete shipwrecks.
>
> There is currently a patch set under discussion to give the network 
> subsystem DMA-buf support. If you are interest in network direct I/O 
> that could help.

Is there a related introduction link for this patch?

>
> Additional to that a lot of GPU drivers support userptr usages, e.g. 
> to import malloced memory into the GPU driver. You can then also do 
> direct I/O on that malloced memory and the kernel will enforce correct 
> handling with the GPU driver through MMU notifiers.
>
> But as far as I know a general DMA-buf based solution isn't possible.

1.The reason we need to use DMABUF memory here is that we need to share 
memory between the CPU and APU. Currently, only DMABUF memory is 
suitable for this purpose. Additionally, we need to read very large files.

2. Are there any other solutions for this? Also, do you have any plans 
to support direct_io for DMABUF memory in the future?

>
> Regards,
> Christian.
>
>>
>> Regards,
>> Lei Liu.
>>
>>>
>>>>
>>>> Lei Liu (2):
>>>>    mm: dmabuf_direct_io: Support direct_io for memory allocated by 
>>>> dmabuf
>>>>    mm: dmabuf_direct_io: Fix memory statistics error for dmabuf 
>>>> allocated
>>>>      memory with direct_io support
>>>>
>>>>   drivers/dma-buf/heaps/system_heap.c |  5 +++--
>>>>   fs/proc/task_mmu.c                  |  8 +++++++-
>>>>   include/linux/mm.h                  |  1 +
>>>>   mm/memory.c                         | 15 ++++++++++-----
>>>>   mm/rmap.c                           |  9 +++++----
>>>>   5 files changed, 26 insertions(+), 12 deletions(-)
>>>>
>>>
>

Re: [PATCH 0/2] Support direct I/O read and write for memory allocated by dmabuf

[T.J. Mercier]

On Wed, Jul 10, 2024 at 8:08 AM Lei Liu <liulei.rjpt@vivo.com> wrote:
>
>
> on 2024/7/10 22:48, Christian König wrote:
> > Am 10.07.24 um 16:35 schrieb Lei Liu:
> >>
> >> on 2024/7/10 22:14, Christian König wrote:
> >>> Am 10.07.24 um 15:57 schrieb Lei Liu:
> >>>> Use vm_insert_page to establish a mapping for the memory allocated
> >>>> by dmabuf, thus supporting direct I/O read and write; and fix the
> >>>> issue of incorrect memory statistics after mapping dmabuf memory.
> >>>
> >>> Well big NAK to that! Direct I/O is intentionally disabled on DMA-bufs.
> >>
> >> Hello! Could you explain why direct_io is disabled on DMABUF? Is
> >> there any historical reason for this?
> >
> > It's basically one of the most fundamental design decision of DMA-Buf.
> > The attachment/map/fence model DMA-buf uses is not really compatible
> > with direct I/O on the underlying pages.
>
> Thank you! Is there any related documentation on this? I would like to
> understand and learn more about the fundamental reasons for the lack of
> support.

Hi Lei and Christian,

This is now the third request I've seen from three different companies
who are interested in this, but the others are not for reasons of read
performance that you mention in the commit message on your first
patch. Someone else at Google ran a comparison between a normal read()
and a direct I/O read() into a preallocated user buffer and found that
with large readahead (16 MB) the throughput can actually be slightly
higher than direct I/O. If you have concerns about read performance,
have you tried increasing the readahead size?

The other motivation is to load a gajillion byte file from disk into a
dmabuf without evicting the entire contents of pagecache while doing
so. Something like this (which does not currently work because read()
tries to GUP on the dmabuf memory as you mention):

static int dmabuf_heap_alloc(int heap_fd, size_t len)
{
    struct dma_heap_allocation_data data = {
        .len = len,
        .fd = 0,
        .fd_flags = O_RDWR | O_CLOEXEC,
        .heap_flags = 0,
    };
    int ret = ioctl(heap_fd, DMA_HEAP_IOCTL_ALLOC, &data);
    if (ret < 0)
        return ret;
    return data.fd;
}

