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

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Date:   Fri, 23 Sep 2022 17:01:26 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
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 2022-09-23 16:58, Jason Gunthorpe wrote:
> On Fri, Sep 23, 2022 at 02:11:03PM -0600, Logan Gunthorpe wrote:
>>
>>
>> On 2022-09-23 13:53, Jason Gunthorpe wrote:
>>> On Fri, Sep 23, 2022 at 01:08:31PM -0600, Logan Gunthorpe wrote:
>>> I'm encouraging Dan to work on better infrastructure in pgmap core
>>> because every pgmap implementation has this issue currently.
>>>
>>> For that reason it is probably not so relavent to this series.
>>>
>>> Perhaps just clarify in the commit message that the FOLL_LONGTERM
>>> restriction is to copy DAX until the pgmap page refcounts are fixed.
>>
>> Ok, I'll add that note.
>>
>> Per the fix for the try_grab_page(), to me it doesn't fit well in 
>> try_grab_page() without doing a bunch of cleanup to change the
>> error handling, and the same would have to be added to try_grab_folio().
>> So I think it's better to leave it where it was, but move it below the 
>> respective grab calls. Does the incremental patch below look correct?
> 
> Oh? I was thinking of just a very simple thing:

Really would like it to return -EREMOTEIO instead of -ENOMEM as that's the
error used for bad P2PDMA page everywhere.

Plus the concern that some of the callsites of try_grab_page() might not have
a get or a pin and thus it's not safe which was the whole point of the change
anyway.

Plus we have to do the same for try_grab_folio().

Logan