Re: [Qemu-devel] [PATCH v4 11/21] mirror: Switch mirror_cow_align() to byte-based

[Kevin Wolf]

Am 05.07.2017 um 23:08 hat Eric Blake geschrieben:
> We are gradually converting to byte-based interfaces, as they are
> easier to reason about than sector-based.  Convert another internal
> function (no semantic change), and add mirror_clip_bytes() as a
> counterpart to mirror_clip_sectors().  Some of the conversion is
> a bit tricky, requiring temporaries to convert between units; it
> will be cleared up in a following patch.
> 
> Signed-off-by: Eric Blake 
> Reviewed-by: John Snow 
> Reviewed-by: Jeff Cody 

> -if (align_nb_sectors > max_sectors) {
> -align_nb_sectors = max_sectors;
> +if (align_bytes > max_bytes) {
> +align_bytes = max_bytes;
>  if (need_cow) {
> -align_nb_sectors = QEMU_ALIGN_DOWN(align_nb_sectors,
> -   s->target_cluster_size >>
> -   BDRV_SECTOR_BITS);
> +align_bytes = QEMU_ALIGN_DOWN(align_bytes,
> +  s->target_cluster_size);

This would fit in a single line, if you have to respin for some reason.

Reviewed-by: Kevin Wolf 

[Qemu-devel] [PATCH v4 11/21] mirror: Switch mirror_cow_align() to byte-based

[Eric Blake]

We are gradually converting to byte-based interfaces, as they are
easier to reason about than sector-based.  Convert another internal
function (no semantic change), and add mirror_clip_bytes() as a
counterpart to mirror_clip_sectors().  Some of the conversion is
a bit tricky, requiring temporaries to convert between units; it
will be cleared up in a following patch.

Signed-off-by: Eric Blake 
Reviewed-by: John Snow 
Reviewed-by: Jeff Cody 

---
v3-v4: no change
v2: tweak mirror_clip_bytes() signature to match previous patch
---
 block/mirror.c | 64 ++
 1 file changed, 38 insertions(+), 26 deletions(-)

diff --git a/block/mirror.c b/block/mirror.c
index 70682b6..374cefd 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -176,6 +176,15 @@ static void mirror_read_complete(void *opaque, int ret)
 aio_context_release(blk_get_aio_context(s->common.blk));
 }

+/* Clip bytes relative to offset to not exceed end-of-file */
+static inline int64_t mirror_clip_bytes(MirrorBlockJob *s,
+int64_t offset,
+int64_t bytes)
+{
+return MIN(bytes, s->bdev_length - offset);
+}
+
+/* Clip nb_sectors relative to sector_num to not exceed end-of-file */
 static inline int mirror_clip_sectors(MirrorBlockJob *s,
   int64_t sector_num,
   int nb_sectors)
@@ -184,44 +193,39 @@ static inline int mirror_clip_sectors(MirrorBlockJob *s,
s->bdev_length / BDRV_SECTOR_SIZE - sector_num);
 }

-/* Round sector_num and/or nb_sectors to target cluster if COW is needed, and
- * return the offset of the adjusted tail sector against original. */
-static int mirror_cow_align(MirrorBlockJob *s,
-int64_t *sector_num,
-int *nb_sectors)
+/* Round offset and/or bytes to target cluster if COW is needed, and
+ * return the offset of the adjusted tail against original. */
+static int mirror_cow_align(MirrorBlockJob *s, int64_t *offset,
+unsigned int *bytes)
 {
 bool need_cow;
 int ret = 0;
-int chunk_sectors = s->granularity >> BDRV_SECTOR_BITS;
-int64_t align_sector_num = *sector_num;
-int align_nb_sectors = *nb_sectors;
-int max_sectors = chunk_sectors * s->max_iov;
+int64_t align_offset = *offset;
+unsigned int align_bytes = *bytes;
+int max_bytes = s->granularity * s->max_iov;

-need_cow = !test_bit(*sector_num / chunk_sectors, s->cow_bitmap);
-need_cow |= !test_bit((*sector_num + *nb_sectors - 1) / chunk_sectors,
+need_cow = !test_bit(*offset / s->granularity, s->cow_bitmap);
+need_cow |= !test_bit((*offset + *bytes - 1) / s->granularity,
   s->cow_bitmap);
 if (need_cow) {
-bdrv_round_sectors_to_clusters(blk_bs(s->target), *sector_num,
-   *nb_sectors, _sector_num,
-   _nb_sectors);
+bdrv_round_to_clusters(blk_bs(s->target), *offset, *bytes,
+   _offset, _bytes);
 }

-if (align_nb_sectors > max_sectors) {
-align_nb_sectors = max_sectors;
+if (align_bytes > max_bytes) {
+align_bytes = max_bytes;
 if (need_cow) {
-align_nb_sectors = QEMU_ALIGN_DOWN(align_nb_sectors,
-   s->target_cluster_size >>
-   BDRV_SECTOR_BITS);
+align_bytes = QEMU_ALIGN_DOWN(align_bytes,
+  s->target_cluster_size);
 }
 }
-/* Clipping may result in align_nb_sectors unaligned to chunk boundary, but
+/* Clipping may result in align_bytes unaligned to chunk boundary, but
  * that doesn't matter because it's already the end of source image. */
-align_nb_sectors = mirror_clip_sectors(s, align_sector_num,
-   align_nb_sectors);
+align_bytes = mirror_clip_bytes(s, align_offset, align_bytes);

-ret = align_sector_num + align_nb_sectors - (*sector_num + *nb_sectors);
-*sector_num = align_sector_num;
-*nb_sectors = align_nb_sectors;
+ret = align_offset + align_bytes - (*offset + *bytes);
+*offset = align_offset;
+*bytes = align_bytes;
 assert(ret >= 0);
 return ret;
 }
@@ -257,10 +261,18 @@ static int mirror_do_read(MirrorBlockJob *s, int64_t 
sector_num,
 nb_sectors = MIN(s->buf_size >> BDRV_SECTOR_BITS, nb_sectors);
 nb_sectors = MIN(max_sectors, nb_sectors);
 assert(nb_sectors);
+assert(nb_sectors < BDRV_REQUEST_MAX_SECTORS);
 ret = nb_sectors;

 if (s->cow_bitmap) {
-ret += mirror_cow_align(s, _num, _sectors);
+int64_t offset = sector_num * BDRV_SECTOR_SIZE;
+unsigned int bytes = nb_sectors *