[PATCH v4 1/5] block/io: fix bdrv_co_block_status_above - Vladimir Sementsov-Ogievskiy

From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
To: qemu-block@nongnu.org
Cc: kwolf@redhat.com, fam@euphon.net, vsementsov@virtuozzo.com,
	qemu-devel@nongnu.org, mreitz@redhat.com, stefanha@redhat.com,
	den@openvz.org
Subject: [PATCH v4 1/5] block/io: fix bdrv_co_block_status_above
Date: Thu, 28 May 2020 13:15:03 +0300	[thread overview]
Message-ID: <20200528101507.151161-2-vsementsov@virtuozzo.com> (raw)
In-Reply-To: <20200528101507.151161-1-vsementsov@virtuozzo.com>

bdrv_co_block_status_above has several design problems with handling
short backing files:

1. With want_zeros=true, it may return ret with BDRV_BLOCK_ZERO but
without BDRV_BLOCK_ALLOCATED flag, when actually short backing file
which produces these after-EOF zeros is inside requested backing
sequence.

2. With want_zero=false, it may return pnum=0 prior to actual EOF,
because of EOF of short backing file.

Fix these things, making logic about short backing files clearer.

With fixed bdrv_block_status_above we also have to improve is_zero in
qcow2 code, otherwise iotest 154 will fail, because with this patch we
stop to merge zeros of different types (produced by fully unallocated
in the whole backing chain regions vs produced by short backing files).

Note also, that this patch leaves for another day the general problem
around block-status: misuse of BDRV_BLOCK_ALLOCATED as is-fs-allocated
vs go-to-backing.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 block/io.c    | 39 ++++++++++++++++++++++++++++++---------
 block/qcow2.c | 16 ++++++++++++++--
 2 files changed, 44 insertions(+), 11 deletions(-)

diff --git a/block/io.c b/block/io.c
index 121ce17a49..412b91b08f 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2461,25 +2461,46 @@ static int coroutine_fn bdrv_co_block_status_above(BlockDriverState *bs,
         ret = bdrv_co_block_status(p, want_zero, offset, bytes, pnum, map,
                                    file);
         if (ret < 0) {
-            break;
+            return ret;
         }
-        if (ret & BDRV_BLOCK_ZERO && ret & BDRV_BLOCK_EOF && !first) {
+        if (*pnum == 0) {
+            if (first) {
+                return ret;
+            }
+
             /*
-             * Reading beyond the end of the file continues to read
-             * zeroes, but we can only widen the result to the
-             * unallocated length we learned from an earlier
-             * iteration.
+             * The top layer deferred to this layer, and because this layer is
+             * short, any zeroes that we synthesize beyond EOF behave as if they
+             * were allocated at this layer
              */
+            assert(ret & BDRV_BLOCK_EOF);
             *pnum = bytes;
+            if (file) {
+                *file = p;
+            }
+            return BDRV_BLOCK_ZERO | BDRV_BLOCK_ALLOCATED;
         }
-        if (ret & (BDRV_BLOCK_ZERO | BDRV_BLOCK_DATA)) {
-            break;
+        if (ret & BDRV_BLOCK_ALLOCATED) {
+            /* We've found the node and the status, we must return. */
+
+            if (ret & BDRV_BLOCK_ZERO && ret & BDRV_BLOCK_EOF && !first) {
+                /*
+                 * This level is also responsible for reads after EOF inside
+                 * the unallocated region in the previous level.
+                 */
+                *pnum = bytes;
+            }
+
+            return ret;
         }
+
         /* [offset, pnum] unallocated on this layer, which could be only
          * the first part of [offset, bytes].  */
-        bytes = MIN(bytes, *pnum);
+        assert(*pnum <= bytes);
+        bytes = *pnum;
         first = false;
     }
+
     return ret;
 }
 
diff --git a/block/qcow2.c b/block/qcow2.c
index dfab8d2f6c..85e49776aa 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -3827,8 +3827,20 @@ static bool is_zero(BlockDriverState *bs, int64_t offset, int64_t bytes)
     if (!bytes) {
         return true;
     }
-    res = bdrv_block_status_above(bs, NULL, offset, bytes, &nr, NULL, NULL);
-    return res >= 0 && (res & BDRV_BLOCK_ZERO) && nr == bytes;
+
+    /*
+     * bdrv_block_status_above doesn't merge different types of zeros, for
+     * example, zeros which come from the region which is unallocated in
+     * the whole backing chain, and zeros which comes because of a short
+     * backing file. So, we need a loop.
+     */
+    do {
+        res = bdrv_block_status_above(bs, NULL, offset, bytes, &nr, NULL, NULL);
+        offset += nr;
+        bytes -= nr;
+    } while (res >= 0 && (res & BDRV_BLOCK_ZERO) && nr && bytes);
+
+    return res >= 0 && (res & BDRV_BLOCK_ZERO) && bytes == 0;
 }
 
 static coroutine_fn int qcow2_co_pwrite_zeroes(BlockDriverState *bs,
-- 
2.18.0


