1 files changed, 612 insertions, 106 deletions
diff --git a/drivers/gpu/drm/ttm/ttm_pool.c b/drivers/gpu/drm/ttm/ttm_pool.c
index 8504dbe19c1a..83b10706ba89 100644
--- a/drivers/gpu/drm/ttm/ttm_pool.c
+++ b/drivers/gpu/drm/ttm/ttm_pool.c
@@ -41,12 +41,20 @@
 #include <asm/set_memory.h>
 #endif
 
+#include <drm/ttm/ttm_backup.h>
 #include <drm/ttm/ttm_pool.h>
 #include <drm/ttm/ttm_tt.h>
 #include <drm/ttm/ttm_bo.h>
 
 #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
  *
@@ -58,6 +66,52 @@ struct ttm_pool_dma {
 	unsigned long vaddr;
 };
 
+/**
+ * struct ttm_pool_alloc_state - Current state of the tt page allocation process
+ * @pages: Pointer to the next tt page pointer to populate.
+ * @caching_divide: Pointer to the first page pointer whose page has a staged but
+ * not committed caching transition from write-back to @tt_caching.
+ * @dma_addr: Pointer to the next tt dma_address entry to populate if any.
+ * @remaining_pages: Remaining pages to populate.
+ * @tt_caching: The requested cpu-caching for the pages allocated.
+ */
+struct ttm_pool_alloc_state {
+	struct page **pages;
+	struct page **caching_divide;
+	dma_addr_t *dma_addr;
+	pgoff_t remaining_pages;
+	enum ttm_caching tt_caching;
+};
+
+/**
+ * struct ttm_pool_tt_restore - State representing restore from backup
+ * @pool: The pool used for page allocation while restoring.
+ * @snapshot_alloc: A snapshot of the most recent struct ttm_pool_alloc_state.
+ * @alloced_page: Pointer to the page most recently allocated from a pool or system.
+ * @first_dma: The dma address corresponding to @alloced_page if dma_mapping
+ * is requested.
+ * @alloced_pages: The number of allocated pages present in the struct ttm_tt
+ * page vector from this restore session.
+ * @restored_pages: The number of 4K pages restored for @alloced_page (which
+ * is typically a multi-order page).
+ * @page_caching: The struct ttm_tt requested caching
+ * @order: The order of @alloced_page.
+ *
+ * Recovery from backup might fail when we've recovered less than the
+ * full ttm_tt. In order not to loose any data (yet), keep information
+ * around that allows us to restart a failed ttm backup recovery.
+ */
+struct ttm_pool_tt_restore {
+	struct ttm_pool *pool;
+	struct ttm_pool_alloc_state snapshot_alloc;
+	struct page *alloced_page;
+	dma_addr_t first_dma;
+	pgoff_t alloced_pages;
+	pgoff_t restored_pages;
+	enum ttm_caching page_caching;
+	unsigned int order;
+};
+
 static unsigned long page_pool_size;
 
 MODULE_PARM_DESC(page_pool_size, "Number of pages in the WC/UC/DMA pool");
@@ -160,34 +214,33 @@ static void ttm_pool_free_page(struct ttm_pool *pool, enum ttm_caching caching,
 	kfree(dma);
 }
 
-/* Apply a new caching to an array of pages */
-static int ttm_pool_apply_caching(struct page **first, struct page **last,
-				  enum ttm_caching caching)
+/* Apply any cpu-caching deferred during page allocation */
+static int ttm_pool_apply_caching(struct ttm_pool_alloc_state *alloc)
 {
 #ifdef CONFIG_X86
-	unsigned int num_pages = last - first;
+	unsigned int num_pages = alloc->pages - alloc->caching_divide;
 
 	if (!num_pages)
 		return 0;
 
-	switch (caching) {
+	switch (alloc->tt_caching) {
 	case ttm_cached:
 		break;
 	case ttm_write_combined:
-		return set_pages_array_wc(first, num_pages);
+		return set_pages_array_wc(alloc->caching_divide, num_pages);
 	case ttm_uncached:
-		return set_pages_array_uc(first, num_pages);
+		return set_pages_array_uc(alloc->caching_divide, num_pages);
 	}
 #endif
+	alloc->caching_divide = alloc->pages;
 	return 0;
 }
 
