[S390] cio: reduce memory consumption of itcw structures

Any list of indirect data adresses (TIDAL) used by a TCW must not
cross a page boundary. The original itcw implementation complies with
this restriction by allocating allmost twice as much memory as
actually needed, so that in any case there is enough room for the full
TIDAL, either above or below the page boundary.
This patch implements an alternative method, by using a TTIC TIDAW to
connect TIDAL parts below and above a page boundary.
Signed-off-by: Stefan Weinhuber <wein@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/cio/itcw.c

@@ -93,6 +93,7 @@ EXPORT_SYMBOL(itcw_get_tcw);
 size_t itcw_calc_size(int intrg, int max_tidaws, int intrg_max_tidaws)
 {
 	size_t len;
+	int cross_count;
 
 	/* Main data. */
 	len = sizeof(struct itcw);
@@ -105,12 +106,27 @@ size_t itcw_calc_size(int intrg, int max_tidaws, int intrg_max_tidaws)
 		       /* TSB */ sizeof(struct tsb) +
 		       /* TIDAL */ intrg_max_tidaws * sizeof(struct tidaw);
 	}
+
 	/* Maximum required alignment padding. */
 	len += /* Initial TCW */ 63 + /* Interrogate TCCB */ 7;
-	/* Maximum padding for structures that may not cross 4k boundary. */
-	if ((max_tidaws > 0) || (intrg_max_tidaws > 0))
-		len += max(max_tidaws, intrg_max_tidaws) *
-		       sizeof(struct tidaw) - 1;
+
+	/* TIDAW lists may not cross a 4k boundary. To cross a
+	 * boundary we need to add a TTIC TIDAW. We need to reserve
+	 * one additional TIDAW for a TTIC that we may need to add due
+	 * to the placement of the data chunk in memory, and a further
+	 * TIDAW for each page boundary that the TIDAW list may cross
+	 * due to it's own size.
+	 */
+	if (max_tidaws) {
+		cross_count = 1 + ((max_tidaws * sizeof(struct tidaw) - 1)
+				   >> PAGE_SHIFT);
+		len += cross_count * sizeof(struct tidaw);
+	}
+	if (intrg_max_tidaws) {
+		cross_count = 1 + ((intrg_max_tidaws * sizeof(struct tidaw) - 1)
+				   >> PAGE_SHIFT);
+		len += cross_count * sizeof(struct tidaw);
+	}
 	return len;
 }
 EXPORT_SYMBOL(itcw_calc_size);
@@ -165,6 +181,7 @@ struct itcw *itcw_init(void *buffer, size_t size, int op, int intrg,
 	void *chunk;
 	addr_t start;
 	addr_t end;
+	int cross_count;
 
 	/* Check for 2G limit. */
 	start = (addr_t) buffer;
@@ -177,8 +194,17 @@ struct itcw *itcw_init(void *buffer, size_t size, int op, int intrg,
 	if (IS_ERR(chunk))
 		return chunk;
 	itcw = chunk;
-	itcw->max_tidaws = max_tidaws;
-	itcw->intrg_max_tidaws = intrg_max_tidaws;
+	/* allow for TTIC tidaws that may be needed to cross a page boundary */
+	cross_count = 0;
+	if (max_tidaws)
+		cross_count = 1 + ((max_tidaws * sizeof(struct tidaw) - 1)
+				   >> PAGE_SHIFT);
+	itcw->max_tidaws = max_tidaws + cross_count;
+	cross_count = 0;
+	if (intrg_max_tidaws)
+		cross_count = 1 + ((intrg_max_tidaws * sizeof(struct tidaw) - 1)
+				   >> PAGE_SHIFT);
+	itcw->intrg_max_tidaws = intrg_max_tidaws + cross_count;
 	/* Main TCW. */
 	chunk = fit_chunk(&start, end, sizeof(struct tcw), 64, 0);
 	if (IS_ERR(chunk))
@@ -198,7 +224,7 @@ struct itcw *itcw_init(void *buffer, size_t size, int op, int intrg,
 	/* Data TIDAL. */
 	if (max_tidaws > 0) {
 		chunk = fit_chunk(&start, end, sizeof(struct tidaw) *
-				  max_tidaws, 16, 1);
+				  itcw->max_tidaws, 16, 0);
 		if (IS_ERR(chunk))
 			return chunk;
 		tcw_set_data(itcw->tcw, chunk, 1);
@@ -206,7 +232,7 @@ struct itcw *itcw_init(void *buffer, size_t size, int op, int intrg,
 	/* Interrogate data TIDAL. */
 	if (intrg && (intrg_max_tidaws > 0)) {
 		chunk = fit_chunk(&start, end, sizeof(struct tidaw) *
-				  intrg_max_tidaws, 16, 1);
+				  itcw->intrg_max_tidaws, 16, 0);
 		if (IS_ERR(chunk))
 			return chunk;
 		tcw_set_data(itcw->intrg_tcw, chunk, 1);
@@ -283,13 +309,29 @@ EXPORT_SYMBOL(itcw_add_dcw);
  * the new tidaw on success or -%ENOSPC if the new tidaw would exceed the
  * available space.
  *
- * Note: the tidaw-list is assumed to be contiguous with no ttics. The
- * last-tidaw flag for the last tidaw in the list will be set by itcw_finalize.
+ * Note: TTIC tidaws are automatically added when needed, so explicitly calling
+ * this interface with the TTIC flag is not supported. The last-tidaw flag
+ * for the last tidaw in the list will be set by itcw_finalize.
  */
 struct tidaw *itcw_add_tidaw(struct itcw *itcw, u8 flags, void *addr, u32 count)
 {
+	struct tidaw *following;
+
 	if (itcw->num_tidaws >= itcw->max_tidaws)
 		return ERR_PTR(-ENOSPC);
+	/*
+	 * Is the tidaw, which follows the one we are about to fill, on the next
+	 * page? Then we have to insert a TTIC tidaw first, that points to the
+	 * tidaw on the new page.
+	 */
+	following = ((struct tidaw *) tcw_get_data(itcw->tcw))
+		+ itcw->num_tidaws + 1;
+	if (itcw->num_tidaws && !((unsigned long) following & ~PAGE_MASK)) {
+		tcw_add_tidaw(itcw->tcw, itcw->num_tidaws++,
+			      TIDAW_FLAGS_TTIC, following, 0);
+		if (itcw->num_tidaws >= itcw->max_tidaws)
+			return ERR_PTR(-ENOSPC);
+	}
 	return tcw_add_tidaw(itcw->tcw, itcw->num_tidaws++, flags, addr, count);
 }
 EXPORT_SYMBOL(itcw_add_tidaw);