In order to save TLB space and CPU usage this patchset enables huge- and giant page-table entries for TTM and TTM-enabled graphics drivers.
Patch 1 introduces a vma_is_special_huge() function to make the mm code take the same path as DAX when splitting huge- and giant page table entries, (which currently is zapping the page-table entry and rely on re-faulting).
Patch 2 makes the mm code split existing huge page-table entries on huge_fault fallbacks. Typically on COW or on buffer-objects that want write-notify. COW and write-notification is always done on the lowest page-table level. See the patch log message for additional considerations.
Patch 3 introduces functions to allow the graphics drivers to manipulate the caching- and encryption flags of huge page-table entries without ugly hacks.
Patch 4 implements the huge_fault handler in TTM. This enables huge page-table entries, provided that the kernel is configured to support transhuge pages, either by default or using madvise(). However, they are unlikely to be inserted unless the kernel buffer object pfns and user-space addresses align perfectly. There are various options here, but since buffer objects that reside in system pages typically start at huge page boundaries if they are backed by huge pages, we try to enforce buffer object starting pfns and user-space addresses to be huge page-size aligned if their size exceeds a huge page-size. If pud-size transhuge ("giant") pages are enabled by the arch, the same holds for those.
Patch 5 implements a specialized huge_fault handler for vmwgfx. The vmwgfx driver may perform dirty-tracking and needs some special code to handle that correctly.
Patch 6 implements a drm helper to align user-space addresses according to the above scheme, if possible.
Patch 7 implements a TTM range manager that does the same for graphics IO memory.
Patch 8 finally hooks up the helpers of patch 6 and 7 to the vmwgfx driver. A similar change is needed for graphics drivers that want a reasonable likelyhood of actually using huge page-table entries.
Finally, if a buffer object size is not huge-page or giant-page aligned, its size will NOT be inflated by this patchset. This means that the buffer object tail will use smaller size page-table entries and thus no memory overhead occurs. Drivers that want to pay the memory overhead price need to implement their own scheme to inflate buffer-object sizes.
PMD size huge page-table-entries have been tested with vmwgfx and found to work well both with system memory backed and IO memory backed buffer objects.
PUD size giant page-table-entries have seen limited (fault and COW) testing using a modified kernel and a fake vmwgfx TTM memory type. The vmwgfx driver does otherwise not support 1GB-size IO memory resources.
Comments and suggestions welcome. Thomas
Changes since RFC: * Check for buffer objects present in contigous IO Memory (Christian König) * Rebased on the vmwgfx emulated coherent memory functionality. That rebase adds patch 5.
Cc: Andrew Morton akpm@linux-foundation.org Cc: Michal Hocko mhocko@suse.com Cc: "Matthew Wilcox (Oracle)" willy@infradead.org Cc: "Kirill A. Shutemov" kirill.shutemov@linux.intel.com Cc: Ralph Campbell rcampbell@nvidia.com Cc: "Jérôme Glisse" jglisse@redhat.com Cc: "Christian König" christian.koenig@amd.com