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

[Logan Gunthorpe]

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