linux-kernel - Re: [PATCH v7 20/21] PCI/P2PDMA: Introduce pci_mmap

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Message-ID: <20220705164019.GB14215@lst.de>
Date:   Tue, 5 Jul 2022 18:40:19 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Christoph Hellwig <hch@....de>,
        Logan Gunthorpe <logang@...tatee.com>,
        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>,
        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>,
        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>,
        Ralph Campbell <rcampbell@...dia.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v7 20/21] PCI/P2PDMA: Introduce pci_mmap_p2pmem()

On Tue, Jul 05, 2022 at 01:29:59PM -0300, Jason Gunthorpe wrote:
> > Making the entire area given by the device to the p2p allocator available
> > to user space seems sensible to me.  That is what the current series does,
> > and what a sysfs interface would do as well.
> 
> That makes openning the mmap exclusive with the in-kernel allocator -
> so it means opening the mmap fails if something else is using a P2P
> page and once the mmap is open all kernel side P2P allocations will
> fail?

No.  Just as in the current patchset you can mmap the file and will get
len / PAGE_SIZE pages from the per-device p2pdma pool, or the mmap will
fail if none are available.  A kernel consumer (or multiple) can use
other pages in the pool at the same time.