linux-kernel - Re: [PATCH v9 11/24] mm/hmm: provide generic DMA managing logic

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Message-ID: <20250423172856.GM1213339@ziepe.ca>
Date: Wed, 23 Apr 2025 14:28:56 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Leon Romanovsky <leon@...nel.org>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>,
	Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
	Keith Busch <kbusch@...nel.org>,
	Leon Romanovsky <leonro@...dia.com>, Jake Edge <jake@....net>,
	Jonathan Corbet <corbet@....net>, Zhu Yanjun <zyjzyj2000@...il.com>,
	Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Sagi Grimberg <sagi@...mberg.me>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Logan Gunthorpe <logang@...tatee.com>,
	Yishai Hadas <yishaih@...dia.com>,
	Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-block@...r.kernel.org, linux-rdma@...r.kernel.org,
	iommu@...ts.linux.dev, linux-nvme@...ts.infradead.org,
	linux-pci@...r.kernel.org, kvm@...r.kernel.org, linux-mm@...ck.org,
	Niklas Schnelle <schnelle@...ux.ibm.com>,
	Chuck Lever <chuck.lever@...cle.com>,
	Luis Chamberlain <mcgrof@...nel.org>,
	Matthew Wilcox <willy@...radead.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Kanchan Joshi <joshi.k@...sung.com>,
	Chaitanya Kulkarni <kch@...dia.com>
Subject: Re: [PATCH v9 11/24] mm/hmm: provide generic DMA managing logic

On Wed, Apr 23, 2025 at 11:13:02AM +0300, Leon Romanovsky wrote:
> From: Leon Romanovsky <leonro@...dia.com>
> 
> HMM callers use PFN list to populate range while calling
> to hmm_range_fault(), the conversion from PFN to DMA address
> is done by the callers with help of another DMA list. However,
> it is wasteful on any modern platform and by doing the right
> logic, that DMA list can be avoided.
> 
> Provide generic logic to manage these lists and gave an interface
> to map/unmap PFNs to DMA addresses, without requiring from the callers
> to be an experts in DMA core API.
> 
> Tested-by: Jens Axboe <axboe@...nel.dk>

I don't think Jens tested the RDMA and hmm parts :)

> +	/*
> +	 * The HMM API violates our normal DMA buffer ownership rules and can't
> +	 * transfer buffer ownership.  The dma_addressing_limited() check is a
> +	 * best approximation to ensure no swiotlb buffering happens.
> +	 */

This is a bit unclear, HMM inherently can't do cache flushing or
swiotlb bounce buffering because its entire purpose is to DMA directly
and coherently to a mm_struct's page tables. There are no sensible
points we could put the required flushing that wouldn't break the
entire model.

FWIW I view that fact that we now fail back to userspace in these
cases instead of quietly malfunction to be a big improvement.

> +bool hmm_dma_unmap_pfn(struct device *dev, struct hmm_dma_map *map, size_t idx)
> +{
> +	struct dma_iova_state *state = &map->state;
> +	dma_addr_t *dma_addrs = map->dma_list;
> +	unsigned long *pfns = map->pfn_list;
> +	unsigned long attrs = 0;
> +
> +#define HMM_PFN_VALID_DMA (HMM_PFN_VALID | HMM_PFN_DMA_MAPPED)
> +	if ((pfns[idx] & HMM_PFN_VALID_DMA) != HMM_PFN_VALID_DMA)
> +		return false;
> +#undef HMM_PFN_VALID_DMA

If a v10 comes I'd put this in a const function level variable:

          const unsigned int HMM_PFN_VALID_DMA = HMM_PFN_VALID | HMM_PFN_DMA_MAPPED;

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason