[Qemu-devel] [PATCH v5 02/23] block: Add flag to avoid wasted work in bdrv_is_allocated()

[Eric Blake <eblake@redhat.com>]

Not all callers care about which BDS owns the mapping for a given
range of the file.  In particular, bdrv_is_allocated() cares more
about finding the largest run of allocated data from the guest
perspective, whether or not that data is consecutive from the
host perspective, and whether or not the data reads as zero.
Therefore, doing subsequent refinements such as checking how much
of the format-layer allocation also satisfies BDRV_BLOCK_ZERO at
the protocol layer is wasted work - in the best case, it just
costs extra CPU cycles during a single bdrv_is_allocated(), but
in the worst case, it results in a smaller *pnum, and forces
callers to iterate through more status probes when visiting the
entire file for even more extra CPU cycles.

This patch only optimizes the block layer (no behavior change when
mapping is true, but skip unnecessary effort when it is false).
Then when subsequent patches tweak the driver callback to be
byte-based, we can also pass this hint through to the driver.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>

---
v5: tweak commit message and one comment, rebase to previous changes,
minor enough to still add R-b
v4: only context changes
v3: s/allocation/mapping/ and flip sense of bool
v2: new patch
---
 block/io.c | 52 ++++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 38 insertions(+), 14 deletions(-)

diff --git a/block/io.c b/block/io.c
index e5a6f63eea..0d8cdab583 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1754,6 +1754,7 @@ typedef struct BdrvCoGetBlockStatusData {
     int nb_sectors;
     int *pnum;
     int64_t ret;
+    bool mapping;
     bool done;
 } BdrvCoGetBlockStatusData;

@@ -1788,6 +1789,11 @@ int64_t coroutine_fn bdrv_co_get_block_status_from_backing(BlockDriverState *bs,
  * Drivers not implementing the functionality are assumed to not support
  * backing files, hence all their sectors are reported as allocated.
  *
+ * If 'mapping' is true, the caller is querying for mapping purposes,
+ * and the result should include BDRV_BLOCK_OFFSET_VALID and
+ * BDRV_BLOCK_ZERO where possible; otherwise, the result may omit those
+ * bits particularly if it allows for a larger value in 'pnum'.
+ *
  * If 'sector_num' is beyond the end of the disk image the return value is
  * BDRV_BLOCK_EOF and 'pnum' is set to 0.
  *
@@ -1804,6 +1810,7 @@ int64_t coroutine_fn bdrv_co_get_block_status_from_backing(BlockDriverState *bs,
  * is allocated in.
  */
 static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
+                                                     bool mapping,
                                                      int64_t sector_num,
                                                      int nb_sectors, int *pnum,
                                                      BlockDriverState **file)
@@ -1854,14 +1861,15 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,

     if (ret & BDRV_BLOCK_RAW) {
         assert(ret & BDRV_BLOCK_OFFSET_VALID && local_file);
-        ret = bdrv_co_get_block_status(local_file, ret >> BDRV_SECTOR_BITS,
+        ret = bdrv_co_get_block_status(local_file, mapping,
+                                       ret >> BDRV_SECTOR_BITS,
                                        local_pnum, &local_pnum, &local_file);
         goto out;
     }

     if (ret & (BDRV_BLOCK_DATA | BDRV_BLOCK_ZERO)) {
         ret |= BDRV_BLOCK_ALLOCATED;
-    } else {
+    } else if (mapping) {
         if (bdrv_unallocated_blocks_are_zero(bs)) {
             ret |= BDRV_BLOCK_ZERO;
         } else if (bs->backing) {
@@ -1873,12 +1881,13 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,
         }
     }

-    if (local_file && local_file != bs &&
+    if (mapping && local_file && local_file != bs &&
         (ret & BDRV_BLOCK_DATA) && !(ret & BDRV_BLOCK_ZERO) &&
         (ret & BDRV_BLOCK_OFFSET_VALID)) {
         int file_pnum;

