Re: [PATCH v3 14/20] mm: introduce FOLL_PCI_P2PDMA to gate getting PCI P2PDMA pages

[Jason Gunthorpe]

On Wed, Sep 29, 2021 at 03:34:22PM -0600, Logan Gunthorpe wrote:
> 
> 
> 
> On 2021-09-28 1:47 p.m., Jason Gunthorpe wrote:
> > On Thu, Sep 16, 2021 at 05:40:54PM -0600, Logan Gunthorpe wrote:
> >> Callers that expect PCI P2PDMA pages can now set FOLL_PCI_P2PDMA to
> >> allow obtaining P2PDMA pages. If a caller does not set this flag
> >> and tries to map P2PDMA pages it will fail.
> >>
> >> This is implemented by adding a flag and a check to get_dev_pagemap().
> > 
> > I would like to see the get_dev_pagemap() deleted from GUP in the
> > first place.
> > 
> > Why isn't this just a simple check of the page->pgmap type after
> > acquiring a valid page reference? See my prior note
> 
> It could be, but that will mean dereferencing the pgmap for every page
> to determine the type of page and then comparing with FOLL_PCI_P2PDMA.

It would be done under the pte devmap test and this is less expensive
than the xarray search.

Jason