[PATCH] block/mirror: inherit the source node discard flag in mirror_top

[PATCH] block/mirror: inherit the source node discard flag in mirror_top

[Haoqian He]

mirror_start_job() inserts a mirror_top filter above the source node
for both mirror and active commit jobs. Previously, the filter was
opened with BDRV_O_RDWR only, even when the source had BDRV_O_UNMAP
enabled.

The block layer checks BDRV_O_UNMAP before invoking a node's discard
callback. Without this flag on mirror_top, discard requests return
success without reaching bdrv_mirror_top_pdiscard(). Consequently,
starting a job can silently suppress guest UNMAP/TRIM requests that
would otherwise reach the discard-enabled source.

Initialize the filter's open flags with BDRV_O_RDWR and conditionally
add BDRV_O_UNMAP from bs->open_flags. Pass the resulting flags to
bdrv_new_open_driver() so that mirror_top follows the source node's
discard setting.

This allows discard requests to reach the existing filter callback,
which forwards them to the source and either tracks the affected
range in the dirty bitmap or synchronously propagates the discard
to the target, depending on the copy mode.

Signed-off-by: Haoqian He <haoqian.he@smartx.com>
---
 block/mirror.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/block/mirror.c b/block/mirror.c
index 089856f4a8..a551d392c6 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -1854,6 +1854,7 @@ static BlockJob *mirror_start_job(
     BlockDriverState *mirror_top_bs;
     bool target_is_backing;
     uint64_t target_perms, target_shared_perms;
+    int flags;
     int ret;
 
     GLOBAL_STATE_CODE();
@@ -1886,8 +1887,13 @@ static BlockJob *mirror_start_job(
     /* In the case of active commit, add dummy driver to provide consistent
      * reads on the top, while disabling it in the intermediate nodes, and make
      * the backing chain writable. */
-    mirror_top_bs = bdrv_new_open_driver(&bdrv_mirror_top, filter_node_name,
-                                         BDRV_O_RDWR, errp);
+    flags = BDRV_O_RDWR;
+    if (bs->open_flags & BDRV_O_UNMAP) {
+        flags |= BDRV_O_UNMAP;
+    }
+    mirror_top_bs = bdrv_new_open_driver(&bdrv_mirror_top,
+                                         filter_node_name,
+                                         flags, errp);
     if (mirror_top_bs == NULL) {
         return NULL;
     }
-- 
2.53.0

Re: [PATCH] block/mirror: inherit the source node discard flag in mirror_top

[Vladimir Sementsov-Ogievskiy]

On 13.10.26 09:05, Haoqian He wrote:
> mirror_start_job() inserts a mirror_top filter above the source node
> for both mirror and active commit jobs. Previously, the filter was
> opened with BDRV_O_RDWR only, even when the source had BDRV_O_UNMAP
> enabled.
> 
> The block layer checks BDRV_O_UNMAP before invoking a node's discard
> callback. Without this flag on mirror_top, discard requests return
> success without reaching bdrv_mirror_top_pdiscard(). Consequently,
> starting a job can silently suppress guest UNMAP/TRIM requests that
> would otherwise reach the discard-enabled source.
> 
> Initialize the filter's open flags with BDRV_O_RDWR and conditionally
> add BDRV_O_UNMAP from bs->open_flags. Pass the resulting flags to
> bdrv_new_open_driver() so that mirror_top follows the source node's
> discard setting.
> 
> This allows discard requests to reach the existing filter callback,
> which forwards them to the source and either tracks the affected
> range in the dirty bitmap or synchronously propagates the discard
> to the target, depending on the copy mode.
> 
> Signed-off-by: Haoqian He <haoqian.he@smartx.com>
> ---
>   block/mirror.c | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index 089856f4a8..a551d392c6 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -1854,6 +1854,7 @@ static BlockJob *mirror_start_job(
>       BlockDriverState *mirror_top_bs;
>       bool target_is_backing;
>       uint64_t target_perms, target_shared_perms;
> +    int flags;
>       int ret;
>   
>       GLOBAL_STATE_CODE();
> @@ -1886,8 +1887,13 @@ static BlockJob *mirror_start_job(
>       /* In the case of active commit, add dummy driver to provide consistent
>        * reads on the top, while disabling it in the intermediate nodes, and make
>        * the backing chain writable. */
> -    mirror_top_bs = bdrv_new_open_driver(&bdrv_mirror_top, filter_node_name,
> -                                         BDRV_O_RDWR, errp);
> +    flags = BDRV_O_RDWR;
> +    if (bs->open_flags & BDRV_O_UNMAP) {
> +        flags |= BDRV_O_UNMAP;
> +    }
> +    mirror_top_bs = bdrv_new_open_driver(&bdrv_mirror_top,
> +                                         filter_node_name,
> +                                         flags, errp);
>       if (mirror_top_bs == NULL) {
>           return NULL;
>       }

Cool find!

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>

Thanks for fixing, applied to my block branch.


-- 
Best regards,
Vladimir

Re: [PATCH] block/mirror: inherit the source node discard flag in mirror_top

[Kevin Wolf]

Am 13.10.2026 um 10:15 hat Vladimir Sementsov-Ogievskiy geschrieben:
> On 13.10.26 09:05, Haoqian He wrote:
> > mirror_start_job() inserts a mirror_top filter above the source node
> > for both mirror and active commit jobs. Previously, the filter was
> > opened with BDRV_O_RDWR only, even when the source had BDRV_O_UNMAP
> > enabled.
> > 
> > The block layer checks BDRV_O_UNMAP before invoking a node's discard
> > callback. Without this flag on mirror_top, discard requests return
> > success without reaching bdrv_mirror_top_pdiscard(). Consequently,
> > starting a job can silently suppress guest UNMAP/TRIM requests that
> > would otherwise reach the discard-enabled source.
> > 
> > Initialize the filter's open flags with BDRV_O_RDWR and conditionally
> > add BDRV_O_UNMAP from bs->open_flags. Pass the resulting flags to
> > bdrv_new_open_driver() so that mirror_top follows the source node's
> > discard setting.
> > 
> > This allows discard requests to reach the existing filter callback,
> > which forwards them to the source and either tracks the affected
> > range in the dirty bitmap or synchronously propagates the discard
> > to the target, depending on the copy mode.
> > 
> > Signed-off-by: Haoqian He <haoqian.he@smartx.com>
> > ---
> >   block/mirror.c | 10 ++++++++--
> >   1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/block/mirror.c b/block/mirror.c
> > index 089856f4a8..a551d392c6 100644
> > --- a/block/mirror.c
> > +++ b/block/mirror.c
> > @@ -1854,6 +1854,7 @@ static BlockJob *mirror_start_job(
> >       BlockDriverState *mirror_top_bs;
> >       bool target_is_backing;
> >       uint64_t target_perms, target_shared_perms;
> > +    int flags;
> >       int ret;
> >       GLOBAL_STATE_CODE();
> > @@ -1886,8 +1887,13 @@ static BlockJob *mirror_start_job(
> >       /* In the case of active commit, add dummy driver to provide consistent
> >        * reads on the top, while disabling it in the intermediate nodes, and make
> >        * the backing chain writable. */
> > -    mirror_top_bs = bdrv_new_open_driver(&bdrv_mirror_top, filter_node_name,
> > -                                         BDRV_O_RDWR, errp);
> > +    flags = BDRV_O_RDWR;
> > +    if (bs->open_flags & BDRV_O_UNMAP) {
> > +        flags |= BDRV_O_UNMAP;
> > +    }
> > +    mirror_top_bs = bdrv_new_open_driver(&bdrv_mirror_top,
> > +                                         filter_node_name,
> > +                                         flags, errp);
> >       if (mirror_top_bs == NULL) {
> >           return NULL;
> >       }
> 
> Cool find!
> 
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> 
> Thanks for fixing, applied to my block branch.

This bug is something that should be relatively easily testable in
qemu-iotests. Can we please add a test at the same time?

Kevin