linux-kernel - Re: [PATCH v9 10/24] mm/hmm: let users to tag specific PFN with DMA mapped bit

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Message-ID: <36891b0e-d5fa-4cf8-a181-599a20af1da3@redhat.com>
Date: Wed, 23 Apr 2025 21:37:24 +0300
From: Mika Penttilä <mpenttil@...hat.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Leon Romanovsky <leon@...nel.org>,
 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 10/24] mm/hmm: let users to tag specific PFN with DMA
 mapped bit


On 4/23/25 21:17, Jason Gunthorpe wrote:
> On Wed, Apr 23, 2025 at 08:54:05PM +0300, Mika Penttilä wrote:
>>> @@ -36,6 +38,13 @@ enum hmm_pfn_flags {
>>>  	HMM_PFN_VALID = 1UL << (BITS_PER_LONG - 1),
>>>  	HMM_PFN_WRITE = 1UL << (BITS_PER_LONG - 2),
>>>  	HMM_PFN_ERROR = 1UL << (BITS_PER_LONG - 3),
>>> +
>>> +	/*
>>> +	 * Sticky flags, carried from input to output,
>>> +	 * don't forget to update HMM_PFN_INOUT_FLAGS
>>> +	 */
>>> +	HMM_PFN_DMA_MAPPED = 1UL << (BITS_PER_LONG - 7),
>>> +
>> How is this playing together with the mapped order usage?
> Order shift starts at bit 8, DMA_MAPPED is at bit 7

hmm bits are the high bits, and order is 5 bits starting from
(BITS_PER_LONG - 8)


> The pfn array is linear and simply indexed. The order is intended for
> page table like HW to be able to build larger entries from the hmm
> data without having to scan for contiguity.
>
> Even if order is present the entry is still replicated across all the
> pfns that are inside the order.
>
> At least this series should replicate the dma_mapped flag as well as
> it doesn't pay attention to order.
>
> I suspect a page table implementation may need to make some small
> changes. Indeed with guarenteed contiguous IOVA there may be a
> significant optimization available to have the HW page table cover all
> the contiguous present pages in the iommu, which would be a higher
> order than the pages themselves. However this would require being able
> to punch non-present holes into contiguous mappings...
>
> Jason
>
--Mika