[PATCH v2 12/16] iomap: add bias for async read requests

[Joanne Koong <joannelkoong@gmail.com>]

Non-block-based filesystems will be using iomap read/readahead. If they
handle reading in ranges asynchronously and fulfill those read requests
on an ongoing basis (instead of all together at the end), then there is
the possibility that the read on the folio may be prematurely ended if
earlier async requests complete before the later ones have been issued.

For example if there is a large folio and a readahead request for 16
pages in that folio, if doing readahead on those 16 pages is split into
4 async requests and the first request is sent off and then completed
before we have sent off the second request, then when the first request
calls iomap_finish_folio_read(), ifs->read_bytes_pending would be 0,
which would end the read and unlock the folio prematurely.

To mitigate this, a "bias" is added to ifs->read_bytes_pending before
the first range is forwarded to the caller and removed after the last
range has been forwarded.

iomap writeback does this with their async requests as well to prevent
prematurely ending writeback.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 fs/iomap/buffered-io.c | 43 ++++++++++++++++++++++++++++++++----------
 1 file changed, 33 insertions(+), 10 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 6fafe3b30563..f673e03f4ffb 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -329,8 +329,8 @@ void iomap_start_folio_read(struct folio *folio, size_t len)
 }
 EXPORT_SYMBOL_GPL(iomap_start_folio_read);
 
-void iomap_finish_folio_read(struct folio *folio, size_t off, size_t len,
-		int error)
+static void __iomap_finish_folio_read(struct folio *folio, size_t off,
+		size_t len, int error, bool update_bitmap)
 {
 	struct iomap_folio_state *ifs = folio->private;
 	bool uptodate = !error;
@@ -340,7 +340,7 @@ void iomap_finish_folio_read(struct folio *folio, size_t off, size_t len,
 		unsigned long flags;
 
 		spin_lock_irqsave(&ifs->state_lock, flags);
-		if (!error)
+		if (!error && update_bitmap)
 			uptodate = ifs_set_range_uptodate(folio, ifs, off, len);
 		ifs->read_bytes_pending -= len;
 		finished = !ifs->read_bytes_pending;
@@ -350,6 +350,12 @@ void iomap_finish_folio_read(struct folio *folio, size_t off, size_t len,
 	if (finished)
 		folio_end_read(folio, uptodate);
 }
+
+void iomap_finish_folio_read(struct folio *folio, size_t off, size_t len,
+		int error)
+{
+	return __iomap_finish_folio_read(folio, off, len, error, true);
+}
 EXPORT_SYMBOL_GPL(iomap_finish_folio_read);
 
 #ifdef CONFIG_BLOCK
@@ -434,9 +440,10 @@ static int iomap_read_folio_iter(struct iomap_iter *iter,
 	loff_t pos = iter->pos;
 	loff_t length = iomap_length(iter);
 	struct folio *folio = ctx->cur_folio;
+	struct iomap_folio_state *ifs;
 	size_t poff, plen;
 	loff_t count;
-	int ret;
+	int ret = 0;
 
 	if (iomap->type == IOMAP_INLINE) {
 		ret = iomap_read_inline_data(iter, folio);
@@ -446,7 +453,14 @@ static int iomap_read_folio_iter(struct iomap_iter *iter,
 	}
 
 	/* zero post-eof blocks as the page may be mapped */
-	ifs_alloc(iter->inode, folio, iter->flags);
+	ifs = ifs_alloc(iter->inode, folio, iter->flags);
+
+	/*
+	 * Add a bias to ifs->read_bytes_pending so that a read is ended only
+	 * after all the ranges have been read in.
+	 */
+	if (ifs)
+		iomap_start_folio_read(folio, 1);
 
 	length = min_t(loff_t, length,
 			folio_size(folio) - offset_in_folio(folio, pos));
@@ -454,8 +468,10 @@ static int iomap_read_folio_iter(struct iomap_iter *iter,
 		iomap_adjust_read_range(iter->inode, folio, &pos,
 				length, &poff, &plen);
 		count = pos - iter->pos + plen;
-		if (plen == 0)
-			return iomap_iter_advance(iter, &count);
+		if (plen == 0) {
+			ret = iomap_iter_advance(iter, &count);
+			break;
+		}
 
 		if (iomap_block_needs_zeroing(iter, pos)) {
 			folio_zero_range(folio, poff, plen);
@@ -465,16 +481,23 @@ static int iomap_read_folio_iter(struct iomap_iter *iter,
 			ret = ctx->ops->read_folio_range(iter, ctx, pos,
 						plen);
 			if (ret)
-				return ret;
+				break;
 		}
 
 		length -= count;
 		ret = iomap_iter_advance(iter, &count);
 		if (ret)
-			return ret;
+			break;
 		pos = iter->pos;
 	}
-	return 0;
+
+	if (ifs) {
+		__iomap_finish_folio_read(folio, 0, 1, ret, false);
+		/* __iomap_finish_folio_read takes care of any unlocking */
+		*cur_folio_owned = true;
+	}
+
+	return ret;
 }
 
 int iomap_read_folio(const struct iomap_ops *ops,
-- 
2.47.3