-/* Map pages of 1 << order size and fill the DMA address array  */
+/* DMA Map pages of 1 << order size and return the resulting dma_address. */
 static int ttm_pool_map(struct ttm_pool *pool, unsigned int order,
-			struct page *p, dma_addr_t **dma_addr)
+			struct page *p, dma_addr_t *dma_addr)
 {
 	dma_addr_t addr;
-	unsigned int i;
 
 	if (pool->use_dma_alloc) {
 		struct ttm_pool_dma *dma = (void *)p->private;
@@ -201,10 +254,7 @@ static int ttm_pool_map(struct ttm_pool *pool, unsigned int order,
 			return -EFAULT;
 	}
 
-	for (i = 1 << order; i ; --i) {
-		*(*dma_addr)++ = addr;
-		addr += PAGE_SIZE;
-	}
+	*dma_addr = addr;
 
 	return 0;
 }
@@ -354,24 +404,235 @@ static unsigned int ttm_pool_page_order(struct ttm_pool *pool, struct page *p)
 	return p->private;
 }
 
-/* Called when we got a page, either from a pool or newly allocated */
+/*
+ * Split larger pages so that we can free each PAGE_SIZE page as soon
+ * as it has been backed up, in order to avoid memory pressure during
+ * reclaim.
+ */
+static void ttm_pool_split_for_swap(struct ttm_pool *pool, struct page *p)
+{
+	unsigned int order = ttm_pool_page_order(pool, p);
+	pgoff_t nr;
+
+	if (!order)
+		return;
+
+	split_page(p, order);
+	nr = 1UL << order;
+	while (nr--)
+		(p++)->private = 0;
+}
+
+/**
+ * DOC: Partial backup and restoration of a struct ttm_tt.
+ *
+ * Swapout using ttm_backup_backup_page() and swapin using
+ * ttm_backup_copy_page() may fail.
+ * The former most likely due to lack of swap-space or memory, the latter due
+ * to lack of memory or because of signal interruption during waits.
+ *
+ * Backup failure is easily handled by using a ttm_tt pages vector that holds
+ * both backup handles and page pointers. This has to be taken into account when
+ * restoring such a ttm_tt from backup, and when freeing it while backed up.
+ * When restoring, for simplicity, new pages are actually allocated from the
+ * pool and the contents of any old pages are copied in and then the old pages
+ * are released.
+ *
+ * For restoration failures, the struct ttm_pool_tt_restore holds sufficient state
+ * to be able to resume an interrupted restore, and that structure is freed once
+ * the restoration is complete. If the struct ttm_tt is destroyed while there
+ * is a valid struct ttm_pool_tt_restore attached, that is also properly taken
+ * care of.
+ */
+
+/* Is restore ongoing for the currently allocated page? */
+static bool ttm_pool_restore_valid(const struct ttm_pool_tt_restore *restore)
+{
+	return restore && restore->restored_pages < (1 << restore->order);
+}
+
+/* DMA unmap and free a multi-order page, either to the relevant pool or to system. */
+static pgoff_t ttm_pool_unmap_and_free(struct ttm_pool *pool, struct page *page,
+				       const dma_addr_t *dma_addr, enum ttm_caching caching)
+{
+	struct ttm_pool_type *pt = NULL;
+	unsigned int order;
+	pgoff_t nr;
+
+	if (pool) {
+		order = ttm_pool_page_order(pool, page);
+		nr = (1UL << order);
+		if (dma_addr)
+			ttm_pool_unmap(pool, *dma_addr, nr);
+
+		pt = ttm_pool_select_type(pool, caching, order);
+	} else {
+		order = page->private;
+		nr = (1UL << order);
+	}
+
+	if (pt)
+		ttm_pool_type_give(pt, page);
+	else
+		ttm_pool_free_page(pool, caching, order, page);
+
+	return nr;
+}
+
+/* Populate the page-array using the most recent allocated multi-order page. */
+static void ttm_pool_allocated_page_commit(struct page *allocated,
+					   dma_addr_t first_dma,
+					   struct ttm_pool_alloc_state *alloc,
+					   pgoff_t nr)
+{
+	pgoff_t i;
+
+	for (i = 0; i < nr; ++i)
+		*alloc->pages++ = allocated++;
