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

commit 53ba78de064b ("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.

[[email protected]: whitespace tweak, per David]
  Link: https://lkml.kernel.org/r/[email protected]
[[email protected]: bypass pofs_get_folio(), per Oscar]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 53ba78de064b ("mm/gup: introduce check_and_migrate_movable_folios()")
Signed-off-by: John Hubbard <[email protected]>
Suggested-by: David Hildenbrand <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Reviewed-by: Oscar Salvador <[email protected]>
Cc: Vivek Kasireddy <[email protected]>
Cc: Dave Airlie <[email protected]>
Cc: 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: Arnd Bergmann <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Dongwon Kim <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Junxiao Chang <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

0 files changed, 0 insertions, 0 deletions