int main(int, char **argv)
{
        const char *file_path = argv[1];
        printf("File: %s\n", file_path);
        int file_fd = open(file_path, O_RDONLY | O_DIRECT);

        struct stat st;
        stat(file_path, &st);
        ssize_t file_size = st.st_size;
        ssize_t aligned_size = (file_size + 4095) & ~4095;

        printf("File size: %zd Aligned size: %zd\n", file_size, aligned_size);
        int heap_fd = open("/dev/dma_heap/system", O_RDONLY);
        int dmabuf_fd = dmabuf_heap_alloc(heap_fd, aligned_size);

        void *vm = mmap(nullptr, aligned_size, PROT_READ | PROT_WRITE,
MAP_SHARED, dmabuf_fd, 0);
        printf("VM at 0x%lx\n", (unsigned long)vm);

        dma_buf_sync sync_flags { DMA_BUF_SYNC_START |
DMA_BUF_SYNC_READ | DMA_BUF_SYNC_WRITE };
        ioctl(dmabuf_fd, DMA_BUF_IOCTL_SYNC, &sync_flags);

        ssize_t rc = read(file_fd, vm, file_size);
        printf("Read: %zd %s\n", rc, rc < 0 ? strerror(errno) : "");

        sync_flags.flags = DMA_BUF_SYNC_END | DMA_BUF_SYNC_READ |
DMA_BUF_SYNC_WRITE;
        ioctl(dmabuf_fd, DMA_BUF_IOCTL_SYNC, &sync_flags);
}

Or replace the mmap() + read() with sendfile().

So I would also like to see the above code (or something else similar)
be able to work and I understand some of the reasons why it currently
does not, but I don't understand why we should actively prevent this
type of behavior entirely.

Best,
T.J.








> >
> >>>
> >>> We already discussed enforcing that in the DMA-buf framework and
> >>> this patch probably means that we should really do that.
> >>>
> >>> Regards,
> >>> Christian.
> >>
> >> Thank you for your response. With the application of AI large model
> >> edgeification, we urgently need support for direct_io on DMABUF to
> >> read some very large files. Do you have any new solutions or plans
> >> for this?
> >
> > We have seen similar projects over the years and all of those turned
> > out to be complete shipwrecks.
> >
> > There is currently a patch set under discussion to give the network
> > subsystem DMA-buf support. If you are interest in network direct I/O
> > that could help.
>
> Is there a related introduction link for this patch?
>
> >
> > Additional to that a lot of GPU drivers support userptr usages, e.g.
> > to import malloced memory into the GPU driver. You can then also do
> > direct I/O on that malloced memory and the kernel will enforce correct
> > handling with the GPU driver through MMU notifiers.
> >
> > But as far as I know a general DMA-buf based solution isn't possible.
>
> 1.The reason we need to use DMABUF memory here is that we need to share
> memory between the CPU and APU. Currently, only DMABUF memory is
> suitable for this purpose. Additionally, we need to read very large files.
>
> 2. Are there any other solutions for this? Also, do you have any plans
> to support direct_io for DMABUF memory in the future?
>
> >
> > Regards,
> > Christian.
> >
> >>
> >> Regards,
> >> Lei Liu.
> >>
> >>>
> >>>>
> >>>> Lei Liu (2):
> >>>>    mm: dmabuf_direct_io: Support direct_io for memory allocated by
> >>>> dmabuf
> >>>>    mm: dmabuf_direct_io: Fix memory statistics error for dmabuf
> >>>> allocated
> >>>>      memory with direct_io support
> >>>>
> >>>>   drivers/dma-buf/heaps/system_heap.c |  5 +++--
> >>>>   fs/proc/task_mmu.c                  |  8 +++++++-
> >>>>   include/linux/mm.h                  |  1 +
> >>>>   mm/memory.c                         | 15 ++++++++++-----
> >>>>   mm/rmap.c                           |  9 +++++----
> >>>>   5 files changed, 26 insertions(+), 12 deletions(-)
> >>>>
> >>>
> >

Re: [PATCH 0/2] Support direct I/O read and write for memory allocated by dmabuf

[Christian König]

