[Devel] [PATCH v5 vz9 1/7] dm-ploop: truncate preallocated space on ploop destruction

[Konstantin Khorenko]

From: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>

Recently we noticed that fstrim does not work properly on ploop images.
The problem is actually worse: current code preallocates excessive space
on the image and loses it later.

Example:
  [root at vz9-demens-1 fstrim]# du -h .
  4.0K  .
  [root at vz9-demens-1 fstrim]# ploop init -s 10G image
  ...
  [root at vz9-demens-1 fstrim]# du -h .
  513M  .
  [root at vz9-demens-1 fstrim]# ploop mount -m /mnt/fstrim/ DiskDescriptor.xml
  ...
  [root at vz9-demens-1 fstrim]# dd if=/dev/urandom of=/mnt/fstrim/file bs=1M count=1024 oflag=direct
  1024+0 records in
  1024+0 records out
  1073741824 bytes (1.1 GB, 1.0 GiB) copied, 4.93694 s, 217 MB/s
  [root at vz9-demens-1 fstrim]# ploop umount DiskDescriptor.xml
  ...
  [root at vz9-demens-1 fstrim]# du -h .
  8.7G  .
  [root at vz9-demens-1 fstrim]# ploop mount -m /mnt/fstrim/ DiskDescriptor.xml
  ...
  [root at vz9-demens-1 fstrim]# rm -rf /mnt/fstrim/file
  [root at vz9-demens-1 fstrim]# fstrim /mnt/fstrim
  [root at vz9-demens-1 fstrim]# ploop umount DiskDescriptor.xml
  ...
  [root at vz9-demens-1 fstrim]# du -h .
  7.7G  .

The image just after the creation was already 512M. After writing 1GB
data it was almost 9GB in size. The fstrim discarded only known blocks, so
final size is 7.7GB.

Mainly two factors contribute to that:
 - file_preallocated_area_start is never updated
 - parallel preallocation requests scale preallocation size by 128M
   each time

To fix this, the patch changes preallocation logic a bit:

 - Update file_preallocated_area_start after allocation starts using
   preallocated space.
 - Truncate preallocated space on ploop destruction.

Following several patches also do the following:

 - Make preallocations absolute instead of relative in
   ploop_req_prealloc(). Thus only request what is really needed.
 - Rename s/ploop_pending_prealloc/ploop_no_pending_prealloc/ as it
   returns true in case of no pending preallocation.
 - Make ploop_preallocate_cluster() return error on unexpected file size
   change. Also simplify the logic a little bit (e.g. more is excess).
 - Preallocation in ploop_allocate_cluster() should not depend on (pos <
   prealloc_start), so always try to preallocate. Also simplify
   ploop_allocate_cluster() a little bit (e.g. old_size is excess).

Same example after the patchset:

[root at ptikh-hci fstrim]# du -h .
4.0K	.
[root at ptikh-hci fstrim]# ploop init -s 10G image
[root at ptikh-hci fstrim]# du -h .
3.2M	.
[root at ptikh-hci fstrim]# ploop mount -m /mnt/fstrim/ DiskDescriptor.xml
...
[root at ptikh-hci fstrim]# dd if=/dev/urandom of=/mnt/fstrim/file bs=1M count=1024 oflag=direct
...
[root at ptikh-hci fstrim]# ploop umount DiskDescriptor.xml
...
[root at ptikh-hci fstrim]# du -h .
1.1G	.
[root at ptikh-hci fstrim]# ploop mount -m /mnt/fstrim/ DiskDescriptor.xml
...
[root at ptikh-hci fstrim]# rm -rf /mnt/fstrim/file
[root at ptikh-hci fstrim]# fstrim /mnt/fstrim
[root at ptikh-hci fstrim]# ploop umount DiskDescriptor.xml
...
[root at ptikh-hci fstrim]# du -h .
4.2M	.

Now it is much better. Empty image size is quite small, write does
not cause excessive preallocation and fstrim therefore reduces the
size.

https://virtuozzo.atlassian.net/browse/VSTOR-108868
Co-developed-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>

Feature: dm-ploop: ploop target driver

---
v3: This is rework of v2 which makes it possible to apply it in
ready-kernel (no ploop structure changes). Also it covers some new
cases, like resetting the state on unexpected file size change. In v2
preemptive_prealloc was not reset and can probably lead to problems.

v4: fix compilation and incorporate v2 description

v5: splitted the patch into several small ones
---
 drivers/md/dm-ploop-map.c    | 2 ++
 drivers/md/dm-ploop-target.c | 6 ++++++
 2 files changed, 8 insertions(+)

diff --git a/drivers/md/dm-ploop-map.c b/drivers/md/dm-ploop-map.c
index 11767dd919503..79c2f4c7db033 100644
--- a/drivers/md/dm-ploop-map.c
+++ b/drivers/md/dm-ploop-map.c
@@ -1339,6 +1339,8 @@ static int ploop_allocate_cluster(struct ploop *ploop, u32 *dst_clu, struct file
 		}
 	}
 
+	if (end > top->file_preallocated_area_start)
+		top->file_preallocated_area_start = end;
 	spin_unlock_irqrestore(&ploop->bat_lock, flags);
 
 	return 0;
diff --git a/drivers/md/dm-ploop-target.c b/drivers/md/dm-ploop-target.c
index 8fa0043a71420..ea5f3a8dab6b6 100644
--- a/drivers/md/dm-ploop-target.c
+++ b/drivers/md/dm-ploop-target.c
@@ -162,6 +162,7 @@ static bool ploop_empty_htable(struct hlist_head head[])
 
 static void ploop_destroy(struct ploop *ploop)
 {
+	struct ploop_delta *top = ploop_top_delta(ploop);
 	int i;
 
 	if (ploop->kt_worker) {
@@ -201,6 +202,11 @@ static void ploop_destroy(struct ploop *ploop)
 
 	for (i = 0; i < 2; i++)
 		percpu_ref_exit(&ploop->inflight_bios_ref[i]);
+
+	if (top->file_preallocated_area_start < top->file_size &&
+	    vfs_truncate2(&top->file->f_path, top->file_preallocated_area_start, top->file))
+		PL_ERR("Unable to truncate preallocated area on destroy");
+
 	/* Nobody uses it after destroy_workqueue() */
 	while (ploop->nr_deltas-- > 0) {
 		if (ploop->deltas[ploop->nr_deltas].file) {
-- 
2.43.0