[qemu-xen staging-4.13] qcow2: Fix the calculation of the maximum L2 cache size

[patchbot@xxxxxxx]

commit c9ffb12754b1575babfef45168b6e1b1af80a95f
Author:     Alberto Garcia <berto@xxxxxxxxxx>
AuthorDate: Fri Aug 16 15:17:42 2019 +0300
Commit:     Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
CommitDate: Mon Oct 28 00:00:03 2019 -0500

    qcow2: Fix the calculation of the maximum L2 cache size
    
    The size of the qcow2 L2 cache defaults to 32 MB, which can be easily
    larger than the maximum amount of L2 metadata that the image can have.
    For example: with 64 KB clusters the user would need a qcow2 image
    with a virtual size of 256 GB in order to have 32 MB of L2 metadata.
    
    Because of that, since commit b749562d9822d14ef69c9eaa5f85903010b86c30
    we forbid the L2 cache to become larger than the maximum amount of L2
    metadata for the image, calculated using this formula:
    
        uint64_t max_l2_cache = virtual_disk_size / (s->cluster_size / 8);
    
    The problem with this formula is that the result should be rounded up
    to the cluster size because an L2 table on disk always takes one full
    cluster.
    
    For example, a 1280 MB qcow2 image with 64 KB clusters needs exactly
    160 KB of L2 metadata, but we need 192 KB on disk (3 clusters) even if
    the last 32 KB of those are not going to be used.
    
    However QEMU rounds the numbers down and only creates 2 cache tables
    (128 KB), which is not enough for the image.
    
    A quick test doing 4KB random writes on a 1280 MB image gives me
    around 500 IOPS, while with the correct cache size I get 16K IOPS.
    
    Cc: qemu-stable@xxxxxxxxxx
    Signed-off-by: Alberto Garcia <berto@xxxxxxxxxx>
    Signed-off-by: Kevin Wolf <kwolf@xxxxxxxxxx>
    (cherry picked from commit b70d08205b2e4044c529eefc21df2c8ab61b473b)
    Signed-off-by: Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
---
 block/qcow2.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 039bdc2f7e..865839682c 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -826,7 +826,11 @@ static void read_cache_sizes(BlockDriverState *bs, 
QemuOpts *opts,
     bool l2_cache_entry_size_set;
     int min_refcount_cache = MIN_REFCOUNT_CACHE_SIZE * s->cluster_size;
     uint64_t virtual_disk_size = bs->total_sectors * BDRV_SECTOR_SIZE;
-    uint64_t max_l2_cache = virtual_disk_size / (s->cluster_size / 8);
+    uint64_t max_l2_entries = DIV_ROUND_UP(virtual_disk_size, s->cluster_size);
+    /* An L2 table is always one cluster in size so the max cache size
+     * should be a multiple of the cluster size. */
+    uint64_t max_l2_cache = ROUND_UP(max_l2_entries * sizeof(uint64_t),
+                                     s->cluster_size);
 
     combined_cache_size_set = qemu_opt_get(opts, QCOW2_OPT_CACHE_SIZE);
     l2_cache_size_set = qemu_opt_get(opts, QCOW2_OPT_L2_CACHE_SIZE);
--
generated by git-patchbot for /home/xen/git/qemu-xen.git#staging-4.13