-        ret2 = bdrv_co_get_block_status(local_file, ret >> BDRV_SECTOR_BITS,
+        ret2 = bdrv_co_get_block_status(local_file, mapping,
+                                        ret >> BDRV_SECTOR_BITS,
                                         local_pnum, &file_pnum, NULL);
         if (ret2 >= 0) {
             /* Ignore errors.  This is just providing extra information, it
@@ -1915,6 +1924,7 @@ static int64_t coroutine_fn bdrv_co_get_block_status(BlockDriverState *bs,

 static int64_t coroutine_fn bdrv_co_get_block_status_above(BlockDriverState *bs,
         BlockDriverState *base,
+        bool mapping,
         int64_t sector_num,
         int nb_sectors,
         int *pnum,
@@ -1926,7 +1936,8 @@ static int64_t coroutine_fn bdrv_co_get_block_status_above(BlockDriverState *bs,

     assert(bs != base);
     for (p = bs; p != base; p = backing_bs(p)) {
-        ret = bdrv_co_get_block_status(p, sector_num, nb_sectors, pnum, file);
+        ret = bdrv_co_get_block_status(p, mapping, sector_num, nb_sectors,
+                                       pnum, file);
         if (ret < 0) {
             break;
         }
@@ -1956,6 +1967,7 @@ static void coroutine_fn bdrv_get_block_status_above_co_entry(void *opaque)
     BdrvCoGetBlockStatusData *data = opaque;

     data->ret = bdrv_co_get_block_status_above(data->bs, data->base,
+                                               data->mapping,
                                                data->sector_num,
                                                data->nb_sectors,
                                                data->pnum,
@@ -1968,11 +1980,12 @@ static void coroutine_fn bdrv_get_block_status_above_co_entry(void *opaque)
  *
  * See bdrv_co_get_block_status_above() for details.
  */
-int64_t bdrv_get_block_status_above(BlockDriverState *bs,
-                                    BlockDriverState *base,
-                                    int64_t sector_num,
-                                    int nb_sectors, int *pnum,
-                                    BlockDriverState **file)
+static int64_t bdrv_common_block_status_above(BlockDriverState *bs,
+                                              BlockDriverState *base,
+                                              bool mapping,
+                                              int64_t sector_num,
+                                              int nb_sectors, int *pnum,
+                                              BlockDriverState **file)
 {
     Coroutine *co;
     BdrvCoGetBlockStatusData data = {
@@ -1982,6 +1995,7 @@ int64_t bdrv_get_block_status_above(BlockDriverState *bs,
         .sector_num = sector_num,
         .nb_sectors = nb_sectors,
         .pnum = pnum,
+        .mapping = mapping,
         .done = false,
     };

@@ -1997,6 +2011,16 @@ int64_t bdrv_get_block_status_above(BlockDriverState *bs,
     return data.ret;
 }

+int64_t bdrv_get_block_status_above(BlockDriverState *bs,
+                                    BlockDriverState *base,
+                                    int64_t sector_num,
+                                    int nb_sectors, int *pnum,
+                                    BlockDriverState **file)
+{
+    return bdrv_common_block_status_above(bs, base, true, sector_num,
+                                          nb_sectors, pnum, file);
+}
+
 int64_t bdrv_get_block_status(BlockDriverState *bs,
                               int64_t sector_num,
                               int nb_sectors, int *pnum,
@@ -2009,15 +2033,15 @@ int64_t bdrv_get_block_status(BlockDriverState *bs,
 int coroutine_fn bdrv_is_allocated(BlockDriverState *bs, int64_t offset,
                                    int64_t bytes, int64_t *pnum)
 {
-    int64_t sector_num = offset >> BDRV_SECTOR_BITS;
-    int nb_sectors = bytes >> BDRV_SECTOR_BITS;
     int64_t ret;
     int psectors;

     assert(QEMU_IS_ALIGNED(offset, BDRV_SECTOR_SIZE));
     assert(QEMU_IS_ALIGNED(bytes, BDRV_SECTOR_SIZE) && bytes < INT_MAX);
-    ret = bdrv_get_block_status(bs, sector_num, nb_sectors, &psectors,
-                                NULL);
+    ret = bdrv_common_block_status_above(bs, backing_bs(bs), false,
+                                         offset >> BDRV_SECTOR_BITS,
+                                         bytes >> BDRV_SECTOR_BITS, &psectors,
+                                         NULL);
     if (ret < 0) {
         return ret;
     }
-- 
2.13.6