+
+	alloc->remaining_pages -= nr;
+
+	if (!alloc->dma_addr)
+		return;
+
+	for (i = 0; i < nr; ++i) {
+		*alloc->dma_addr++ = first_dma;
+		first_dma += PAGE_SIZE;
+	}
+}
+
+/*
+ * When restoring, restore backed-up content to the newly allocated page and
+ * if successful, populate the page-table and dma-address arrays.
+ */
+static int ttm_pool_restore_commit(struct ttm_pool_tt_restore *restore,
+				   struct ttm_backup *backup,
+				   const struct ttm_operation_ctx *ctx,
+				   struct ttm_pool_alloc_state *alloc)
+
+{
+	pgoff_t i, nr = 1UL << restore->order;
+	struct page **first_page = alloc->pages;
+	struct page *p;
+	int ret = 0;
+
+	for (i = restore->restored_pages; i < nr; ++i) {
+		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++;
+				continue;
+			}
+
+			ret = ttm_backup_copy_page(backup, restore->alloced_page + i,
+						   handle, ctx->interruptible);
+			if (ret)
+				break;
+
+			ttm_backup_drop(backup, handle);
+		} else if (p) {
+			/*
+			 * We could probably avoid splitting the old page
+			 * using clever logic, but ATM we don't care, as
+			 * we prioritize releasing memory ASAP. Note that
+			 * here, the old retained page is always write-back
+			 * cached.
+			 */
+			ttm_pool_split_for_swap(restore->pool, p);
+			copy_highpage(restore->alloced_page + i, p);
+			__free_pages(p, 0);
+		}
+
+		restore->restored_pages++;
+		first_page[i] = ttm_backup_handle_to_page_ptr(0);
+	}
+
+	if (ret) {
+		if (!restore->restored_pages) {
+			dma_addr_t *dma_addr = alloc->dma_addr ? &restore->first_dma : NULL;
+
+			ttm_pool_unmap_and_free(restore->pool, restore->alloced_page,
+						dma_addr, restore->page_caching);
+			restore->restored_pages = nr;
+		}
+		return ret;
+	}
+
+	ttm_pool_allocated_page_commit(restore->alloced_page, restore->first_dma,
+				       alloc, nr);
+	if (restore->page_caching == alloc->tt_caching || PageHighMem(restore->alloced_page))
+		alloc->caching_divide = alloc->pages;
+	restore->snapshot_alloc = *alloc;
+	restore->alloced_pages += nr;
+
+	return 0;
+}
+
+/* If restoring, save information needed for ttm_pool_restore_commit(). */
+static void
+ttm_pool_page_allocated_restore(struct ttm_pool *pool, unsigned int order,
+				struct page *p,
+				enum ttm_caching page_caching,
+				dma_addr_t first_dma,
+				struct ttm_pool_tt_restore *restore,
+				const struct ttm_pool_alloc_state *alloc)
+{
+	restore->pool = pool;
+	restore->order = order;
+	restore->restored_pages = 0;
+	restore->page_caching = page_caching;
+	restore->first_dma = first_dma;
+	restore->alloced_page = p;
+	restore->snapshot_alloc = *alloc;
+}
+
+/*
+ * Called when we got a page, either from a pool or newly allocated.
+ * if needed, dma map the page and populate the dma address array.
+ * Populate the page address array.
+ * If the caching is consistent, update any deferred caching. Otherwise
+ * stage this page for an upcoming deferred caching update.
+ */
 static int ttm_pool_page_allocated(struct ttm_pool *pool, unsigned int order,
-				   struct page *p, dma_addr_t **dma_addr,
-				   unsigned long *num_pages,
-				   struct page ***pages)
+				   struct page *p, enum ttm_caching page_caching,
+				   struct ttm_pool_alloc_state *alloc,
+				   struct ttm_pool_tt_restore *restore)
 {
-	unsigned int i;
-	int r;
+	bool caching_consistent;
+	dma_addr_t first_dma;
+	int r = 0;
+
+	caching_consistent = (page_caching == alloc->tt_caching) || PageHighMem(p);
 
-	if (*dma_addr) {
-		r = ttm_pool_map(pool, order, p, dma_addr);
+	if (caching_consistent) {
+		r = ttm_pool_apply_caching(alloc);
 		if (r)
 			return r;
 	}
 