Am 10.07.24 um 18:34 schrieb T.J. Mercier:
> On Wed, Jul 10, 2024 at 8:08 AM Lei Liu <liulei.rjpt@vivo.com> wrote:
>> on 2024/7/10 22:48, Christian König wrote:
>>> Am 10.07.24 um 16:35 schrieb Lei Liu:
>>>> on 2024/7/10 22:14, Christian König wrote:
>>>>> Am 10.07.24 um 15:57 schrieb Lei Liu:
>>>>>> Use vm_insert_page to establish a mapping for the memory allocated
>>>>>> by dmabuf, thus supporting direct I/O read and write; and fix the
>>>>>> issue of incorrect memory statistics after mapping dmabuf memory.
>>>>> Well big NAK to that! Direct I/O is intentionally disabled on DMA-bufs.
>>>> Hello! Could you explain why direct_io is disabled on DMABUF? Is
>>>> there any historical reason for this?
>>> It's basically one of the most fundamental design decision of DMA-Buf.
>>> The attachment/map/fence model DMA-buf uses is not really compatible
>>> with direct I/O on the underlying pages.
>> Thank you! Is there any related documentation on this? I would like to
>> understand and learn more about the fundamental reasons for the lack of
>> support.
> Hi Lei and Christian,
>
> This is now the third request I've seen from three different companies
> who are interested in this,

Yeah, completely agree. This is a re-occurring pattern :)

Maybe we should document the preferred solution for that.

> but the others are not for reasons of read
> performance that you mention in the commit message on your first
> patch. Someone else at Google ran a comparison between a normal read()
> and a direct I/O read() into a preallocated user buffer and found that
> with large readahead (16 MB) the throughput can actually be slightly
> higher than direct I/O. If you have concerns about read performance,
> have you tried increasing the readahead size?
>
> The other motivation is to load a gajillion byte file from disk into a
> dmabuf without evicting the entire contents of pagecache while doing
> so. Something like this (which does not currently work because read()
> tries to GUP on the dmabuf memory as you mention):
>
> static int dmabuf_heap_alloc(int heap_fd, size_t len)
> {
>      struct dma_heap_allocation_data data = {
>          .len = len,
>          .fd = 0,
>          .fd_flags = O_RDWR | O_CLOEXEC,
>          .heap_flags = 0,
>      };
>      int ret = ioctl(heap_fd, DMA_HEAP_IOCTL_ALLOC, &data);
>      if (ret < 0)
>          return ret;
>      return data.fd;
> }
>
> int main(int, char **argv)
> {
>          const char *file_path = argv[1];
>          printf("File: %s\n", file_path);
>          int file_fd = open(file_path, O_RDONLY | O_DIRECT);
>
>          struct stat st;
>          stat(file_path, &st);
>          ssize_t file_size = st.st_size;
>          ssize_t aligned_size = (file_size + 4095) & ~4095;
>
>          printf("File size: %zd Aligned size: %zd\n", file_size, aligned_size);
>          int heap_fd = open("/dev/dma_heap/system", O_RDONLY);
>          int dmabuf_fd = dmabuf_heap_alloc(heap_fd, aligned_size);
>
>          void *vm = mmap(nullptr, aligned_size, PROT_READ | PROT_WRITE,
> MAP_SHARED, dmabuf_fd, 0);
>          printf("VM at 0x%lx\n", (unsigned long)vm);
>
>          dma_buf_sync sync_flags { DMA_BUF_SYNC_START |
> DMA_BUF_SYNC_READ | DMA_BUF_SYNC_WRITE };
>          ioctl(dmabuf_fd, DMA_BUF_IOCTL_SYNC, &sync_flags);
>
>          ssize_t rc = read(file_fd, vm, file_size);
>          printf("Read: %zd %s\n", rc, rc < 0 ? strerror(errno) : "");
>
>          sync_flags.flags = DMA_BUF_SYNC_END | DMA_BUF_SYNC_READ |
> DMA_BUF_SYNC_WRITE;
>          ioctl(dmabuf_fd, DMA_BUF_IOCTL_SYNC, &sync_flags);
> }
>
> Or replace the mmap() + read() with sendfile().

