iomap: Add per-block dirty state tracking to improve performance

When filesystem blocksize is less than folio size (either with
mapping_large_folio_support() or with blocksize < pagesize) and when the
folio is uptodate in pagecache, then even a byte write can cause
an entire folio to be written to disk during writeback. This happens
because we currently don't have a mechanism to track per-block dirty
state within struct iomap_folio_state. We currently only track uptodate
state.

This patch implements support for tracking per-block dirty state in
iomap_folio_state->state bitmap. This should help improve the filesystem
write performance and help reduce write amplification.

Performance testing of below fio workload reveals ~16x performance
improvement using nvme with XFS (4k blocksize) on Power (64K pagesize)
FIO reported write bw scores improved from around ~28 MBps to ~452 MBps.

1. <test_randwrite.fio>
[global]
	ioengine=psync
	rw=randwrite
	overwrite=1
	pre_read=1
	direct=0
	bs=4k
	size=1G
	dir=./
	numjobs=8
	fdatasync=1
	runtime=60
	iodepth=64
	group_reporting=1

[fio-run]

2. Also our internal performance team reported that this patch improves
   their database workload performance by around ~83% (with XFS on Power)

Reported-by: Aravinda Herle <araherle@in.ibm.com>
Reported-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

1 files changed, 150 insertions, 11 deletions

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 1ec4ef44851f..283fb96f6609 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -25,13 +25,19 @@
 
 typedef int (*iomap_punch_t)(struct inode *inode, loff_t offset, loff_t length);
 /*
- * Structure allocated for each folio to track per-block uptodate state
+ * Structure allocated for each folio to track per-block uptodate, dirty state
  * and I/O completions.
  */
 struct iomap_folio_state {
 	atomic_t		read_bytes_pending;
 	atomic_t		write_bytes_pending;
 	spinlock_t		state_lock;
+
+	/*
+	 * Each block has two bits in this bitmap:
+	 * Bits [0..blocks_per_folio) has the uptodate status.
+	 * Bits [b_p_f...(2*b_p_f))   has the dirty status.
+	 */
 	unsigned long		state[];
 };
 
@@ -78,6 +84,61 @@ static void iomap_set_range_uptodate(struct folio *folio, size_t off,
 		folio_mark_uptodate(folio);
 }
 
+static inline bool ifs_block_is_dirty(struct folio *folio,
+		struct iomap_folio_state *ifs, int block)
+{
+	struct inode *inode = folio->mapping->host;
+	unsigned int blks_per_folio = i_blocks_per_folio(inode, folio);
+
+	return test_bit(block + blks_per_folio, ifs->state);
+}
+
+static void ifs_clear_range_dirty(struct folio *folio,
+		struct iomap_folio_state *ifs, size_t off, size_t len)
+{
+	struct inode *inode = folio->mapping->host;
+	unsigned int blks_per_folio = i_blocks_per_folio(inode, folio);
+	unsigned int first_blk = (off >> inode->i_blkbits);
+	unsigned int last_blk = (off + len - 1) >> inode->i_blkbits;
+	unsigned int nr_blks = last_blk - first_blk + 1;
+	unsigned long flags;
+
+	spin_lock_irqsave(&ifs->state_lock, flags);
+	bitmap_clear(ifs->state, first_blk + blks_per_folio, nr_blks);
+	spin_unlock_irqrestore(&ifs->state_lock, flags);
+}
+
+static void iomap_clear_range_dirty(struct folio *folio, size_t off, size_t len)
+{
+	struct iomap_folio_state *ifs = folio->private;
+
+	if (ifs)
+		ifs_clear_range_dirty(folio, ifs, off, len);
+}
+
+static void ifs_set_range_dirty(struct folio *folio,
+		struct iomap_folio_state *ifs, size_t off, size_t len)
+{
+	struct inode *inode = folio->mapping->host;
+	unsigned int blks_per_folio = i_blocks_per_folio(inode, folio);
+	unsigned int first_blk = (off >> inode->i_blkbits);
+	unsigned int last_blk = (off + len - 1) >> inode->i_blkbits;
+	unsigned int nr_blks = last_blk - first_blk + 1;
+	unsigned long flags;
+
+	spin_lock_irqsave(&ifs->state_lock, flags);
+	bitmap_set(ifs->state, first_blk + blks_per_folio, nr_blks);
+	spin_unlock_irqrestore(&ifs->state_lock, flags);
+}
+
+static void iomap_set_range_dirty(struct folio *folio, size_t off, size_t len)
+{
+	struct iomap_folio_state *ifs = folio->private;
+
+	if (ifs)
+		ifs_set_range_dirty(folio, ifs, off, len);
+}
+
 static struct iomap_folio_state *ifs_alloc(struct inode *inode,
 		struct folio *folio, unsigned int flags)
 {
@@ -93,14 +154,24 @@ static struct iomap_folio_state *ifs_alloc(struct inode *inode,
 	else
 		gfp = GFP_NOFS | __GFP_NOFAIL;
 
