mm/gup: avoid an unnecessary allocation call for FOLL_LONGTERM cases

commit 53ba78de ("mm/gup: introduce
check_and_migrate_movable_folios()") created a new constraint on the
pin_user_pages*() API family: a potentially large internal allocation must
now occur, for FOLL_LONGTERM cases.

A user-visible consequence has now appeared: user space can no longer pin
more than 2GB of memory anymore on x86_64.  That's because, on a 4KB
PAGE_SIZE system, when user space tries to (indirectly, via a device
driver that calls pin_user_pages()) pin 2GB, this requires an allocation
of a folio pointers array of MAX_PAGE_ORDER size, which is the limit for
kmalloc().

In addition to the directly visible effect described above, there is also
the problem of adding an unnecessary allocation.  The **pages array
argument has already been allocated, and there is no need for a redundant
**folios array allocation in this case.

Fix this by avoiding the new allocation entirely.  This is done by
referring to either the original page[i] within **pages, or to the
associated folio.  Thanks to David Hildenbrand for suggesting this
approach and for providing the initial implementation (which I've tested
and adjusted slightly) as well.

[jhubbard@nvidia.com: whitespace tweak, per David]
  Link: https://lkml.kernel.org/r/131cf9c8-ebc0-4cbb-b722-22fa8527bf3c@nvidia.com
[jhubbard@nvidia.com: bypass pofs_get_folio(), per Oscar]
  Link: https://lkml.kernel.org/r/c1587c7f-9155-45be-bd62-1e36c0dd6923@nvidia.com
Link: https://lkml.kernel.org/r/20241105032944.141488-2-jhubbard@nvidia.com


Fixes: 53ba78de ("mm/gup: introduce check_and_migrate_movable_folios()")
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dongwon Kim <dongwon.kim@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Junxiao Chang <junxiao.chang@intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>