[Question] Missing data after DMA read transfer - mm issue with transparent huge page?

[Nicolas Morey Chaisemartin]

Hi everyone,

This is a repost from a different address as it seems the previous one ended in Gmail junk due to a domain error..
I added more info found while blindly debugging the issue.

Short version:
I'm having an issue with direct DMA transfer from a device to host memory.
It seems some of the data is not transferring to the appropriate page.

Some more details:
I'm debugging a home made PCI driver for our board (Kalray), attached to a x86_64 host running centos7 (3.10.0-327.el7.x86_64)

In the current case, a userland application transfers back and forth data through read/write operations on a file.
On the kernel side, it triggers DMA transfers through the PCI to/from our board memory.

We followed what pretty much all docs said about direct I/O to user buffers:

1) get_user_pages() (in the current case, it's at most 16 pages at once)
2) convert to a scatterlist
3) pci_map_sg
4) eventually coalesce sg (Intel IOMMU is enabled, so it's usually possible)
4) A lot of DMA engine handling code, using the dmaengine layer and virt-dma
5) wait for transfer complete, in the mean time, go back to (1) to schedule more work, if any
6) pci_unmap_sg
7) for read (card2host) transfer, set_page_dirty_lock
8) page_cache_release

In 99,9999% it works perfectly.
However, I have one userland application where a few pages are not written by a read (card2host) transfer.
The buffer is memset them to a different value so I can check that nothing has overwritten them.

I know (PCI protocol analyser) that the data left our board for the "right" address (the one set in the sg by pci_map_sg).
I tried reading the data between the pci_unmap_sg and the set_page_dirty, using
        uint32_t *addr = page_address(trans->pages[0]);
        dev_warn(&pdata->pdev->dev, "val = %x\n", *addr);
and it has the expected value.
But if I try to copy_from_user (using the address coming from userland, the one passed to get_user_pages), the data has not been written and I see the memset value.

New infos:

The issue happens with IOMMU on or off.
I compiled a kernel with DMA_API_DEBUG enabled and got no warnings or errors.

I digged a little bit deeper with my very small understanding of linux mm and I discovered that:
 * we are using transparent huge pages
 * the page 'not transferred' are the last few of a huge page
More precisely:
- We have several transfer in flight from the same user buffer
- Each transfer is 16 pages long
- At one point in time, we start transferring from another huge page (transfers are still in flight from the previous one)
- When a transfer from the previous huge page completes, I dumped at the mapcount of the pages from the previous transfers,
  they are all to 0. The pages are still mapped to dma at this point.
- A get_user_page to the address of the completed transfer returns return a different struct page * then the on I had.
But this is before I have unmapped/put_page them back. From my understanding this should not have happened.

I tried the same code with a kernel 4.5 and encountered the same issue

Disabling transparent huge pages makes the issue disapear

Thanks in advance

Nicolas