drivers/mic/scif: convert get_user_pages() --> pin_user_pages()

This code was using get_user_pages*(), in a "Case 2" scenario
(DMA/RDMA), using the categorization from [1]. That means that it's
time to convert the get_user_pages*() + put_page() calls to
pin_user_pages*() + unpin_user_pages() calls.

There is some helpful background in [2]: basically, this is a small
part of fixing a long-standing disconnect between pinning pages, and
file systems' use of those pages.

Note that this effectively changes the code's behavior as well: it now
ultimately calls set_page_dirty_lock(), instead of SetPageDirty(). This
is probably more accurate.

As Christoph Hellwig put it, "set_page_dirty() is only safe if we are
dealing with a file backed page where we have reference on the inode it
hangs off." [3]

[1] Documentation/core-api/pin_user_pages.rst

[2] "Explicit pinning of user-space pages":
    https://lwn.net/Articles/807108/

[3] https://lore.kernel.org/r/20190723153640.GB720@lst.deSigned-off-by: John Hubbard <jhubbard@nvidia.com>
Link: https://lore.kernel.org/r/20200518041307.1987328-1-jhubbard@nvidia.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/mic/scif/scif_rma.c

@@ -113,14 +113,17 @@ static int scif_destroy_pinned_pages(struct scif_pinned_pages *pin)
 	int writeable = pin->prot & SCIF_PROT_WRITE;
 	int kernel = SCIF_MAP_KERNEL & pin->map_flags;
 
-	for (j = 0; j < pin->nr_pages; j++) {
-		if (pin->pages[j] && !kernel) {
-			if (writeable)
-				SetPageDirty(pin->pages[j]);
-			put_page(pin->pages[j]);
+	if (kernel) {
+		for (j = 0; j < pin->nr_pages; j++) {
+			if (pin->pages[j] && !kernel) {
+				if (writeable)
+					set_page_dirty_lock(pin->pages[j]);
+				put_page(pin->pages[j]);
+			}
 		}
-	}
-
+	} else
+		unpin_user_pages_dirty_lock(pin->pages, pin->nr_pages,
+					    writeable);
 	scif_free(pin->pages,
 		  pin->nr_pages * sizeof(*pin->pages));
 	scif_free(pin, sizeof(*pin));
@@ -1375,7 +1378,7 @@ int __scif_pin_pages(void *addr, size_t len, int *out_prot,
 			}
 		}
 
-		pinned_pages->nr_pages = get_user_pages_fast(
+		pinned_pages->nr_pages = pin_user_pages_fast(
 				(u64)addr,
 				nr_pages,
 				(prot & SCIF_PROT_WRITE) ? FOLL_WRITE : 0,
@@ -1385,11 +1388,8 @@ int __scif_pin_pages(void *addr, size_t len, int *out_prot,
 				if (ulimit)
 					__scif_dec_pinned_vm_lock(mm, nr_pages);
 				/* Roll back any pinned pages */
-				for (i = 0; i < pinned_pages->nr_pages; i++) {
-					if (pinned_pages->pages[i])
-						put_page(
-						pinned_pages->pages[i]);
-				}
+				unpin_user_pages(pinned_pages->pages,
+						 pinned_pages->nr_pages);
 				prot &= ~SCIF_PROT_WRITE;
 				try_upgrade = false;
 				goto retry;