-	ifs = kzalloc(struct_size(ifs, state, BITS_TO_LONGS(nr_blocks)),
-		      gfp);
-	if (ifs) {
-		spin_lock_init(&ifs->state_lock);
-		if (folio_test_uptodate(folio))
-			bitmap_fill(ifs->state, nr_blocks);
-		folio_attach_private(folio, ifs);
-	}
+	/*
+	 * ifs->state tracks two sets of state flags when the
+	 * filesystem block size is smaller than the folio size.
+	 * The first state tracks per-block uptodate and the
+	 * second tracks per-block dirty state.
+	 */
+	ifs = kzalloc(struct_size(ifs, state,
+		      BITS_TO_LONGS(2 * nr_blocks)), gfp);
+	if (!ifs)
+		return ifs;
+
+	spin_lock_init(&ifs->state_lock);
+	if (folio_test_uptodate(folio))
+		bitmap_set(ifs->state, 0, nr_blocks);
+	if (folio_test_dirty(folio))
+		bitmap_set(ifs->state, nr_blocks, nr_blocks);
+	folio_attach_private(folio, ifs);
+
 	return ifs;
 }
 
@@ -519,6 +590,17 @@ void iomap_invalidate_folio(struct folio *folio, size_t offset, size_t len)
 }
 EXPORT_SYMBOL_GPL(iomap_invalidate_folio);
 
+bool iomap_dirty_folio(struct address_space *mapping, struct folio *folio)
+{
+	struct inode *inode = mapping->host;
+	size_t len = folio_size(folio);
+
+	ifs_alloc(inode, folio, 0);
+	iomap_set_range_dirty(folio, 0, len);
+	return filemap_dirty_folio(mapping, folio);
+}
+EXPORT_SYMBOL_GPL(iomap_dirty_folio);
+
 static void
 iomap_write_failed(struct inode *inode, loff_t pos, unsigned len)
 {
@@ -723,6 +805,7 @@ static size_t __iomap_write_end(struct inode *inode, loff_t pos, size_t len,
 	if (unlikely(copied < len && !folio_test_uptodate(folio)))
 		return 0;
 	iomap_set_range_uptodate(folio, offset_in_folio(folio, pos), len);
+	iomap_set_range_dirty(folio, offset_in_folio(folio, pos), copied);
 	filemap_dirty_folio(inode->i_mapping, folio);
 	return copied;
 }
@@ -892,6 +975,43 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i,
 }
 EXPORT_SYMBOL_GPL(iomap_file_buffered_write);
 
