[Devel] [PATCH hci-8.1 v11 01/10] 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 a later patch in the series.

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>
Reviewed-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
---
 drivers/md/dm-qcow2-target.c | 48 ++++++++++++++++++++++++++++++++----
 drivers/md/dm-qcow2.h        |  2 ++
 2 files changed, 45 insertions(+), 5 deletions(-)

diff --git a/drivers/md/dm-qcow2-target.c b/drivers/md/dm-qcow2-target.c
index 7c572bd66e1c..0cbbd8e4f493 100644
--- a/drivers/md/dm-qcow2-target.c
+++ b/drivers/md/dm-qcow2-target.c
@@ -418,6 +418,48 @@ 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. Sets *md to NULL on error.
+ *
+ * The caller must ensure nobody else looks this page up until this
+ * returns (e.g., the device is suspended). A lookup before MD_UPTODATE
+ * is set queues its qio on md->wait_list, which is never woken here,
+ * and the page is freed with the qio still queued on read error.
+ * A cached page that is not MD_UPTODATE yet makes the insert fail.
+ */
+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)
+		goto err;
+
+	ret = qcow2_rw_page_sync(READ, qcow2, id, (*md)->page);
+	if (ret) {
+		spin_lock_irq(&qcow2->md_pages_lock);
+		qcow2_md_page_erase(qcow2, *md);
+		spin_unlock_irq(&qcow2->md_pages_lock);
+		qcow2_free_md_page(*md);
+		goto err;
+	}
+
+	spin_lock_irq(&qcow2->md_pages_lock);
+	(*md)->status |= MD_UPTODATE;
+	spin_unlock_irq(&qcow2->md_pages_lock);
+	return 0;
+err:
+	*md = NULL;
+	return ret;
+}
+
 static void inflight_ref_exit0(struct percpu_ref *ref)
 {
 	struct qcow2_target *tgt = container_of(ref, struct qcow2_target,
@@ -851,13 +893,9 @@ 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 ad5286b16883..5abc7f5ad239 100644
--- a/drivers/md/dm-qcow2.h
+++ b/drivers/md/dm-qcow2.h
@@ -388,6 +388,8 @@ 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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