Or copy_file_range(). That's pretty much exactly what I suggested on the 
other mail thread around that topic as well.

> So I would also like to see the above code (or something else similar)
> be able to work and I understand some of the reasons why it currently
> does not, but I don't understand why we should actively prevent this
> type of behavior entirely.

+1

Regards,
Christian.

>
> Best,
> T.J.
>
>
>
>
>
>
>
>
>>>>> We already discussed enforcing that in the DMA-buf framework and
>>>>> this patch probably means that we should really do that.
>>>>>
>>>>> Regards,
>>>>> Christian.
>>>> Thank you for your response. With the application of AI large model
>>>> edgeification, we urgently need support for direct_io on DMABUF to
>>>> read some very large files. Do you have any new solutions or plans
>>>> for this?
>>> We have seen similar projects over the years and all of those turned
>>> out to be complete shipwrecks.
>>>
>>> There is currently a patch set under discussion to give the network
>>> subsystem DMA-buf support. If you are interest in network direct I/O
>>> that could help.
>> Is there a related introduction link for this patch?
>>
>>> Additional to that a lot of GPU drivers support userptr usages, e.g.
>>> to import malloced memory into the GPU driver. You can then also do
>>> direct I/O on that malloced memory and the kernel will enforce correct
>>> handling with the GPU driver through MMU notifiers.
>>>
>>> But as far as I know a general DMA-buf based solution isn't possible.
>> 1.The reason we need to use DMABUF memory here is that we need to share
>> memory between the CPU and APU. Currently, only DMABUF memory is
>> suitable for this purpose. Additionally, we need to read very large files.
>>
>> 2. Are there any other solutions for this? Also, do you have any plans
>> to support direct_io for DMABUF memory in the future?
>>
>>> Regards,
>>> Christian.
>>>
>>>> Regards,
>>>> Lei Liu.
>>>>
>>>>>> Lei Liu (2):
>>>>>>     mm: dmabuf_direct_io: Support direct_io for memory allocated by
>>>>>> dmabuf
>>>>>>     mm: dmabuf_direct_io: Fix memory statistics error for dmabuf
>>>>>> allocated
>>>>>>       memory with direct_io support
>>>>>>
>>>>>>    drivers/dma-buf/heaps/system_heap.c |  5 +++--
>>>>>>    fs/proc/task_mmu.c                  |  8 +++++++-
>>>>>>    include/linux/mm.h                  |  1 +
>>>>>>    mm/memory.c                         | 15 ++++++++++-----
>>>>>>    mm/rmap.c                           |  9 +++++----
>>>>>>    5 files changed, 26 insertions(+), 12 deletions(-)
>>>>>>

Re: [PATCH 0/2] Support direct I/O read and write for memory allocated by dmabuf

[Daniel Vetter]

On Wed, Jul 10, 2024 at 04:14:18PM +0200, Christian König wrote:
> Am 10.07.24 um 15:57 schrieb Lei Liu:
> > Use vm_insert_page to establish a mapping for the memory allocated
> > by dmabuf, thus supporting direct I/O read and write; and fix the
> > issue of incorrect memory statistics after mapping dmabuf memory.
> 
> Well big NAK to that! Direct I/O is intentionally disabled on DMA-bufs.
> 
> We already discussed enforcing that in the DMA-buf framework and this patch
> probably means that we should really do that.

Last time I looked dma_mmap doesn't guarantee that the vma end sup with
VM_SPECIAL, and that's pretty much the only reason why we can't enforce
this. But we might be able to enforce this at least on some architectures,
I didn't check for that ... if at least x86-64 and arm64 could have the
check, that would be great. So might be worth it to re-audit this all.