-	*num_pages -= 1 << order;
-	for (i = 1 << order; i; --i, ++(*pages), ++p)
-		**pages = p;
+	if (alloc->dma_addr) {
+		r = ttm_pool_map(pool, order, p, &first_dma);
+		if (r)
+			return r;
+	}
+
+	if (restore) {
+		ttm_pool_page_allocated_restore(pool, order, p, page_caching,
+						first_dma, restore, alloc);
+	} else {
+		ttm_pool_allocated_page_commit(p, first_dma, alloc, 1UL << order);
+
+		if (caching_consistent)
+			alloc->caching_divide = alloc->pages;
+	}
 
 	return 0;
 }
@@ -394,53 +655,62 @@ static void ttm_pool_free_range(struct ttm_pool *pool, struct ttm_tt *tt,
 				pgoff_t start_page, pgoff_t end_page)
 {
 	struct page **pages = &tt->pages[start_page];
-	unsigned int order;
+	struct ttm_backup *backup = tt->backup;
 	pgoff_t i, nr;
 
 	for (i = start_page; i < end_page; i += nr, pages += nr) {
-		struct ttm_pool_type *pt = NULL;
+		struct page *p = *pages;
 
-		order = ttm_pool_page_order(pool, *pages);
-		nr = (1UL << order);
-		if (tt->dma_address)
-			ttm_pool_unmap(pool, tt->dma_address[i], nr);
+		nr = 1;
+		if (ttm_backup_page_ptr_is_handle(p)) {
+			unsigned long handle = ttm_backup_page_ptr_to_handle(p);
 
-		pt = ttm_pool_select_type(pool, caching, order);
-		if (pt)
-			ttm_pool_type_give(pt, *pages);
-		else
-			ttm_pool_free_page(pool, caching, order, *pages);
+			if (handle != 0)
+				ttm_backup_drop(backup, handle);
+		} else if (p) {
+			dma_addr_t *dma_addr = tt->dma_address ?
+				tt->dma_address + i : NULL;
+
+			nr = ttm_pool_unmap_and_free(pool, p, dma_addr, caching);
+		}
 	}
 }
 
-/**
- * ttm_pool_alloc - Fill a ttm_tt object
- *
- * @pool: ttm_pool to use
- * @tt: ttm_tt object to fill
- * @ctx: operation context
- *
- * Fill the ttm_tt object with pages and also make sure to DMA map them when
- * necessary.
- *
- * Returns: 0 on successe, negative error code otherwise.
+static void ttm_pool_alloc_state_init(const struct ttm_tt *tt,
+				      struct ttm_pool_alloc_state *alloc)
+{
+	alloc->pages = tt->pages;
+	alloc->caching_divide = tt->pages;
+	alloc->dma_addr = tt->dma_address;
+	alloc->remaining_pages = tt->num_pages;
+	alloc->tt_caching = tt->caching;
+}
+
+/*
+ * Find a suitable allocation order based on highest desired order
+ * and number of remaining pages
  */
-int ttm_pool_alloc(struct ttm_pool *pool, struct ttm_tt *tt,
-		   struct ttm_operation_ctx *ctx)
+static unsigned int ttm_pool_alloc_find_order(unsigned int highest,
+					      const struct ttm_pool_alloc_state *alloc)
+{
+	return min_t(unsigned int, highest, __fls(alloc->remaining_pages));
+}
+
+static int __ttm_pool_alloc(struct ttm_pool *pool, struct ttm_tt *tt,
+			    const struct ttm_operation_ctx *ctx,
+			    struct ttm_pool_alloc_state *alloc,
+			    struct ttm_pool_tt_restore *restore)
 {
-	pgoff_t num_pages = tt->num_pages;
-	dma_addr_t *dma_addr = tt->dma_address;
-	struct page **caching = tt->pages;
-	struct page **pages = tt->pages;
 	enum ttm_caching page_caching;
 	gfp_t gfp_flags = GFP_USER;
 	pgoff_t caching_divide;
 	unsigned int order;
+	bool allow_pools;
 	struct page *p;
 	int r;
 
-	WARN_ON(!num_pages || ttm_tt_is_populated(tt));
-	WARN_ON(dma_addr && !pool->dev);
+	WARN_ON(!alloc->remaining_pages || ttm_tt_is_populated(tt));
+	WARN_ON(alloc->dma_addr && !pool->dev);
 
 	if (tt->page_flags & TTM_TT_FLAG_ZERO_ALLOC)
 		gfp_flags |= __GFP_ZERO;
@@ -453,86 +723,155 @@ int ttm_pool_alloc(struct ttm_pool *pool, struct ttm_tt *tt,
 	else
 		gfp_flags |= GFP_HIGHUSER;
 
