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

[Jason Gunthorpe]

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.

> > 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.

Jason