linux-kernel - Re: [PATCH v10 1/8] mm: introduce FOLL_PCI_P2PDMA to gate getting PCI P2PDMA pages

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Date:   Mon, 26 Sep 2022 19:57:14 -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, Christoph Hellwig <hch@....de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        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>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [PATCH v10 1/8] mm: introduce FOLL_PCI_P2PDMA to gate getting
 PCI P2PDMA pages

On Fri, Sep 23, 2022 at 05:51:49PM -0600, Logan Gunthorpe wrote:

> And on further consideration I really think the correct error return is 
> important here. This will be a user facing error that'll be easy enough
> to hit: think code that might be run on any file and if the file is 
> hosted on a block device that doesn't support P2PDMA then the user
> will see the very uninformative "Cannot allocate memory" error.
> 
> Userspace code that's written for purpose can look at the EREMOTEIO error
> and tell the user something useful, if we return the correct error.
> If we return ENOMEM in this case, that is not possible because
> lots of things might have caused that error.

That is reasonable, but I'd still prefer to see it done more
centrally.

>> If we know PIN/GET is not set then we don't even need to call the
>> function because it is a NOP.

> That's not what the documentation for the function says:

> "Either FOLL_PIN or FOLL_GET (or neither) may be set... Return: true for success,
>  or if no action was required (if neither FOLL_PIN nor FOLL_GET was set, nothing
>  is done)."

I mean the way the code is structured is at the top of the call chain
the PIN/GET/0 is decided and then the callchain is run. All the
callsites of try_grab_page() must be safe to call under FOLL_PIN
because their caller is making the decision what flag to use.

Jason