Re: [Qemu-devel] [RFC V3 6/7] block: Create authorizations mechanism for external snapshots.

["Benoît Canet" <benoit.canet@irqsave.net>]

Le Wednesday 04 Dec 2013 à 11:47:22 (+0800), Fam Zheng a écrit :
> On 2013年12月03日 21:26, Benoît Canet wrote:
> >---
> >  block.c                   | 64 +++++++++++++++++++++++++++++++++++++++++------
> >  block/blkverify.c         |  2 +-
> >  include/block/block.h     | 16 +++++++++---
> >  include/block/block_int.h |  9 ++++---
> >  4 files changed, 75 insertions(+), 16 deletions(-)
> >
> >diff --git a/block.c b/block.c
> >index 8016ff2..0569cb2 100644
> >--- a/block.c
> >+++ b/block.c
> >@@ -4945,21 +4945,69 @@ int bdrv_amend_options(BlockDriverState *bs, QEMUOptionParameter *options)
> >      return bs->drv->bdrv_amend_options(bs, options);
> >  }
> >
> >-ExtSnapshotPerm bdrv_check_ext_snapshot(BlockDriverState *bs)
> >+/* will be used to recurse on single child block filter until first format
> >+ * (single child block filter will store their child in bs->file)
> >+ */
> >+ExtSnapshotPerm bdrv_generic_check_ext_snapshot(BlockDriverState *bs,
> >+                                                BlockDriverState *candidate)
> >  {
> >-    if (bs->drv->bdrv_check_ext_snapshot) {
> >-        return bs->drv->bdrv_check_ext_snapshot(bs);
> >+    if (!bs->drv) {
> >+        return EXT_SNAPSHOT_FORBIDDEN;
> >      }
> >
> >-    if (bs->file && bs->file->drv && bs->file->drv->bdrv_check_ext_snapshot) {
> >-        return bs->file->drv->bdrv_check_ext_snapshot(bs);
> >+    if (!bs->drv->authorizations[BS_CANT_SNAPSHOT]) {
> 
> This double negative feels hard to read for me.
> 
> >+        if (bs == candidate) {
> >+             return EXT_SNAPSHOT_ALLOWED;
> >+        } else {
> >+             return EXT_SNAPSHOT_FORBIDDEN;
> >+        }
> >      }
> >
> >-    /* external snapshots are allowed by default */
> >-    return EXT_SNAPSHOT_ALLOWED;
> >+    if (!bs->drv->authorizations[BS_FILTER_PASS_DOWN]) {
> >+        return EXT_SNAPSHOT_FORBIDDEN;
> >+    }
> >+
> >+    if (!bs->file) {
> >+        return EXT_SNAPSHOT_FORBIDDEN;
> >+    }
> >+
> >+    return bdrv_recurse_check_ext_snapshot(bs->file, candidate);
> >  }
> >
> >-ExtSnapshotPerm bdrv_check_ext_snapshot_forbidden(BlockDriverState *bs)
> >+ExtSnapshotPerm bdrv_recurse_check_ext_snapshot(BlockDriverState *bs,
> >+                                                BlockDriverState *candidate)
> >  {
> >+    if (bs->drv && bs->drv->bdrv_check_ext_snapshot) {
> >+        return bs->drv->bdrv_check_ext_snapshot(bs, candidate);
> >+    }
> 
> Maybe I'm missing something, but if a driver always returns positive
> permit, despite of what candidate is (or even it's relevant to bs),
> then doesn't it also affect other devices? because...
> 
> >+
> >+    return bdrv_generic_check_ext_snapshot(bs, candidate);
> >+}
> >+
> >+/* This function check if the candidate bs has snapshots authorized by going
> >+ * down the forest of bs, skipping filters and stopping on the the first bses
> >+ * authorizing snapshots
> >+ */
> >+ExtSnapshotPerm bdrv_check_ext_snapshot(BlockDriverState *candidate)
> >+{
> >+    BlockDriverState *bs;
> >+
> >+    /* walk down the bs forest recursively */
> >+    QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
> 
> this iterates through all the known graph trees (device_list),
> instead of limiting to only the device that candidate belongs to.

The recursion termination success is candidate == bs.
This make sure that the scan of the other tree of the forest will not return any
spurious success.

> 
> Why not just check candidate's permission bitmap and go down from
> it? If an ancestor need to disable its descendants, it could simply
> set permission bits of its children and recurse down.

Yes I initially though about updating permission bit of each bs.
The problem is that the graph will evolve (taking a snapshot, adding throttling
filter) and it would be a mess to constansly update the permission bits.

Best regards

Benoît

> 
> Fam