drm/pagemap: Enable THP support for GPU memory migration

drm/pagemap: Enable THP support for GPU memory migration

This enables support for Transparent Huge Pages (THP) for device pages by
using MIGRATE_VMA_SELECT_COMPOUND during migration. It removes the need to
split folios and loop multiple times over all pages to perform required
operations at page level. Instead, we rely on newly introduced support for
higher orders in drm_pagemap and folio-level API.

In Xe, this drastically improves performance when using SVM. The GT stats
below collected after a 2MB page fault show overall servicing is more than
7 times faster, and thanks to reduced CPU overhead the time spent on the
actual copy goes from 23% without THP to 80% with THP:

Without THP:

    svm_2M_pagefault_us: 966
    svm_2M_migrate_us: 942
    svm_2M_device_copy_us: 223
    svm_2M_get_pages_us: 9
    svm_2M_bind_us: 10

With THP:

    svm_2M_pagefault_us: 132
    svm_2M_migrate_us: 128
    svm_2M_device_copy_us: 106
    svm_2M_get_pages_us: 1
    svm_2M_bind_us: 2

v2:
- Fix one occurrence of drm_pagemap_get_devmem_page() (Matthew Brost)

v3:
- Remove migrate_device_split_page() and folio_split_lock, instead rely on
  free_zone_device_folio() to split folios before freeing (Matthew Brost)
- Assert folio order is HPAGE_PMD_ORDER (Matthew Brost)
- Always use folio_set_zone_device_data() in split (Matthew Brost)

v4:
- Warn on compound device page, s/continue/goto next/ (Matthew Brost)

v5:
- Revert warn on compound device page
- s/zone_device_page_init()/zone_device_folio_init() (Matthew Brost)

Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Michal Mrozek <michal.mrozek@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: linux-mm@kvack.org
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260312192126.2024853-5-francois.dugast@intel.com