-	for (order = min_t(unsigned int, MAX_PAGE_ORDER, __fls(num_pages));
-	     num_pages;
-	     order = min_t(unsigned int, order, __fls(num_pages))) {
+	page_caching = tt->caching;
+	allow_pools = true;
+	for (order = ttm_pool_alloc_find_order(MAX_PAGE_ORDER, alloc);
+	     alloc->remaining_pages;
+	     order = ttm_pool_alloc_find_order(order, alloc)) {
 		struct ttm_pool_type *pt;
 
-		page_caching = tt->caching;
-		pt = ttm_pool_select_type(pool, tt->caching, order);
-		p = pt ? ttm_pool_type_take(pt) : NULL;
-		if (p) {
-			r = ttm_pool_apply_caching(caching, pages,
-						   tt->caching);
-			if (r)
-				goto error_free_page;
-
-			caching = pages;
-			do {
-				r = ttm_pool_page_allocated(pool, order, p,
-							    &dma_addr,
-							    &num_pages,
-							    &pages);
-				if (r)
-					goto error_free_page;
-
-				caching = pages;
-				if (num_pages < (1 << order))
-					break;
-
-				p = ttm_pool_type_take(pt);
-			} while (p);
-		}
-
-		page_caching = ttm_cached;
-		while (num_pages >= (1 << order) &&
-		       (p = ttm_pool_alloc_page(pool, gfp_flags, order))) {
-
-			if (PageHighMem(p)) {
-				r = ttm_pool_apply_caching(caching, pages,
-							   tt->caching);
-				if (r)
-					goto error_free_page;
-				caching = pages;
-			}
-			r = ttm_pool_page_allocated(pool, order, p, &dma_addr,
-						    &num_pages, &pages);
-			if (r)
-				goto error_free_page;
-			if (PageHighMem(p))
-				caching = pages;
+		/* First, try to allocate a page from a pool if one exists. */
+		p = NULL;
+		pt = ttm_pool_select_type(pool, page_caching, order);
+		if (pt && allow_pools)
+			p = ttm_pool_type_take(pt);
+		/*
+		 * If that fails or previously failed, allocate from system.
+		 * Note that this also disallows additional pool allocations using
+		 * write-back cached pools of the same order. Consider removing
+		 * that behaviour.
+		 */
+		if (!p) {
+			page_caching = ttm_cached;
+			allow_pools = false;
+			p = ttm_pool_alloc_page(pool, gfp_flags, order);
 		}
-
+		/* If that fails, lower the order if possible and retry. */
 		if (!p) {
 			if (order) {
 				--order;
+				page_caching = tt->caching;
+				allow_pools = true;
 				continue;
 			}
 			r = -ENOMEM;
 			goto error_free_all;
 		}
+		r = ttm_pool_page_allocated(pool, order, p, page_caching, alloc,
+					    restore);
+		if (r)
+			goto error_free_page;
+
+		if (ttm_pool_restore_valid(restore)) {
+			r = ttm_pool_restore_commit(restore, tt->backup, ctx, alloc);
+			if (r)
+				goto error_free_all;
+		}
 	}
 
-	r = ttm_pool_apply_caching(caching, pages, tt->caching);
+	r = ttm_pool_apply_caching(alloc);
 	if (r)
 		goto error_free_all;
 
+	kfree(tt->restore);
+	tt->restore = NULL;
+
 	return 0;
 
 error_free_page:
 	ttm_pool_free_page(pool, page_caching, order, p);
 
 error_free_all:
-	num_pages = tt->num_pages - num_pages;
-	caching_divide = caching - tt->pages;
+	if (tt->restore)
+		return r;
+
+	caching_divide = alloc->caching_divide - tt->pages;
 	ttm_pool_free_range(pool, tt, tt->caching, 0, caching_divide);
-	ttm_pool_free_range(pool, tt, ttm_cached, caching_divide, num_pages);
+	ttm_pool_free_range(pool, tt, ttm_cached, caching_divide,
+			    tt->num_pages - alloc->remaining_pages);
 
