drm/shmem-helper: Map huge pages in fault handler

Attempt a PMD sized PFN insertion into the VMA if the faulty address
of the fault handler is part of a huge page.

On builds with CONFIG_TRANSPARENT_HUGEPAGE enabled, if the mmap() user
address is PMD size aligned, if the GEM object is backed by shmem
buffers on mountpoints setting the 'huge=' option and if the shmem
backing store manages to allocate a huge folio, CPU mapping would then
benefit from significantly increased memcpy() performance. When these
conditions are met on a system with 2 MiB huge pages, an aligned copy
of 2 MiB would raise a single page fault instead of 4096.

v4:
- implement map_pages instead of huge_fault

v6:
- get rid of map_pages handler for now (keep it for another series
  along with arm64 contpte support)

v11:
- remove page fault validity check helper
- rename drm_gem_shmem_map_pmd() to drm_gem_shmem_try_map_pmd()
- add Boris R-b

v12:
- move up ret var decl in fault handler to minimize diff

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patch.msgid.link/20251205182231.194072-3-loic.molinari@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index e47ab3ebb5c2206926e98d723c1e6d3a4991abb6..e67216cbb469cf5bfe6ba0b8bf4901abe61316b6 100644 (file)
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -567,6 +567,26 @@ int drm_gem_shmem_dumb_create(struct drm_file *file, struct drm_device *dev,
 }
 EXPORT_SYMBOL_GPL(drm_gem_shmem_dumb_create);
 
+static bool drm_gem_shmem_try_map_pmd(struct vm_fault *vmf, unsigned long addr,
+                                     struct page *page)
+{
+#ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP
+       unsigned long pfn = page_to_pfn(page);
+       unsigned long paddr = pfn << PAGE_SHIFT;
+       bool aligned = (addr & ~PMD_MASK) == (paddr & ~PMD_MASK);
+
+       if (aligned &&
+           pmd_none(*vmf->pmd) &&
+           folio_test_pmd_mappable(page_folio(page))) {
+               pfn &= PMD_MASK >> PAGE_SHIFT;
+               if (vmf_insert_pfn_pmd(vmf, pfn, false) == VM_FAULT_NOPAGE)
+                       return true;
+       }
+#endif
+
+       return false;
+}
+
 static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
 {
        struct vm_area_struct *vma = vmf->vma;
@@ -574,8 +594,9 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
        struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
        loff_t num_pages = obj->size >> PAGE_SHIFT;
        vm_fault_t ret;
-       struct page *page;
+       struct page **pages = shmem->pages;
        pgoff_t page_offset;
+       unsigned long pfn;
 
        /* Offset to faulty address in the VMA. */
        page_offset = vmf->pgoff - vma->vm_pgoff;
@@ -586,12 +607,18 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
            drm_WARN_ON_ONCE(obj->dev, !shmem->pages) ||
            shmem->madv < 0) {
                ret = VM_FAULT_SIGBUS;
-       } else {
-               page = shmem->pages[page_offset];
+               goto out;
+       }
 
-               ret = vmf_insert_pfn(vma, vmf->address, page_to_pfn(page));
+       if (drm_gem_shmem_try_map_pmd(vmf, vmf->address, pages[page_offset])) {
+               ret = VM_FAULT_NOPAGE;
+               goto out;
        }
 
+       pfn = page_to_pfn(pages[page_offset]);
+       ret = vmf_insert_pfn(vma, vmf->address, pfn);
+
+ out:
        dma_resv_unlock(shmem->base.resv);
 
        return ret;