mm: introduce and use mapping_empty()

Patch series "Remove nrexceptional tracking", v2.

We actually use nrexceptional for very little these days.  It's a minor
pain to keep in sync with nrpages, but the pain becomes much bigger with
the THP patches because we don't know how many indices a shadow entry
occupies.  It's easier to just remove it than keep it accurate.

Also, we save 8 bytes per inode which is nothing to sneeze at; on my
laptop, it would improve shmem_inode_cache from 22 to 23 objects per
16kB, and inode_cache from 26 to 27 objects.  Combined, that saves
a megabyte of memory from a combined usage of 25MB for both caches.
Unfortunately, ext4 doesn't cross a magic boundary, so it doesn't save
any memory for ext4.

This patch (of 4):

Instead of checking the two counters (nrpages and nrexceptional), we can
just check whether i_pages is empty.

Link: https://lkml.kernel.org/r/20201026151849.24232-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20201026151849.24232-2-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: Vishal Verma <vishal.l.verma@intel.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/block_dev.c

@@ -79,7 +79,7 @@ static void kill_bdev(struct block_device *bdev)
 {
 	struct address_space *mapping = bdev->bd_inode->i_mapping;
 
-	if (mapping->nrpages == 0 && mapping->nrexceptional == 0)
+	if (mapping_empty(mapping))
 		return;
 
 	invalidate_bh_lrus();

fs/dax.c

@@ -965,7 +965,7 @@ int dax_writeback_mapping_range(struct address_space *mapping,
 	if (WARN_ON_ONCE(inode->i_blkbits != PAGE_SHIFT))
 		return -EIO;
 
-	if (!mapping->nrexceptional || wbc->sync_mode != WB_SYNC_ALL)
+	if (mapping_empty(mapping) || wbc->sync_mode != WB_SYNC_ALL)
 		return 0;
 
 	trace_dax_writeback_range(inode, xas.xa_index, end_index);

fs/gfs2/glock.c

@@ -273,8 +273,7 @@ static void __gfs2_glock_put(struct gfs2_glock *gl)
 	if (mapping) {
 		truncate_inode_pages_final(mapping);
 		if (!gfs2_withdrawn(sdp))
-			GLOCK_BUG_ON(gl, mapping->nrpages ||
+			GLOCK_BUG_ON(gl, !mapping_empty(mapping));
-				     mapping->nrexceptional);
 	}
 	trace_gfs2_glock_put(gl);
 	sdp->sd_lockstruct.ls_ops->lm_put_lock(gl);

include/linux/pagemap.h

@@ -18,6 +18,11 @@
 
 struct pagevec;
 
+static inline bool mapping_empty(struct address_space *mapping)
+{
+	return xa_empty(&mapping->i_pages);
+}
+
 /*
  * Bits in mapping->flags.
  */

mm/truncate.c

@@ -295,7 +295,7 @@ void truncate_inode_pages_range(struct address_space *mapping,
 	pgoff_t		index;
 	int		i;
 
-	if (mapping->nrpages == 0 && mapping->nrexceptional == 0)
+	if (mapping_empty(mapping))
 		goto out;
 
 	/* Offsets within partial pages */
@@ -440,9 +440,6 @@ EXPORT_SYMBOL(truncate_inode_pages);
  */
 void truncate_inode_pages_final(struct address_space *mapping)
 {
-	unsigned long nrexceptional;
-	unsigned long nrpages;
-
 	/*
 	 * Page reclaim can not participate in regular inode lifetime
 	 * management (can't call iput()) and thus can race with the
@@ -452,16 +449,7 @@ void truncate_inode_pages_final(struct address_space *mapping)
 	 */
 	mapping_set_exiting(mapping);
 
-	/*
+	if (!mapping_empty(mapping)) {
-	 * When reclaim installs eviction entries, it increases
-	 * nrexceptional first, then decreases nrpages.  Make sure we see
-	 * this in the right order or we might miss an entry.
-	 */
-	nrpages = mapping->nrpages;
-	smp_rmb();
-	nrexceptional = mapping->nrexceptional;
-
-	if (nrpages || nrexceptional) {
 		/*
 		 * As truncation uses a lockless tree lookup, cycle
 		 * the tree lock to make sure any ongoing tree
@@ -633,7 +621,7 @@ int invalidate_inode_pages2_range(struct address_space *mapping,
 	int ret2 = 0;
 	int did_range_unmap = 0;
 
-	if (mapping->nrpages == 0 && mapping->nrexceptional == 0)
+	if (mapping_empty(mapping))
 		goto out;
 
 	pagevec_init(&pvec);