linux-kernel - Re: [PATCH v11 2/9] mm: introduce FOLL_PCI_P2PDMA to gate getting PCI P2PDMA pages

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Message-ID: <42903f08-ee07-8d72-dd35-ada320695e20@nvidia.com>
Date:   Tue, 25 Oct 2022 01:09:15 +0000
From:   Chaitanya Kulkarni <chaitanyak@...dia.com>
To:     Logan Gunthorpe <logang@...tatee.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
CC:     Christoph Hellwig <hch@....de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        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>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [PATCH v11 2/9] mm: introduce FOLL_PCI_P2PDMA to gate getting PCI
 P2PDMA pages

On 10/21/22 10:41, Logan Gunthorpe wrote:
> GUP Callers that expect PCI P2PDMA pages can now set FOLL_PCI_P2PDMA to
> allow obtaining P2PDMA pages. If GUP is called without the flag and a
> P2PDMA page is found, it will return an error in try_grab_page() or
> try_grab_folio().
> 
> The check is safe to do before taking the reference to the page in both
> cases seeing the page should be protected by either the appropriate
> ptl or mmap_lock; or the gup fast guarantees preventing TLB flushes.
> 
> try_grab_folio() has one call site that WARNs on failure and cannot
> actually deal with the failure of this function (it seems it will
> get into an infinite loop). Expand the comment there to document a
> couple more conditions on why it will not fail.
> 
> FOLL_PCI_P2PDMA cannot be set if FOLL_LONGTERM is set. This is to copy
> fsdax until pgmap refcounts are fixed (see the link below for more
> information).
> 
> Link: https://lkml.kernel.org/r/Yy4Ot5MoOhsgYLTQ@ziepe.ca
> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>

Reviewed-by: Chaitanya Kulkarni <kch@...dia.com>

-ck