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Re: [PATCH] block/rbd: fix write zeroes with growing images

From: Hanna Reitz
Subject: Re: [PATCH] block/rbd: fix write zeroes with growing images
Date: Tue, 22 Mar 2022 10:38:44 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 
On 21.03.22 09:31, Stefano Garzarella wrote:

On Sat, Mar 19, 2022 at 04:15:33PM +0100, Peter Lieven wrote:
Am 18.03.2022 um 17:47 schrieb Stefano Garzarella <sgarzare@redhat.com>: 

On Fri, Mar 18, 2022 at 04:48:18PM +0100, Peter Lieven wrote:
Am 18.03.2022 um 09:25 schrieb Stefano Garzarella <sgarzare@redhat.com>: 

On Thu, Mar 17, 2022 at 07:27:05PM +0100, Peter Lieven wrote:
Am 17.03.2022 um 17:26 schrieb Stefano Garzarella <sgarzare@redhat.com>:
Commit d24f80234b ("block/rbd: increase dynamically the image size") 
added a workaround to support growing images (eg. qcow2), resizing
the image before write operations that exceed the current size.

We recently added support for write zeroes and without the
workaround we can have problems with qcow2.

So let's move the resize into qemu_rbd_start_co() and do it when
the command is RBD_AIO_WRITE or RBD_AIO_WRITE_ZEROES.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2020993
Fixes: c56ac27d2a ("block/rbd: add write zeroes support")
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
block/rbd.c | 26 ++++++++++++++------------
1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/block/rbd.c b/block/rbd.c
index 8f183eba2a..6caf35cbba 100644
--- a/block/rbd.c
+++ b/block/rbd.c
@@ -1107,6 +1107,20 @@ static int coroutine_fn qemu_rbd_start_co(BlockDriverState *bs, 

  assert(!qiov || qiov->size == bytes);

+    if (cmd == RBD_AIO_WRITE || cmd == RBD_AIO_WRITE_ZEROES) {
+        /*
+         * RBD APIs don't allow us to write more than actual size, so in order +         * to support growing images, we resize the image before write 
+         * operations that exceed the current size.
+         */
+        if (offset + bytes > s->image_size) {
+            int r = qemu_rbd_resize(bs, offset + bytes);
+            if (r < 0) {
+                return r;
+            }
+        }
+    }
+
  r = rbd_aio_create_completion(&task,
                                (rbd_callback_t) qemu_rbd_completion_cb, &c); 
  if (r < 0) {
@@ -1182,18 +1196,6 @@ coroutine_fn qemu_rbd_co_pwritev(BlockDriverState *bs, int64_t offset, 
                               int64_t bytes, QEMUIOVector *qiov,
                               BdrvRequestFlags flags)
{
-    BDRVRBDState *s = bs->opaque;
-    /*
-     * RBD APIs don't allow us to write more than actual size, so in order 
-     * to support growing images, we resize the image before write
-     * operations that exceed the current size.
-     */
-    if (offset + bytes > s->image_size) {
-        int r = qemu_rbd_resize(bs, offset + bytes);
-        if (r < 0) {
-            return r;
-        }
-    }
  return qemu_rbd_start_co(bs, offset, bytes, qiov, flags, RBD_AIO_WRITE); 
}

--
2.35.1



Do we really have a use case for growing rbd images?


The use case is to have a qcow2 image on rbd.
I don't think it's very common, but some people use it and here [1] we had a little discussion about features that could be interesting (e.g.  persistent dirty bitmaps for incremental backup).
In any case the support is quite simple and does not affect other use cases since we only increase the size when we go beyond the current size. 

IMHO we can have it in :-)
The QCOW2 alone doesn’t make much sense, but additional metadata might be a use case. 

Yep.
Be aware that the current approach will serialize requests. If there is a real use case, we might think of a better solution.
Good point, but it only happens when we have to resize, so maybe it's okay for now, but I agree we could do better ;-)
There might also be a problem if a write for a higher offset past eof will be executed shortly before a write to a slightly lower offset past eof. The second resize will fail as it would shrink the image. We would need proper locking to avoid this. Maybe we need to check if we write past eof. If yes, take a lock around the resize op and then check again if it’s still eof and only resize if true.
I thought rbd_resize() was synchronous. Indeed when you said this could serialize writes it sounded like confirmation to me.
Since we call rbd_resize() before rbd_aio_writev(), I thought this case could not occur. 

Can you please elaborate?
Seconding this request, because if rbd_resize() is allowed to shrink data, it being asynchronous might cause data corruption.
I’ll keep your patch because I find this highly unlikely, though: qemu_rbd_resize() itself is definitely synchronous, it can’t invoke qemu_coroutine_yield().
The only other possibility that comes to my mind is that rbd_resize() might delay the actual resize operation, but I would still expect consecutive resize requests to be executed in order, and since we call rbd_aio_writev()/rbd_aio_write_zeroes() immediately after the rbd_resize() (with no yielding in between), everything should be executed in the order that we expect. 

Hanna
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