[v7,06/20] fs/dax: Always remove DAX page-cache entries when breaking layouts

Commit Message

Alistair Popple Feb. 4, 2025, 10:48 p.m. UTC

Prior to any truncation operations file systems call
dax_break_mapping() to ensure pages in the range are not under going
DMA. Later DAX page-cache entries will be removed by
truncate_folio_batch_exceptionals() in the generic page-cache code.

However this makes it possible for folios to be removed from the
page-cache even though they are still DMA busy if the file-system
hasn't called dax_break_mapping(). It also means they can never be
waited on in future because FS DAX will lose track of them once the
page-cache entry has been deleted.

Instead it is better to delete the FS DAX entry when the file-system
calls dax_break_mapping() as part of it's truncate operation. This
ensures only idle pages can be removed from the FS DAX page-cache and
makes it easy to detect if a file-system hasn't called
dax_break_mapping() prior to a truncate operation.

Signed-off-by: Alistair Popple <[e-mail]>
Reviewed-by: Dan Williams <[e-mail]>

---

Changes for v7:

 - s/dax_break_mapping/dax_break_layout/ suggested by Dan.
 - Rework dax_break_mapping() to take a NULL callback for NOWAIT
   behaviour as suggested by Dan.
---
 fs/dax.c            | 40 ++++++++++++++++++++++++++++++++++++++++
 fs/xfs/xfs_inode.c  |  5 ++---
 include/linux/dax.h |  2 ++
 mm/truncate.c       | 16 +++++++++++++++-
 4 files changed, 59 insertions(+), 4 deletions(-)

Patch

diff --git a/fs/dax.c b/fs/dax.c
index 710b280..39f1dc0 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -845,6 +845,36 @@  int dax_delete_mapping_entry(struct address_space *mapping, pgoff_t index)
 	return ret;
 }
 
+void dax_delete_mapping_range(struct address_space *mapping,
+				loff_t start, loff_t end)
+{
+	void *entry;
+	pgoff_t start_idx = start >> PAGE_SHIFT;
+	pgoff_t end_idx;
+	XA_STATE(xas, &mapping->i_pages, start_idx);
+
+	/* If end == LLONG_MAX, all pages from start to till end of file */
+	if (end == LLONG_MAX)
+		end_idx = ULONG_MAX;
+	else
+		end_idx = end >> PAGE_SHIFT;
+
+	xas_lock_irq(&xas);
+	xas_for_each(&xas, entry, end_idx) {
+		if (!xa_is_value(entry))
+			continue;
+		entry = wait_entry_unlocked_exclusive(&xas, entry);
+		if (!entry)
+			continue;
+		dax_disassociate_entry(entry, mapping, true);
+		xas_store(&xas, NULL);
+		mapping->nrpages -= 1UL << dax_entry_order(entry);
+		put_unlocked_entry(&xas, entry, WAKE_ALL);
+	}
+	xas_unlock_irq(&xas);
+}
+EXPORT_SYMBOL_GPL(dax_delete_mapping_range);
+
 static int wait_page_idle(struct page *page,
 			void (cb)(struct inode *),
 			struct inode *inode)
@@ -856,6 +886,9 @@  static int wait_page_idle(struct page *page,
 /*
  * Unmaps the inode and waits for any DMA to complete prior to deleting the
  * DAX mapping entries for the range.
+ *
+ * For NOWAIT behavior, pass @cb as NULL to early-exit on first found
+ * busy page
  */
 int dax_break_layout(struct inode *inode, loff_t start, loff_t end,
 		void (cb)(struct inode *))
@@ -870,10 +903,17 @@  int dax_break_layout(struct inode *inode, loff_t start, loff_t end,
 		page = dax_layout_busy_page_range(inode->i_mapping, start, end);
 		if (!page)
 			break;
+		if (!cb) {
+			error = -ERESTARTSYS;
+			break;
+		}
 
 		error = wait_page_idle(page, cb, inode);
 	} while (error == 0);
 
+	if (!page)
+		dax_delete_mapping_range(inode->i_mapping, start, end);
+
 	return error;
 }
 EXPORT_SYMBOL_GPL(dax_break_layout);
diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
index 62c2ae3..c9ffabe 100644
--- a/fs/xfs/xfs_inode.c
+++ b/fs/xfs/xfs_inode.c
@@ -2732,7 +2732,6 @@  xfs_mmaplock_two_inodes_and_break_dax_layout(
 	struct xfs_inode	*ip2)
 {
 	int			error;
-	struct page		*page;
 
 	if (ip1->i_ino > ip2->i_ino)
 		swap(ip1, ip2);
@@ -2756,8 +2755,8 @@  xfs_mmaplock_two_inodes_and_break_dax_layout(
 	 * need to unlock & lock the XFS_MMAPLOCK_EXCL which is not suitable
 	 * for this nested lock case.
 	 */
-	page = dax_layout_busy_page(VFS_I(ip2)->i_mapping);
-	if (!dax_page_is_idle(page)) {
+	error = dax_break_layout(VFS_I(ip2), 0, -1, NULL);
+	if (error) {
 		xfs_iunlock(ip2, XFS_MMAPLOCK_EXCL);
 		xfs_iunlock(ip1, XFS_MMAPLOCK_EXCL);
 		goto again;
diff --git a/include/linux/dax.h b/include/linux/dax.h
index a6b277f..2fbb262 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -255,6 +255,8 @@  vm_fault_t dax_iomap_fault(struct vm_fault *vmf, unsigned int order,
 vm_fault_t dax_finish_sync_fault(struct vm_fault *vmf,
 		unsigned int order, pfn_t pfn);
 int dax_delete_mapping_entry(struct address_space *mapping, pgoff_t index);
+void dax_delete_mapping_range(struct address_space *mapping,
+				loff_t start, loff_t end);
 int dax_invalidate_mapping_entry_sync(struct address_space *mapping,
 				      pgoff_t index);
 int __must_check dax_break_layout(struct inode *inode, loff_t start,
diff --git a/mm/truncate.c b/mm/truncate.c
index e2e115a..0395e57 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -78,8 +78,22 @@  static void truncate_folio_batch_exceptionals(struct address_space *mapping,
 
 	if (dax_mapping(mapping)) {
 		for (i = j; i < nr; i++) {
-			if (xa_is_value(fbatch->folios[i]))
+			if (xa_is_value(fbatch->folios[i])) {
+				/*
+				 * File systems should already have called
+				 * dax_break_layout_entry() to remove all DAX
+				 * entries while holding a lock to prevent
+				 * establishing new entries. Therefore we
+				 * shouldn't find any here.
+				 */
+				WARN_ON_ONCE(1);
+
+				/*
+				 * Delete the mapping so truncate_pagecache()
+				 * doesn't loop forever.
+				 */
 				dax_delete_mapping_entry(mapping, indices[i]);
+			}
 		}
 		goto out;
 	}