Yonatan Maman <ymaman@xxxxxxxxxx> writes:
> From: Yonatan Maman <Ymaman@xxxxxxxxxx>
>
> hmm_range_fault() natively triggers a page fault on device private
> pages, migrating them to RAM. In some cases, such as with RDMA devices,
> the migration overhead between the device (e.g., GPU) and the CPU, and
> vice-versa, significantly damages performance. Thus, enabling Peer-to-
> Peer (P2P) DMA access for device private page might be crucial for
> minimizing data transfer overhead.
>
> This change introduces an API to support P2P connections for device
> private pages by implementing the following:
>
> - Leveraging the struct pagemap_ops for P2P Page Callbacks. This
> callback involves mapping the page to MMIO and returning the
> corresponding PCI_P2P page.
>
> - Utilizing hmm_range_fault for Initializing P2P Connections. The API
> also adds the HMM_PFN_REQ_TRY_P2P flag option for the
> hmm_range_fault caller to initialize P2P. If set, hmm_range_fault
> attempts initializing the P2P connection first, if the owner device
> supports P2P, using p2p_page. In case of failure or lack of support,
> hmm_range_fault will continue with the regular flow of migrating the
> page to RAM.
>
> This change does not affect previous use-cases of hmm_range_fault,
> because both the caller and the page owner must explicitly request and
> support it to initialize P2P connection.
>
> Signed-off-by: Yonatan Maman <Ymaman@xxxxxxxxxx>
> Reviewed-by: Gal Shalom <GalShalom@xxxxxxxxxx>
> ---
> include/linux/hmm.h | 2 ++
> include/linux/memremap.h | 7 +++++++
> mm/hmm.c | 28 ++++++++++++++++++++++++++++
> 3 files changed, 37 insertions(+)
>
> diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> index 126a36571667..7154f5ed73a1 100644
> --- a/include/linux/hmm.h
> +++ b/include/linux/hmm.h
> @@ -41,6 +41,8 @@ enum hmm_pfn_flags {
> /* Input flags */
> HMM_PFN_REQ_FAULT = HMM_PFN_VALID,
> HMM_PFN_REQ_WRITE = HMM_PFN_WRITE,
> + /* allow returning PCI P2PDMA pages */
> + HMM_PFN_REQ_ALLOW_P2P = 1,
>
> HMM_PFN_FLAGS = 0xFFUL << HMM_PFN_ORDER_SHIFT,
> };
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index 3f7143ade32c..0ecfd3d191fa 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -89,6 +89,13 @@ struct dev_pagemap_ops {
> */
> vm_fault_t (*migrate_to_ram)(struct vm_fault *vmf);
>
> + /*
> + * Used for private (un-addressable) device memory only. Return a
> + * corresponding struct page, that can be mapped to device
> + * (e.g using dma_map_page)
> + */
> + struct page *(*get_dma_page_for_device)(struct page *private_page);
It would be nice to add some documentation about this feature to
Documentation/mm/hmm.rst. In particular some notes on the page
lifetime/refcounting rules.
On that note how is the refcounting of the returned p2pdma page expected
to work? We don't want the driver calling hmm_range_fault() to be able
to pin the page with eg. get_page(), so the returned p2pdma page should
have a zero refcount to enforce that.
> +
> /*
> * Handle the memory failure happens on a range of pfns. Notify the
> * processes who are using these pfns, and try to recover the data on
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 7e0229ae4a5a..987dd143d697 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -230,6 +230,8 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
> unsigned long cpu_flags;
> pte_t pte = ptep_get(ptep);
> uint64_t pfn_req_flags = *hmm_pfn;
> + struct page *(*get_dma_page_handler)(struct page *private_page);
> + struct page *dma_page;
>
> if (pte_none_mostly(pte)) {
> required_fault =
> @@ -257,6 +259,32 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
> return 0;
> }
>
> + /*
> + * P2P for supported pages, and according to caller request
> + * translate the private page to the match P2P page if it fails
> + * continue with the regular flow
> + */
> + if (is_device_private_entry(entry)) {
> + get_dma_page_handler =
> + pfn_swap_entry_to_page(entry)
> + ->pgmap->ops->get_dma_page_for_device;
> + if ((hmm_vma_walk->range->default_flags &
> + HMM_PFN_REQ_ALLOW_P2P) &&
> + get_dma_page_handler) {
> + dma_page = get_dma_page_handler(
> + pfn_swap_entry_to_page(entry));
> + if (!IS_ERR(dma_page)) {
> + cpu_flags = HMM_PFN_VALID;
> + if (is_writable_device_private_entry(
> + entry))
> + cpu_flags |= HMM_PFN_WRITE;
> + *hmm_pfn = page_to_pfn(dma_page) |
> + cpu_flags;
> + return 0;
> + }
> + }
> + }
> +
> required_fault =
> hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, 0);
> if (!required_fault) {
