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Date:   Fri, 23 Sep 2022 17:35:22 -0600
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To:     Jason Gunthorpe <jgg@ziepe.ca>
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From:   Logan Gunthorpe <logang@deltatee.com>
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Subject: Re: [PATCH v10 1/8] mm: introduce FOLL_PCI_P2PDMA to gate getting PCI
 P2PDMA pages
Precedence: bulk


On 2022-09-23 17:21, Jason Gunthorpe wrote:
> On Fri, Sep 23, 2022 at 05:14:11PM -0600, Logan Gunthorpe wrote:
>>
>>
>> On 2022-09-23 17:07, Jason Gunthorpe wrote:
>>> On Fri, Sep 23, 2022 at 05:01:26PM -0600, Logan Gunthorpe wrote:
>>>>
>>>>
>>>>
>>>> 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.
>>>
>>> I'd rather not see GUP made more fragile just for that..
>>
>> Not sure how that's more fragile... You're way seems more dangerous given
>> the large number of call sites we are adding it to when it might not
>> apply.
> 
> No, that is the point, it *always* applies. A devmap struct page of
> the wrong type should never exit gup, from any path, no matter what.
> 
> We have two central functions that validate a page is OK to return,
> that *everyone* must call.
> 
> If you don't put it there then we will probably miss copying it into a
> call site eventually.

Most of the call sites don't apply though, with huge pages and gate pages...

>>> try_grab_page() calls folio_ref_inc(), that is only legal if it knows
>>> the page is already a valid pointer under the PTLs, so it is safe to
>>> check the pgmap as well.
>>
>> My point is it doesn't get a reference or a pin unless FOLL_PIN or FOLL_GET is
>> set and the documentation states that neither might be set, in which case 
>> folio_ref_inc() will not be called...
> 
> That isn't how GUP is structured, all the calls to try_grab_page() are
> in places where PIN/GET might be set and are safe for that usage.
> 
> 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)."

https://elixir.bootlin.com/linux/v6.0-rc6/source/mm/gup.c#L194

Logan