drm/ttm: Use fault-injection to test error paths

Use fault-injection to test partial TTM swapout and interrupted swapin.
Return -EINTR for swapin to test the callers ability to handle and
restart the swapin, and on swapout perform a partial swapout to test that
the swapin and release_shrunken functionality.

v8:
- Use the core fault-injection system.
v9:
- Fix compliation failure for !CONFIG_FAULT_INJECTION

Cc: Christian König <[e-mail]>
Cc: Somalapuram Amaranath <[e-mail]>
Cc: Matthew Brost <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Thomas Hellström <[e-mail]>
Reviewed-by: Matthew Brost <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Link: https://lore.kernel.org/intel-xe/[e-mail]

drivers/gpu/drm/ttm/ttm_pool.c

···
 
 #include "ttm_module.h"
 
+#ifdef CONFIG_FAULT_INJECTION
+#include <linux/fault-inject.h>
+static DECLARE_FAULT_ATTR(backup_fault_inject);
+#else
+#define should_fail(...) false
+#endif
+
 /**
  * struct ttm_pool_dma - Helper object for coherent DMA mappings
  *
···
 		p = first_page[i];
 		if (ttm_backup_page_ptr_is_handle(p)) {
 			unsigned long handle = ttm_backup_page_ptr_to_handle(p);
+
+			if (IS_ENABLED(CONFIG_FAULT_INJECTION) && ctx->interruptible &&
+			    should_fail(&backup_fault_inject, 1)) {
+				ret = -EINTR;
+				break;
+			}
 
 			if (handle == 0) {
 				restore->restored_pages++;
···
 
 	alloc_gfp = GFP_KERNEL | __GFP_HIGH | __GFP_NOWARN | __GFP_RETRY_MAYFAIL;
 
-	for (i = 0; i < tt->num_pages; ++i) {
+	num_pages = tt->num_pages;
+
+	/* Pretend doing fault injection by shrinking only half of the pages. */
+	if (IS_ENABLED(CONFIG_FAULT_INJECTION) && should_fail(&backup_fault_inject, 1))
+		num_pages = DIV_ROUND_UP(num_pages, 2);
+
+	for (i = 0; i < num_pages; ++i) {
 		s64 shandle;
 
 		page = tt->pages[i];
···
 			    &ttm_pool_debugfs_globals_fops);
 	debugfs_create_file("page_pool_shrink", 0400, ttm_debugfs_root, NULL,
 			    &ttm_pool_debugfs_shrink_fops);
+#ifdef CONFIG_FAULT_INJECTION
+	fault_create_debugfs_attr("backup_fault_inject", ttm_debugfs_root,
+				  &backup_fault_inject);
+#endif
 #endif
 
 	mm_shrinker = shrinker_alloc(0, "drm-ttm_pool");