+static int iomap_write_delalloc_ifs_punch(struct inode *inode,
+		struct folio *folio, loff_t start_byte, loff_t end_byte,
+		iomap_punch_t punch)
+{
+	unsigned int first_blk, last_blk, i;
+	loff_t last_byte;
+	u8 blkbits = inode->i_blkbits;
+	struct iomap_folio_state *ifs;
+	int ret = 0;
+
+	/*
+	 * When we have per-block dirty tracking, there can be
+	 * blocks within a folio which are marked uptodate
+	 * but not dirty. In that case it is necessary to punch
+	 * out such blocks to avoid leaking any delalloc blocks.
+	 */
+	ifs = folio->private;
+	if (!ifs)
+		return ret;
+
+	last_byte = min_t(loff_t, end_byte - 1,
+			folio_pos(folio) + folio_size(folio) - 1);
+	first_blk = offset_in_folio(folio, start_byte) >> blkbits;
+	last_blk = offset_in_folio(folio, last_byte) >> blkbits;
+	for (i = first_blk; i <= last_blk; i++) {
+		if (!ifs_block_is_dirty(folio, ifs, i)) {
+			ret = punch(inode, folio_pos(folio) + (i << blkbits),
+				    1 << blkbits);
+			if (ret)
+				return ret;
+		}
+	}
+
+	return ret;
+}
+
+
 static int iomap_write_delalloc_punch(struct inode *inode, struct folio *folio,
 		loff_t *punch_start_byte, loff_t start_byte, loff_t end_byte,
 		iomap_punch_t punch)
@@ -909,6 +1029,12 @@ static int iomap_write_delalloc_punch(struct inode *inode, struct folio *folio,
 			return ret;
 	}
 
+	/* Punch non-dirty blocks within folio */
+	ret = iomap_write_delalloc_ifs_punch(inode, folio, start_byte,
+			end_byte, punch);
+	if (ret)
+		return ret;
+
 	/*
 	 * Make sure the next punch start is correctly bound to
 	 * the end of this data range, not the end of the folio.
@@ -1639,7 +1765,7 @@ iomap_writepage_map(struct iomap_writepage_ctx *wpc,
 		struct writeback_control *wbc, struct inode *inode,
 		struct folio *folio, u64 end_pos)
 {
-	struct iomap_folio_state *ifs = ifs_alloc(inode, folio, 0);
+	struct iomap_folio_state *ifs = folio->private;
 	struct iomap_ioend *ioend, *next;
 	unsigned len = i_blocksize(inode);
 	unsigned nblocks = i_blocks_per_folio(inode, folio);
@@ -1647,6 +1773,13 @@ iomap_writepage_map(struct iomap_writepage_ctx *wpc,
 	int error = 0, count = 0, i;
 	LIST_HEAD(submit_list);
 
+	WARN_ON_ONCE(end_pos <= pos);
+
+	if (!ifs && nblocks > 1) {
+		ifs = ifs_alloc(inode, folio, 0);
+		iomap_set_range_dirty(folio, 0, end_pos - pos);
+	}
+
 	WARN_ON_ONCE(ifs && atomic_read(&ifs->write_bytes_pending) != 0);
 
 	/*
@@ -1655,7 +1788,7 @@ iomap_writepage_map(struct iomap_writepage_ctx *wpc,
 	 * invalid, grab a new one.
 	 */
 	for (i = 0; i < nblocks && pos < end_pos; i++, pos += len) {
-		if (ifs && !ifs_block_is_uptodate(ifs, i))
+		if (ifs && !ifs_block_is_dirty(folio, ifs, i))
 			continue;
 
 		error = wpc->ops->map_blocks(wpc, inode, pos);
@@ -1699,6 +1832,12 @@ iomap_writepage_map(struct iomap_writepage_ctx *wpc,
 		}
 	}
 
+	/*
+	 * We can have dirty bits set past end of file in page_mkwrite path
+	 * while mapping the last partial folio. Hence it's better to clear
+	 * all the dirty bits in the folio here.
+	 */
+	iomap_clear_range_dirty(folio, 0, folio_size(folio));
 	folio_start_writeback(folio);
 	folio_unlock(folio);