mm/gup: introduce unpin_folio/unpin_folios helpers

Patch series "mm/gup: Introduce memfd_pin_folios() for pinning memfd
folios", v16.

Currently, some drivers (e.g, Udmabuf) that want to longterm-pin the
pages/folios associated with a memfd, do so by simply taking a reference
on them.  This is not desirable because the pages/folios may reside in
Movable zone or CMA block.

Therefore, having drivers use memfd_pin_folios() API ensures that the
folios are appropriately pinned via FOLL_PIN for longterm DMA.

This patchset also introduces a few helpers and converts the Udmabuf
driver to use folios and memfd_pin_folios() API to longterm-pin the folios
for DMA.  Two new Udmabuf selftests are also included to test the driver
and the new API.


This patch (of 9):

These helpers are the folio versions of unpin_user_page/unpin_user_pages. 
They are currently only useful for unpinning folios pinned by
memfd_pin_folios() or other associated routines.  However, they could find
new uses in the future, when more and more folio-only helpers are added to
GUP.

We should probably sanity check the folio as part of unpin similar to how
it is done in unpin_user_page/unpin_user_pages but we cannot cleanly do
that at the moment without also checking the subpage.  Therefore, sanity
checking needs to be added to these routines once we have a way to
determine if any given folio is anon-exclusive (via a per folio
AnonExclusive flag).

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Vivek Kasireddy <[email protected]>
Suggested-by: David Hildenbrand <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Acked-by: Dave Airlie <[email protected]>
Acked-by: Gerd Hoffmann <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Dongwon Kim <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Junxiao Chang <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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