Re: [Qemu-devel] [Qemu-block] [PATCH v3 19/20] block: Minimize raw use of bds->total_sectors

[Manos Pitsidianakis <el13635@mail.ntua.gr>]

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On Tue, Jun 27, 2017 at 02:24:57PM -0500, Eric Blake wrote:
>bdrv_is_allocated_above() was relying on intermediate->total_sectors,
>which is a field that can have stale contents depending on the value
>of intermediate->has_variable_length.  An audit shows that we are safe
>(we were first calling through bdrv_co_get_block_status() which in
>turn calls bdrv_nb_sectors() and therefore just refreshed the current
>length), but it's nicer to favor our accessor functions to avoid having
>to repeat such an audit, even if it means refresh_total_sectors() is
>called more frequently.
>
>Suggested-by: John Snow <jsnow@redhat.com>
>Signed-off-by: Eric Blake <eblake@redhat.com>
>
>---
>v2: new patch
>---
> block/io.c | 12 +++++++-----
> 1 file changed, 7 insertions(+), 5 deletions(-)
>
>diff --git a/block/io.c b/block/io.c
>index 0545180..5bbf153 100644
>--- a/block/io.c
>+++ b/block/io.c
>@@ -1924,6 +1924,7 @@ int bdrv_is_allocated_above(BlockDriverState *top,
>     intermediate = top;
>     while (intermediate && intermediate != base) {
>         int64_t pnum_inter;
>+        int64_t size_inter;
>         int psectors_inter;
>
>         ret = bdrv_is_allocated(intermediate, sector_num * BDRV_SECTOR_SIZE,
>@@ -1941,13 +1942,14 @@ int bdrv_is_allocated_above(BlockDriverState *top,
>
>         /*
>          * [sector_num, nb_sectors] is unallocated on top but intermediate
>-         * might have
>-         *
>-         * [sector_num+x, nr_sectors] allocated.
>+         * might have [sector_num+x, nb_sectors-x] allocated.
>          */
>+        size_inter = bdrv_nb_sectors(intermediate);
>+        if (size_inter < 0) {
>+            return size_inter;
>+        }
>         if (n > psectors_inter &&
>-            (intermediate == top ||
>-             sector_num + psectors_inter < intermediate->total_sectors)) {
>+            (intermediate == top || sector_num + psectors_inter < size_inter)) {
>             n = psectors_inter;
>         }
>
>-- 
>2.9.4

Reviewed-by: Manos Pitsidianakis <el13635@mail.ntua.gr>

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