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

[Jason Gunthorpe]

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

Jason