[2/2] mm: Use multi-index entries in the page cache
diff mbox series

Message ID 20200629152033.16175-3-willy@infradead.org
State New
Headers show
Series
  • Use multi-index entries in the page cache
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Commit Message

Matthew Wilcox June 29, 2020, 3:20 p.m. UTC
We currently store order-N THPs as 2^N consecutive entries.  While this
consumes rather more memory than necessary, it also turns out to be
buggy for filesystems which track dirty pages as a writeback operation
which starts in the middle of a dirty THP will not notice as the dirty
bit is only set on the head index.  With multi-index entries, the dirty
bit will be found on the head index.

This does end up simplifying the page cache slightly, although not as
much as I had hoped.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 mm/filemap.c     | 42 +++++++++++++++++++-----------------------
 mm/huge_memory.c | 21 +++++++++++++++++----
 mm/khugepaged.c  | 12 +++++++++++-
 mm/shmem.c       | 11 ++---------
 4 files changed, 49 insertions(+), 37 deletions(-)

Patch
diff mbox series

diff --git a/mm/filemap.c b/mm/filemap.c
index 80ce3658b147..28859bc43a3a 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -126,13 +126,12 @@  static void page_cache_delete(struct address_space *mapping,
 
 	/* hugetlb pages are represented by a single entry in the xarray */
 	if (!PageHuge(page)) {
-		xas_set_order(&xas, page->index, compound_order(page));
-		nr = compound_nr(page);
+		xas_set_order(&xas, page->index, thp_order(page));
+		nr = thp_nr_pages(page);
 	}
 
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 	VM_BUG_ON_PAGE(PageTail(page), page);
-	VM_BUG_ON_PAGE(nr != 1 && shadow, page);
 
 	xas_store(&xas, shadow);
 	xas_init_marks(&xas);
@@ -322,19 +321,12 @@  static void page_cache_delete_batch(struct address_space *mapping,
 
 		WARN_ON_ONCE(!PageLocked(page));
 
-		if (page->index == xas.xa_index)
-			page->mapping = NULL;
+		page->mapping = NULL;
 		/* Leave page->index set: truncation lookup relies on it */
 
-		/*
-		 * Move to the next page in the vector if this is a regular
-		 * page or the index is of the last sub-page of this compound
-		 * page.
-		 */
-		if (page->index + compound_nr(page) - 1 == xas.xa_index)
-			i++;
+		i++;
 		xas_store(&xas, NULL);
-		total_pages++;
+		total_pages += thp_nr_pages(page);
 	}
 	mapping->nrpages -= total_pages;
 }
@@ -851,20 +843,24 @@  static int __add_to_page_cache_locked(struct page *page,
 	}
 
 	do {
-		xas_lock_irq(&xas);
-		old = xas_load(&xas);
-		if (old && !xa_is_value(old))
-			xas_set_err(&xas, -EEXIST);
-		xas_store(&xas, page);
-		if (xas_error(&xas))
-			goto unlock;
+		unsigned long exceptional = 0;
 
-		if (xa_is_value(old)) {
-			mapping->nrexceptional--;
+		xas_lock_irq(&xas);
+		xas_for_each_conflict(&xas, old) {
+			if (!xa_is_value(old)) {
+				xas_set_err(&xas, -EEXIST);
+				goto unlock;
+			}
+			exceptional++;
 			if (shadowp)
 				*shadowp = old;
 		}
-		mapping->nrpages++;
+
+		xas_store(&xas, page);
+		if (xas_error(&xas))
+			goto unlock;
+		mapping->nrexceptional -= exceptional;
+		mapping->nrpages += nr;
 
 		/* hugetlb pages do not participate in page cache accounting */
 		if (!huge)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 78c84bee7e29..7e5ff05ceeaa 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2603,6 +2603,8 @@  int split_huge_page_to_list(struct page *page, struct list_head *list)
 	struct page *head = compound_head(page);
 	struct pglist_data *pgdata = NODE_DATA(page_to_nid(head));
 	struct deferred_split *ds_queue = get_deferred_split_queue(head);
+	XA_STATE_ORDER(xas, &head->mapping->i_pages, head->index,
+				compound_order(head));
 	struct anon_vma *anon_vma = NULL;
 	struct address_space *mapping = NULL;
 	int count, mapcount, extra_pins, ret;
@@ -2667,19 +2669,28 @@  int split_huge_page_to_list(struct page *page, struct list_head *list)
 	unmap_page(head);
 	VM_BUG_ON_PAGE(compound_mapcount(head), head);
 