I think all other dma-buf exporters/allocators do only create VM_SPECIAL
vmas.
-Sima
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [PATCH 0/2] Support direct I/O read and write for memory allocated by dmabuf

[Lei Liu]

On 2024/7/11 22:25, Christian König wrote:
> Am 10.07.24 um 18:34 schrieb T.J. Mercier:
>> On Wed, Jul 10, 2024 at 8:08 AM Lei Liu <liulei.rjpt@vivo.com> wrote:
>>> on 2024/7/10 22:48, Christian König wrote:
>>>> Am 10.07.24 um 16:35 schrieb Lei Liu:
>>>>> on 2024/7/10 22:14, Christian König wrote:
>>>>>> Am 10.07.24 um 15:57 schrieb Lei Liu:
>>>>>>> Use vm_insert_page to establish a mapping for the memory allocated
>>>>>>> by dmabuf, thus supporting direct I/O read and write; and fix the
>>>>>>> issue of incorrect memory statistics after mapping dmabuf memory.
>>>>>> Well big NAK to that! Direct I/O is intentionally disabled on 
>>>>>> DMA-bufs.
>>>>> Hello! Could you explain why direct_io is disabled on DMABUF? Is
>>>>> there any historical reason for this?
>>>> It's basically one of the most fundamental design decision of DMA-Buf.
>>>> The attachment/map/fence model DMA-buf uses is not really compatible
>>>> with direct I/O on the underlying pages.
>>> Thank you! Is there any related documentation on this? I would like to
>>> understand and learn more about the fundamental reasons for the lack of
>>> support.
>> Hi Lei and Christian,
>>
>> This is now the third request I've seen from three different companies
>> who are interested in this,
>
> Yeah, completely agree. This is a re-occurring pattern :)
>
> Maybe we should document the preferred solution for that.
>
>> but the others are not for reasons of read
>> performance that you mention in the commit message on your first
>> patch. Someone else at Google ran a comparison between a normal read()
>> and a direct I/O read() into a preallocated user buffer and found that
>> with large readahead (16 MB) the throughput can actually be slightly
>> higher than direct I/O. If you have concerns about read performance,
>> have you tried increasing the readahead size?
>>
>> The other motivation is to load a gajillion byte file from disk into a
>> dmabuf without evicting the entire contents of pagecache while doing
>> so. Something like this (which does not currently work because read()
>> tries to GUP on the dmabuf memory as you mention):
>>
>> static int dmabuf_heap_alloc(int heap_fd, size_t len)
>> {
>>      struct dma_heap_allocation_data data = {
>>          .len = len,
>>          .fd = 0,
>>          .fd_flags = O_RDWR | O_CLOEXEC,
>>          .heap_flags = 0,
>>      };
>>      int ret = ioctl(heap_fd, DMA_HEAP_IOCTL_ALLOC, &data);
>>      if (ret < 0)
>>          return ret;
>>      return data.fd;
>> }
>>
>> int main(int, char **argv)
>> {
>>          const char *file_path = argv[1];
>>          printf("File: %s\n", file_path);
>>          int file_fd = open(file_path, O_RDONLY | O_DIRECT);
>>
>>          struct stat st;
>>          stat(file_path, &st);
>>          ssize_t file_size = st.st_size;
>>          ssize_t aligned_size = (file_size + 4095) & ~4095;
>>
>>          printf("File size: %zd Aligned size: %zd\n", file_size, 
>> aligned_size);
>>          int heap_fd = open("/dev/dma_heap/system", O_RDONLY);
>>          int dmabuf_fd = dmabuf_heap_alloc(heap_fd, aligned_size);
>>
>>          void *vm = mmap(nullptr, aligned_size, PROT_READ | PROT_WRITE,
>> MAP_SHARED, dmabuf_fd, 0);
>>          printf("VM at 0x%lx\n", (unsigned long)vm);
>>
>>          dma_buf_sync sync_flags { DMA_BUF_SYNC_START |
>> DMA_BUF_SYNC_READ | DMA_BUF_SYNC_WRITE };
>>          ioctl(dmabuf_fd, DMA_BUF_IOCTL_SYNC, &sync_flags);
>>
>>          ssize_t rc = read(file_fd, vm, file_size);
>>          printf("Read: %zd %s\n", rc, rc < 0 ? strerror(errno) : "");
>>
>>          sync_flags.flags = DMA_BUF_SYNC_END | DMA_BUF_SYNC_READ |
>> DMA_BUF_SYNC_WRITE;
>>          ioctl(dmabuf_fd, DMA_BUF_IOCTL_SYNC, &sync_flags);
>> }
>>
>> Or replace the mmap() + read() with sendfile().
>
> Or copy_file_range(). That's pretty much exactly what I suggested on 
> the other mail thread around that topic as well.

