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: <20250424084626.GQ48485@unreal>
Date: Thu, 24 Apr 2025 11:46:26 +0300
From: Leon Romanovsky <leon@...nel.org>
To: Christoph Hellwig <hch@....de>
Cc: Jason Gunthorpe <jgg@...pe.ca>,
	Mika Penttilä <mpenttil@...hat.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Jens Axboe <axboe@...nel.dk>, Keith Busch <kbusch@...nel.org>,
	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 Thu, Apr 24, 2025 at 10:11:01AM +0200, Christoph Hellwig wrote:
> On Thu, Apr 24, 2025 at 11:07:44AM +0300, Leon Romanovsky wrote:
> > > 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.
> > 
> > Thanks for the fix.
> 
> Maybe we can use the chance to make the scheme less fragile?  i.e.
> put flags in the high bits and derive the first valid bit from the
> pfn order?
> 

It can be done too. This is what I got:

   38 enum hmm_pfn_flags {
   39         /* Output fields and flags */
   40         HMM_PFN_VALID = 1UL << (BITS_PER_LONG - 1),
   41         HMM_PFN_WRITE = 1UL << (BITS_PER_LONG - 2),
   42         HMM_PFN_ERROR = 1UL << (BITS_PER_LONG - 3),
   43         /*
   44          * Sticky flags, carried from input to output,
   45          * don't forget to update HMM_PFN_INOUT_FLAGS
   46          */
   47         HMM_PFN_DMA_MAPPED = 1UL << (BITS_PER_LONG - 4),
   48         HMM_PFN_P2PDMA     = 1UL << (BITS_PER_LONG - 5),
   49         HMM_PFN_P2PDMA_BUS = 1UL << (BITS_PER_LONG - 6),
   50
   51         HMM_PFN_ORDER_SHIFT = (BITS_PER_LONG - 11),
   52
   53         /* Input flags */
   54         HMM_PFN_REQ_FAULT = HMM_PFN_VALID,
   55         HMM_PFN_REQ_WRITE = HMM_PFN_WRITE,
   56
   57         HMM_PFN_FLAGS = ~((1UL << HMM_PFN_ORDER_SHIFT) - 1),
   58 };

So now, we just need to move HMM_PFN_ORDER_SHIFT if we add new flags
and HMM_PFN_FLAGS will be updated automatically.

Thanks