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

[Jason Gunthorpe]

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