+	if (mapping) {
+		/* XXX: Need better GFP flags here */
+		xas_split_alloc(&xas, head, 0, GFP_ATOMIC);
+		if (xas_error(&xas)) {
+			ret = xas_error(&xas);
+			goto out_unlock;
+		}
+	}
+
 	/* prevent PageLRU to go away from under us, and freeze lru stats */
 	spin_lock_irqsave(&pgdata->lru_lock, flags);
 
 	if (mapping) {
-		XA_STATE(xas, &mapping->i_pages, page_index(head));
-
 		/*
 		 * Check if the head page is present in page cache.
 		 * We assume all tail are present too, if head is there.
 		 */
-		xa_lock(&mapping->i_pages);
+		xas_lock(&xas);
+		xas_reset(&xas);
 		if (xas_load(&xas) != head)
 			goto fail;
+		xas_split(&xas, head, 0);
 	}
 
 	/* Prevent deferred_split_scan() touching ->_refcount */
@@ -2717,7 +2728,7 @@  int split_huge_page_to_list(struct page *page, struct list_head *list)
 		}
 		spin_unlock(&ds_queue->split_queue_lock);
 fail:		if (mapping)
-			xa_unlock(&mapping->i_pages);
+			xas_unlock(&xas);
 		spin_unlock_irqrestore(&pgdata->lru_lock, flags);
 		remap_page(head);
 		ret = -EBUSY;
@@ -2731,6 +2742,8 @@  fail:		if (mapping)
 	if (mapping)
 		i_mmap_unlock_read(mapping);
 out:
+	/* Free any memory we didn't use */
+	xas_nomem(&xas, 0);
 	count_vm_event(!ret ? THP_SPLIT_PAGE : THP_SPLIT_PAGE_FAILED);
 	return ret;
 }
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index b043c40a21d4..52dcec90e1c3 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1638,7 +1638,10 @@  static void collapse_file(struct mm_struct *mm,
 	}
 	count_memcg_page_event(new_page, THP_COLLAPSE_ALLOC);
 
-	/* This will be less messy when we use multi-index entries */
+	/*
+	 * Ensure we have slots for all the pages in the range.  This is
+	 * almost certainly a no-op because most of the pages must be present
+	 */
 	do {
 		xas_lock_irq(&xas);
 		xas_create_range(&xas);
@@ -1844,6 +1847,9 @@  static void collapse_file(struct mm_struct *mm,
 			__mod_lruvec_page_state(new_page, NR_SHMEM, nr_none);
 	}
 
+	/* Join all the small entries into a single multi-index entry */
+	xas_set_order(&xas, start, HPAGE_PMD_ORDER);
+	xas_store(&xas, new_page);
 xa_locked:
 	xas_unlock_irq(&xas);
 xa_unlocked:
@@ -1965,6 +1971,10 @@  static void khugepaged_scan_file(struct mm_struct *mm,
 			continue;
 		}
 
+		/*
+		 * XXX: khugepaged should compact smaller compound pages
+		 * into a PMD sized page
+		 */
 		if (PageTransCompound(page)) {
 			result = SCAN_PAGE_COMPOUND;
 			break;
diff --git a/mm/shmem.c b/mm/shmem.c
index a0dbe62f8042..030cc483dd3f 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -608,7 +608,6 @@  static int shmem_add_to_page_cache(struct page *page,
 				   struct mm_struct *charge_mm)
 {
 	XA_STATE_ORDER(xas, &mapping->i_pages, index, compound_order(page));
-	unsigned long i = 0;
 	unsigned long nr = compound_nr(page);
 	int error;
 
@@ -638,17 +637,11 @@  static int shmem_add_to_page_cache(struct page *page,
 		void *entry;
 		xas_lock_irq(&xas);
 		entry = xas_find_conflict(&xas);
-		if (entry != expected)
+		if (entry != expected) {
 			xas_set_err(&xas, -EEXIST);
-		xas_create_range(&xas);
-		if (xas_error(&xas))
 			goto unlock;
-next:
-		xas_store(&xas, page);
-		if (++i < nr) {
-			xas_next(&xas);
-			goto next;
 		}
+		xas_store(&xas, page);
 		if (PageTransHuge(page)) {
 			count_vm_event(THP_FILE_ALLOC);
 			__inc_node_page_state(page, NR_SHMEM_THPS);