[Devel] [PATCH hci-8.1 v5 1/3] dm-qcow2: add qcow2_md_page_read_sync() helper

[Alexander Ivanov <alexander.ivanov@virtuozzo.com>]

Add a helper, which returns an up to date md page from md cache,
or allocates a new one, inserts it and reads it synchronously.
On read failure the page is erased from cache and freed.

Use it in qcow2_parse_header() instead of open-coded allocation
and read of page 0. It will also be used by the next patch.

https://virtuozzo.atlassian.net/browse/VSTOR-148171
Feature: dm-qcow2: block device over QCOW2 files driver

Signed-off-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>
Reviewed-by: Denis V. Lunev <den@openvz.org>
---
 drivers/md/dm-qcow2-target.c | 41 +++++++++++++++++++++++++++++++-----
 drivers/md/dm-qcow2.h        |  2 ++
 2 files changed, 38 insertions(+), 5 deletions(-)

diff --git a/drivers/md/dm-qcow2-target.c b/drivers/md/dm-qcow2-target.c
index be09615eb8e2..311a23d63a5c 100644
--- a/drivers/md/dm-qcow2-target.c
+++ b/drivers/md/dm-qcow2-target.c
@@ -418,6 +418,41 @@ int qcow2_alloc_and_insert_md_page(struct qcow2 *qcow2, u64 index, struct md_pag
 }
 ALLOW_ERROR_INJECTION(qcow2_alloc_and_insert_md_page, ERRNO);
 
+/*
+ * Returns up to date md page from cache, or allocates
+ * a new one and reads it synchronously.
+ *
+ * The caller must ensure no read of this page is in flight
+ * (e.g., the device is suspended): if the page is cached but
+ * not MD_UPTODATE yet, qcow2_md_page_find_or_postpone() returns
+ * NULL and the following insert fails.
+ */
+int qcow2_md_page_read_sync(struct qcow2 *qcow2, u64 id, struct md_page **md)
+{
+	int ret;
+
+	spin_lock_irq(&qcow2->md_pages_lock);
+	*md = qcow2_md_page_find_or_postpone(qcow2, id, NULL);
+	spin_unlock_irq(&qcow2->md_pages_lock);
+	if (*md)
+		return 0;
+
+	ret = qcow2_alloc_and_insert_md_page(qcow2, id, md);
+	if (ret)
+		return ret;
+
+	ret = qcow2_rw_page_sync(READ, qcow2, id, (*md)->page);
+	spin_lock_irq(&qcow2->md_pages_lock);
+	if (ret)
+		qcow2_md_page_erase(qcow2, *md);
+	else
+		(*md)->status |= MD_UPTODATE;
+	spin_unlock_irq(&qcow2->md_pages_lock);
+	if (ret)
+		qcow2_free_md_page(*md);
+	return ret;
+}
+
 static void inflight_ref_exit0(struct percpu_ref *ref)
 {
 	struct qcow2_target *tgt = container_of(ref, struct qcow2_target,
@@ -851,13 +886,9 @@ static int qcow2_parse_header(struct dm_target *ti, struct qcow2 *qcow2,
 	}
 	qcow2->file_preallocated_area_start = qcow2->file_size;
 
-	ret = qcow2_alloc_and_insert_md_page(qcow2, 0, &md);
-	if (ret)
-		return ret;
-	ret = qcow2_rw_page_sync(READ, qcow2, md->id, md->page);
+	ret = qcow2_md_page_read_sync(qcow2, 0, &md);
 	if (ret)
 		return ret;
-	md->status |= MD_UPTODATE;
 
 	raw_hdr = kmap(md->page);
 	min_len = to_bytes(ti->len);
diff --git a/drivers/md/dm-qcow2.h b/drivers/md/dm-qcow2.h
index d9b8c38e093f..69d3ae6fca8a 100644
--- a/drivers/md/dm-qcow2.h
+++ b/drivers/md/dm-qcow2.h
@@ -386,6 +386,8 @@ void do_qcow2_work(struct work_struct *ws);
 void do_qcow2_fsync_work(struct work_struct *ws);
 int qcow2_alloc_and_insert_md_page(struct qcow2 *qcow2, u64 index,
 					struct md_page **md);
+int qcow2_md_page_read_sync(struct qcow2 *qcow2, u64 id,
+			    struct md_page **md);
 struct md_page *qcow2_md_page_find_or_postpone(struct qcow2 *qcow2,
 					       u64 id, struct qio **qio);
 struct md_page *qcow2_md_page_renumber(struct qcow2 *qcow2, u64 id,
-- 
2.53.0

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