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: <20250423233335.GW1213339@ziepe.ca>
Date: Wed, 23 Apr 2025 20:33:35 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Mika Penttilä <mpenttil@...hat.com>
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 Wed, Apr 23, 2025 at 09:37:24PM +0300, Mika Penttilä wrote:
> 
> 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)

I see, so yes order occupies 5 bits [-4,-5,-6,-7,-8] and the
DMA_MAPPED overlaps, it should be 9 not 7 because of the backwardness.

Probably testing didn't hit this because the usual 2M order of 9 only
sets bits -4 and -8 .. The way the order works it doesn't clear the
0 bits, which I wonder if is a little bug..

Jason