[PATCH v2 08/12] mm, swap: free backing pages in xswap_unmap_clusters

[Baoquan He]

vm_area_unmap_pages() clears PTEs and frees intermediate page table
pages. It does not free the backing physical pages that
xswap_map_clusters() allocated. Those pages were leaked on every
unmap.

Collect the backing pages before the unmap. Walk the page table with
apply_to_existing_page_range() and a small callback that stores
pte_page() of every present PTE into a dynamically allocated array.
After vm_area_unmap_pages() clears the PTEs, free the collected pages
with __free_page().

The array allocation runs under memalloc_noreclaim_save(). Reclaim
must not recurse into the caller, which holds xswap_lock. If the
allocation fails, return -ENOMEM without unmapping, so that no backing
page is leaked.

xswap_unmap_clusters() now returns an error code.

Signed-off-by: Baoquan He <hebaoquan@xxxxxxxxxx>
---
 mm/swapfile.c | 57 ++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 50 insertions(+), 7 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index abe3d0ee5aba..e10bb6d245fe 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -67,8 +67,8 @@
 
 static int xswap_map_clusters(struct swap_info_struct *si,
 			      unsigned long start_idx, unsigned long nr);
-static void xswap_unmap_clusters(struct swap_info_struct *si,
-				 unsigned long start_idx, unsigned long nr);
+static int xswap_unmap_clusters(struct swap_info_struct *si,
+				unsigned long start_idx, unsigned long nr);
 static int xswap_mapped_end(pte_t *pte, unsigned long addr, void *data);
 static void xswap_try_shrink(struct swap_info_struct *si);
 
@@ -3889,8 +3889,26 @@ static int xswap_map_clusters(struct swap_info_struct *si,
 	return -ENOMEM;
 }
 
-static void xswap_unmap_clusters(struct swap_info_struct *si,
-				 unsigned long start_idx, unsigned long nr)
+struct xswap_page_data {
+	struct page **pages;
+	int nr;
+	int max;
+};
+
+static int xswap_collect_page(pte_t *pte, unsigned long addr, void *data)
+{
+	struct xswap_page_data *xpd = data;
+	pte_t pteval = ptep_get(pte);
+
+	if (!pte_present(pteval))
+		return 0;
+	if (xpd->nr < xpd->max)
+		xpd->pages[xpd->nr++] = pte_page(pteval);
+	return 0;
+}
+
+static int xswap_unmap_clusters(struct swap_info_struct *si,
+				unsigned long start_idx, unsigned long nr)
 {
 	unsigned long start_addr = (unsigned long)si->cluster_info +
 				   (size_t)start_idx * sizeof(struct swap_cluster_info);
@@ -3898,13 +3916,18 @@ static void xswap_unmap_clusters(struct swap_info_struct *si,
 	/* Round to page boundaries for vm_area_unmap_pages(). */
 	unsigned long vm_start = PAGE_ALIGN(start_addr);
 	unsigned long vm_end = PAGE_ALIGN(end_addr);
+	unsigned long size;
+	unsigned long npages;
+	struct xswap_page_data xpd;
+	unsigned int noreclaim_flags;
+	int i;
 
 	mutex_lock(&si->xswap_lock);
 
 	if (vm_start >= vm_end) {
 		WRITE_ONCE(si->nr_clusters_mapped, start_idx);
 		mutex_unlock(&si->xswap_lock);
-		return;
+		return 0;
 	}
 
 	/*
@@ -3916,13 +3939,33 @@ static void xswap_unmap_clusters(struct swap_info_struct *si,
 	flush_percpu_swap_cluster(si);
 	synchronize_rcu();
 
+	size = vm_end - vm_start;
+	npages = size >> PAGE_SHIFT;
+
+	noreclaim_flags = memalloc_noreclaim_save();
+	xpd.pages = kmalloc_array(npages, sizeof(*xpd.pages),
+				  __GFP_HIGH | __GFP_NOMEMALLOC | GFP_KERNEL);
+	memalloc_noreclaim_restore(noreclaim_flags);
+	if (!xpd.pages) {
+		mutex_unlock(&si->xswap_lock);
+		return -ENOMEM;
+	}
+
+	xpd.nr = 0;
+	xpd.max = npages;
+	apply_to_existing_page_range(&init_mm, vm_start, size,
+				     xswap_collect_page, &xpd);
+
 	vm_area_unmap_pages(si->cluster_vm, vm_start, vm_end);
-	/* vm_area_unmap_pages() clears PTEs but does not free pages. */
-	/* TODO: free backing pages via page table walk or tracking bitmap */
+
+	for (i = 0; i < xpd.nr; i++)
+		__free_page(xpd.pages[i]);
+	kfree(xpd.pages);
 
 	/* Pairs with READ_ONCE() in shrink/grow paths. */
 	WRITE_ONCE(si->nr_clusters_mapped, start_idx);
 	mutex_unlock(&si->xswap_lock);
+	return 0;
 }
 
 /* Track the end of the run of pages that is already mapped. */
-- 
2.54.0