 	return r;
 }
+
+/**
+ * ttm_pool_alloc - Fill a ttm_tt object
+ *
+ * @pool: ttm_pool to use
+ * @tt: ttm_tt object to fill
+ * @ctx: operation context
+ *
+ * Fill the ttm_tt object with pages and also make sure to DMA map them when
+ * necessary.
+ *
+ * Returns: 0 on successe, negative error code otherwise.
+ */
+int ttm_pool_alloc(struct ttm_pool *pool, struct ttm_tt *tt,
+		   struct ttm_operation_ctx *ctx)
+{
+	struct ttm_pool_alloc_state alloc;
+
+	if (WARN_ON(ttm_tt_is_backed_up(tt)))
+		return -EINVAL;
+
+	ttm_pool_alloc_state_init(tt, &alloc);
+
+	return __ttm_pool_alloc(pool, tt, ctx, &alloc, NULL);
+}
 EXPORT_SYMBOL(ttm_pool_alloc);
 
 /**
+ * ttm_pool_restore_and_alloc - Fill a ttm_tt, restoring previously backed-up
+ * content.
+ *
+ * @pool: ttm_pool to use
+ * @tt: ttm_tt object to fill
+ * @ctx: operation context
+ *
+ * Fill the ttm_tt object with pages and also make sure to DMA map them when
+ * necessary. Read in backed-up content.
+ *
+ * Returns: 0 on successe, negative error code otherwise.
+ */
+int ttm_pool_restore_and_alloc(struct ttm_pool *pool, struct ttm_tt *tt,
+			       const struct ttm_operation_ctx *ctx)
+{
+	struct ttm_pool_alloc_state alloc;
+
+	if (WARN_ON(!ttm_tt_is_backed_up(tt)))
+		return -EINVAL;
+
+	if (!tt->restore) {
+		gfp_t gfp = GFP_KERNEL | __GFP_NOWARN;
+
+		ttm_pool_alloc_state_init(tt, &alloc);
+		if (ctx->gfp_retry_mayfail)
+			gfp |= __GFP_RETRY_MAYFAIL;
+
+		tt->restore = kzalloc(sizeof(*tt->restore), gfp);
+		if (!tt->restore)
+			return -ENOMEM;
+
+		tt->restore->snapshot_alloc = alloc;
+		tt->restore->pool = pool;
+		tt->restore->restored_pages = 1;
+	} else {
+		struct ttm_pool_tt_restore *restore = tt->restore;
+		int ret;
+
+		alloc = restore->snapshot_alloc;
+		if (ttm_pool_restore_valid(tt->restore)) {
+			ret = ttm_pool_restore_commit(restore, tt->backup, ctx, &alloc);
+			if (ret)
+				return ret;
+		}
+		if (!alloc.remaining_pages)
+			return 0;
+	}
+
+	return __ttm_pool_alloc(pool, tt, ctx, &alloc, tt->restore);
+}
+
+/**
  * ttm_pool_free - Free the backing pages from a ttm_tt object
  *
  * @pool: Pool to give pages back to.
@@ -550,6 +889,169 @@ void ttm_pool_free(struct ttm_pool *pool, struct ttm_tt *tt)
 EXPORT_SYMBOL(ttm_pool_free);
 
