[PATCH v2 2/4] drm/gem-shmem: Map pages in mmap fault handler

[Thomas Zimmermann <tzimmermann@suse.de>]

Gem-shmem operates on pages instead of I/O memory ranges, so use them
for mmap. This will allow for tracking page dirty/accessed flags. If
hugepage support is available, insert the page's folio if possible.
Otherwise fall back to mapping individual pages.

As the PFN is no longer required for hugepage mappings, simplify the
related code and make it depend on CONFIG_TRANSPARENT_HUGEPAGE. Prepare
for tracking folio status.

v2:
- do not look up the page before testing page-array bounds (Matthew)
- simplify error handling in drm_gem_shmem_fault() (Boris)
- adapt to changes in drm_gem_shmem_try_mmap_pmd()

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 49 +++++++++++++++-----------
 1 file changed, 29 insertions(+), 20 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index e7316dc7e921..24553dec070d 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -553,16 +553,17 @@ EXPORT_SYMBOL_GPL(drm_gem_shmem_dumb_create);
 static vm_fault_t 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;
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	phys_addr_t paddr = page_to_phys(page);
 	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;
-		return vmf_insert_pfn_pmd(vmf, pfn, false);
+	if (aligned && pmd_none(*vmf->pmd)) {
+		struct folio *folio = page_folio(page);
+
+		if (folio_test_pmd_mappable(folio)) {
+			/* Read-only mapping; split upon write fault */
+			return vmf_insert_folio_pmd(vmf, folio, false);
+		}
 	}
 #endif
 
@@ -575,13 +576,10 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
 	struct drm_gem_object *obj = vma->vm_private_data;
 	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 **pages = shmem->pages;
-	pgoff_t page_offset;
-	unsigned long pfn;
-
-	/* Offset to faulty address in the VMA. */
-	page_offset = vmf->pgoff - vma->vm_pgoff;
+	pgoff_t page_offset = vmf->pgoff - vma->vm_pgoff; /* page offset within VMA */
+	struct page *page;
+	vm_fault_t ret;
 
 	dma_resv_lock(shmem->base.resv, NULL);
 
@@ -592,14 +590,25 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
 		goto out;
 	}
 
-	ret = drm_gem_shmem_try_map_pmd(vmf, vmf->address, pages[page_offset]);
-	if (ret == VM_FAULT_NOPAGE)
+	page = pages[page_offset];
+	if (!page) {
+		ret = VM_FAULT_SIGBUS;
 		goto out;
+	}
+
+	ret = drm_gem_shmem_try_map_pmd(vmf, vmf->address, page);
+	if (ret != VM_FAULT_NOPAGE) {
+		struct folio *folio = page_folio(page);
+
+		get_page(page);
 
-	pfn = page_to_pfn(pages[page_offset]);
-	ret = vmf_insert_pfn(vma, vmf->address, pfn);
+		folio_lock(folio);
+
+		vmf->page = page;
+		ret = VM_FAULT_LOCKED;
+	}
 
- out:
+out:
 	dma_resv_unlock(shmem->base.resv);
 
 	return ret;
@@ -689,7 +698,7 @@ int drm_gem_shmem_mmap(struct drm_gem_shmem_object *shmem, struct vm_area_struct
 	if (ret)
 		return ret;
 
-	vm_flags_set(vma, VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP);
+	vm_flags_mod(vma, VM_DONTEXPAND | VM_DONTDUMP, VM_PFNMAP);
 	vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
 	if (shmem->map_wc)
 		vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
-- 
2.52.0