udmabuf: Add support for mapping hugepages (v4)

If the VMM's (Qemu) memory backend is backed up by memfd + Hugepages
(hugetlbfs and not THP), we have to first find the hugepage(s) where
the Guest allocations are located and then extract the regular 4k
sized subpages from them.

v2: Ensure that the subpage and hugepage offsets are calculated correctly
when the range of subpage allocations cuts across multiple hugepages.

v3: Instead of repeatedly looking up the hugepage for each subpage,
only do it when the subpage allocation crosses over into a different
hugepage. (suggested by Gerd and DW)

v4: Fix the following warning identified by checkpatch:
CHECK:OPEN_ENDED_LINE: Lines should not end with a '('

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20210609182915.592743-1-vivek.kasireddy@intel.com
[ kraxel: one more checkpatch format tweak ]
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/dma-buf/udmabuf.c

@@ -11,6 +11,7 @@
 #include <linux/shmem_fs.h>
 #include <linux/slab.h>
 #include <linux/udmabuf.h>
+#include <linux/hugetlb.h>
 
 static const u32    list_limit = 1024;  /* udmabuf_create_list->count limit */
 static const size_t size_limit_mb = 64; /* total dmabuf size, in megabytes  */
@@ -160,10 +161,13 @@ static long udmabuf_create(struct miscdevice *device,
 {
 	DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
 	struct file *memfd = NULL;
+	struct address_space *mapping = NULL;
 	struct udmabuf *ubuf;
 	struct dma_buf *buf;
 	pgoff_t pgoff, pgcnt, pgidx, pgbuf = 0, pglimit;
-	struct page *page;
+	struct page *page, *hpage = NULL;
+	pgoff_t subpgoff, maxsubpgs;
+	struct hstate *hpstate;
 	int seals, ret = -EINVAL;
 	u32 i, flags;
 
@@ -194,7 +198,8 @@ static long udmabuf_create(struct miscdevice *device,
 		memfd = fget(list[i].memfd);
 		if (!memfd)
 			goto err;
-		if (!shmem_mapping(file_inode(memfd)->i_mapping))
+		mapping = file_inode(memfd)->i_mapping;
+		if (!shmem_mapping(mapping) && !is_file_hugepages(memfd))
 			goto err;
 		seals = memfd_fcntl(memfd, F_GET_SEALS, 0);
 		if (seals == -EINVAL)
@@ -205,17 +210,48 @@ static long udmabuf_create(struct miscdevice *device,
 			goto err;
 		pgoff = list[i].offset >> PAGE_SHIFT;
 		pgcnt = list[i].size   >> PAGE_SHIFT;
+		if (is_file_hugepages(memfd)) {
+			hpstate = hstate_file(memfd);
+			pgoff = list[i].offset >> huge_page_shift(hpstate);
+			subpgoff = (list[i].offset &
+				    ~huge_page_mask(hpstate)) >> PAGE_SHIFT;
+			maxsubpgs = huge_page_size(hpstate) >> PAGE_SHIFT;
+		}
 		for (pgidx = 0; pgidx < pgcnt; pgidx++) {
-			page = shmem_read_mapping_page(
-				file_inode(memfd)->i_mapping, pgoff + pgidx);
+			if (is_file_hugepages(memfd)) {
+				if (!hpage) {
+					hpage = find_get_page_flags(mapping, pgoff,
+								    FGP_ACCESSED);
+					if (IS_ERR(hpage)) {
+						ret = PTR_ERR(hpage);
+						goto err;
+					}
+				}
+				page = hpage + subpgoff;
+				get_page(page);
+				subpgoff++;
+				if (subpgoff == maxsubpgs) {
+					put_page(hpage);
+					hpage = NULL;
+					subpgoff = 0;
+					pgoff++;
+				}
+			} else {
+				page = shmem_read_mapping_page(mapping,
+							       pgoff + pgidx);
 				if (IS_ERR(page)) {
 					ret = PTR_ERR(page);
 					goto err;
 				}
+			}
 			ubuf->pages[pgbuf++] = page;
 		}
 		fput(memfd);
 		memfd = NULL;
+		if (hpage) {
+			put_page(hpage);
+			hpage = NULL;
+		}
 	}
 
 	exp_info.ops  = &udmabuf_ops;