 /**
+ * ttm_pool_drop_backed_up() - Release content of a swapped-out struct ttm_tt
+ * @tt: The struct ttm_tt.
+ *
+ * Release handles with associated content or any remaining pages of
+ * a backed-up struct ttm_tt.
+ */
+void ttm_pool_drop_backed_up(struct ttm_tt *tt)
+{
+	struct ttm_pool_tt_restore *restore;
+	pgoff_t start_page = 0;
+
+	WARN_ON(!ttm_tt_is_backed_up(tt));
+
+	restore = tt->restore;
+
+	/*
+	 * Unmap and free any uncommitted restore page.
+	 * any tt page-array backup entries already read back has
+	 * been cleared already
+	 */
+	if (ttm_pool_restore_valid(restore)) {
+		dma_addr_t *dma_addr = tt->dma_address ? &restore->first_dma : NULL;
+
+		ttm_pool_unmap_and_free(restore->pool, restore->alloced_page,
+					dma_addr, restore->page_caching);
+		restore->restored_pages = 1UL << restore->order;
+	}
+
+	/*
+	 * If a restore is ongoing, part of the tt pages may have a
+	 * caching different than writeback.
+	 */
+	if (restore) {
+		pgoff_t mid = restore->snapshot_alloc.caching_divide - tt->pages;
+
+		start_page = restore->alloced_pages;
+		WARN_ON(mid > start_page);
+		/* Pages that might be dma-mapped and non-cached */
+		ttm_pool_free_range(restore->pool, tt, tt->caching,
+				    0, mid);
+		/* Pages that might be dma-mapped but cached */
+		ttm_pool_free_range(restore->pool, tt, ttm_cached,
+				    mid, restore->alloced_pages);
+		kfree(restore);
+		tt->restore = NULL;
+	}
+
+	ttm_pool_free_range(NULL, tt, ttm_cached, start_page, tt->num_pages);
+}
+
+/**
+ * ttm_pool_backup() - Back up or purge a struct ttm_tt
+ * @pool: The pool used when allocating the struct ttm_tt.
+ * @tt: The struct ttm_tt.
+ * @flags: Flags to govern the backup behaviour.
+ *
+ * Back up or purge a struct ttm_tt. If @purge is true, then
+ * all pages will be freed directly to the system rather than to the pool
+ * they were allocated from, making the function behave similarly to
+ * ttm_pool_free(). If @purge is false the pages will be backed up instead,
+ * exchanged for handles.
+ * A subsequent call to ttm_pool_restore_and_alloc() will then read back the content and
+ * a subsequent call to ttm_pool_drop_backed_up() will drop it.
+ * If backup of a page fails for whatever reason, @ttm will still be
+ * partially backed up, retaining those pages for which backup fails.
+ * In that case, this function can be retried, possibly after freeing up
+ * memory resources.
+ *
+ * Return: Number of pages actually backed up or freed, or negative
+ * error code on error.
+ */
+long ttm_pool_backup(struct ttm_pool *pool, struct ttm_tt *tt,
+		     const struct ttm_backup_flags *flags)
+{
+	struct ttm_backup *backup = tt->backup;
+	struct page *page;
+	unsigned long handle;
+	gfp_t alloc_gfp;
+	gfp_t gfp;
+	int ret = 0;
+	pgoff_t shrunken = 0;
+	pgoff_t i, num_pages;
+
+	if (WARN_ON(ttm_tt_is_backed_up(tt)))
+		return -EINVAL;
+
+	if ((!ttm_backup_bytes_avail() && !flags->purge) ||
+	    pool->use_dma_alloc || ttm_tt_is_backed_up(tt))
+		return -EBUSY;
+
+#ifdef CONFIG_X86
+	/* Anything returned to the system needs to be cached. */
+	if (tt->caching != ttm_cached)
+		set_pages_array_wb(tt->pages, tt->num_pages);
+#endif
+
+	if (tt->dma_address || flags->purge) {
+		for (i = 0; i < tt->num_pages; i += num_pages) {
+			unsigned int order;
+
+			page = tt->pages[i];
+			if (unlikely(!page)) {
+				num_pages = 1;
+				continue;
+			}
+
+			order = ttm_pool_page_order(pool, page);
+			num_pages = 1UL << order;
+			if (tt->dma_address)
+				ttm_pool_unmap(pool, tt->dma_address[i],
+					       num_pages);
+			if (flags->purge) {
+				shrunken += num_pages;
+				page->private = 0;
+				__free_pages(page, order);
+				memset(tt->pages + i, 0,
+				       num_pages * sizeof(*tt->pages));
+			}
+		}
+	}
+
+	if (flags->purge)
+		return shrunken;
+
+	if (pool->use_dma32)
+		gfp = GFP_DMA32;
+	else
+		gfp = GFP_HIGHUSER;
+
+	alloc_gfp = GFP_KERNEL | __GFP_HIGH | __GFP_NOWARN | __GFP_RETRY_MAYFAIL;
+
+	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];
+		if (unlikely(!page))
+			continue;
+
+		ttm_pool_split_for_swap(pool, page);
+
+		shandle = ttm_backup_backup_page(backup, page, flags->writeback, i,
+						 gfp, alloc_gfp);
+		if (shandle < 0) {
+			/* We allow partially shrunken tts */
+			ret = shandle;
+			break;
+		}
+		handle = shandle;
+		tt->pages[i] = ttm_backup_handle_to_page_ptr(handle);
+		put_page(page);
+		shrunken++;
+	}
+
+	return shrunken ? shrunken : ret;
+}
+
+/**
  * ttm_pool_init - Initialize a pool
  *
  * @pool: the pool to initialize
@@ -810,6 +1312,10 @@ int ttm_pool_mgr_init(unsigned long num_pages)
 			    &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");