linux-kernel - Re: [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap

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Message-ID: <20210930003652.GH3544071@ziepe.ca>
Date:   Wed, 29 Sep 2021 21:36:52 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Martin Oliveira <martin.oliveira@...eticom.com>,
        Chaitanya Kulkarni <ckulkarnilinux@...il.com>
Subject: Re: [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap_p2pmem()

On Wed, Sep 29, 2021 at 05:49:36PM -0600, Logan Gunthorpe wrote:

> Some of this seems out of date. Pretty sure the pages are not refcounted
> with vmf_insert_mixed() and vmf_insert_mixed() is currently the only way
> to use VM_MIXEDMAP mappings.

Hum.

vmf_insert_mixed() boils down to insert_pfn() which always sets the
special bit, so vm_normal_page() returns NULL and thus the pages are
not freed during zap.

So, if the pages are always special and not refcounted all the docs
seem really out of date - or rather they describe the situation
without the special bit, I think.

Why would DAX want to do this in the first place?? This means the
address space zap is much more important that just speeding up
destruction, it is essential for correctness since the PTEs are not
holding refcounts naturally...

Sigh.

Jason