Thank you for your suggestion. I will study the method you suggested 
with Yang. Using copy_file_range() might be a good solution approach.

Regards,
Lei Liu.

>
>> So I would also like to see the above code (or something else similar)
>> be able to work and I understand some of the reasons why it currently
>> does not, but I don't understand why we should actively prevent this
>> type of behavior entirely.
>
> +1
>
> Regards,
> Christian.
>
>>
>> Best,
>> T.J.
>>
>>
>>
>>
>>
>>
>>
>>
>>>>>> We already discussed enforcing that in the DMA-buf framework and
>>>>>> this patch probably means that we should really do that.
>>>>>>
>>>>>> Regards,
>>>>>> Christian.
>>>>> Thank you for your response. With the application of AI large model
>>>>> edgeification, we urgently need support for direct_io on DMABUF to
>>>>> read some very large files. Do you have any new solutions or plans
>>>>> for this?
>>>> We have seen similar projects over the years and all of those turned
>>>> out to be complete shipwrecks.
>>>>
>>>> There is currently a patch set under discussion to give the network
>>>> subsystem DMA-buf support. If you are interest in network direct I/O
>>>> that could help.
>>> Is there a related introduction link for this patch?
>>>
>>>> Additional to that a lot of GPU drivers support userptr usages, e.g.
>>>> to import malloced memory into the GPU driver. You can then also do
>>>> direct I/O on that malloced memory and the kernel will enforce correct
>>>> handling with the GPU driver through MMU notifiers.
>>>>
>>>> But as far as I know a general DMA-buf based solution isn't possible.
>>> 1.The reason we need to use DMABUF memory here is that we need to share
>>> memory between the CPU and APU. Currently, only DMABUF memory is
>>> suitable for this purpose. Additionally, we need to read very large 
>>> files.
>>>
>>> 2. Are there any other solutions for this? Also, do you have any plans
>>> to support direct_io for DMABUF memory in the future?
>>>
>>>> Regards,
>>>> Christian.
>>>>
>>>>> Regards,
>>>>> Lei Liu.
>>>>>
>>>>>>> Lei Liu (2):
>>>>>>>     mm: dmabuf_direct_io: Support direct_io for memory allocated by
>>>>>>> dmabuf
>>>>>>>     mm: dmabuf_direct_io: Fix memory statistics error for dmabuf
>>>>>>> allocated
>>>>>>>       memory with direct_io support
>>>>>>>
>>>>>>>    drivers/dma-buf/heaps/system_heap.c |  5 +++--
>>>>>>>    fs/proc/task_mmu.c                  |  8 +++++++-
>>>>>>>    include/linux/mm.h                  |  1 +
>>>>>>>    mm/memory.c                         | 15 ++++++++++-----
>>>>>>>    mm/rmap.c                           |  9 +++++----
>>>>>>>    5 files changed, 26 insertions(+), 12 deletions(-)
>>>>>>>
>

[PATCH 0/2] Support direct I/O read and write for memory allocated by dmabuf

[Lei Liu]

Use vm_insert_page to establish a mapping for the memory allocated
by dmabuf, thus supporting direct I/O read and write; and fix the
issue of incorrect memory statistics after mapping dmabuf memory.

Lei Liu (2):
  mm: dmabuf_direct_io: Support direct_io for memory allocated by dmabuf
  mm: dmabuf_direct_io: Fix memory statistics error for dmabuf allocated
    memory with direct_io support

 drivers/dma-buf/heaps/system_heap.c |  5 +++--
 fs/proc/task_mmu.c                  |  8 +++++++-
 include/linux/mm.h                  |  1 +
 mm/memory.c                         | 15 ++++++++++-----
 mm/rmap.c                           |  9 +++++----
 5 files changed, 26 insertions(+), 12 deletions(-)

-- 
2.34.1