mm: remove the extra ZONE_DEVICE struct page refcount

mm: remove the extra ZONE_DEVICE struct page refcount

ZONE_DEVICE struct pages have an extra reference count that complicates
the code for put_page() and several places in the kernel that need to
check the reference count to see that a page is not being used (gup,
compaction, migration, etc.). Clean up the code so the reference count
doesn't need to be treated specially for ZONE_DEVICE pages.

Note that this excludes the special idle page wakeup for fsdax pages,
which still happens at refcount 1.  This is a separate issue and will
be sorted out later.  Given that only fsdax pages require the
notifiacation when the refcount hits 1 now, the PAGEMAP_OPS Kconfig
symbol can go away and be replaced with a FS_DAX check for this hook
in the put_page fastpath.

Based on an earlier patch from Ralph Campbell <[email protected]>.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Logan Gunthorpe <[email protected]>
Reviewed-by: Ralph Campbell <[email protected]>
Reviewed-by: Jason Gunthorpe <[email protected]>
Reviewed-by: Dan Williams <[email protected]>
Acked-by: Felix Kuehling <[email protected]>
Tested-by: "Sierra Guiza, Alejandro (Alex)" <[email protected]>
Cc: Alex Deucher <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Ben Skeggs <[email protected]>
Cc: Chaitanya Kulkarni <[email protected]>
Cc: Christian Knig <[email protected]>
Cc: Karol Herbst <[email protected]>
Cc: Lyude Paul <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: "Pan, Xinhui" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>