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qcow2: make refcount size calculation conservative

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The refcount metadata size calculation is inaccurate and can produce
numbers that are too small.  This is bad because we should calculate a
conservative number - one that is guaranteed to be large enough.

This patch switches the approach to a fixed point calculation because
the existing equation is hard to solve when inaccuracies are taken care
of.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20170705125738.8777-5-stefanha@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2017-07-05 13:57:33 +01:00 committed by Max Reitz
parent 95c67e3bd7
commit 7c5bcc4212
1 changed files with 42 additions and 41 deletions
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83
block/qcow2.c
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@ -2537,6 +2537,43 @@ static int preallocate(BlockDriverState *bs)
return 0;
}
/* qcow2_refcount_metadata_size:
* @clusters: number of clusters to refcount (including data and L1/L2 tables)
* @cluster_size: size of a cluster, in bytes
* @refcount_order: refcount bits power-of-2 exponent
*
* Returns: Number of bytes required for refcount blocks and table metadata.
*/
static int64_t qcow2_refcount_metadata_size(int64_t clusters,
size_t cluster_size,
int refcount_order)
{
/*
* Every host cluster is reference-counted, including metadata (even
* refcount metadata is recursively included).
*
* An accurate formula for the size of refcount metadata size is difficult
* to derive. An easier method of calculation is finding the fixed point
* where no further refcount blocks or table clusters are required to
* reference count every cluster.
*/
int64_t blocks_per_table_cluster = cluster_size / sizeof(uint64_t);
int64_t refcounts_per_block = cluster_size * 8 / (1 << refcount_order);
int64_t table = 0; /* number of refcount table clusters */
int64_t blocks = 0; /* number of refcount block clusters */
int64_t last;
int64_t n = 0;
do {
last = n;
blocks = DIV_ROUND_UP(clusters + table + blocks, refcounts_per_block);
table = DIV_ROUND_UP(blocks, blocks_per_table_cluster);
n = clusters + blocks + table;
} while (n != last);
return (blocks + table) * cluster_size;
}
/**
* qcow2_calc_prealloc_size:
* @total_size: virtual disk size in bytes
@ -2550,22 +2587,9 @@ static int64_t qcow2_calc_prealloc_size(int64_t total_size,
size_t cluster_size,
int refcount_order)
{
/* Note: The following calculation does not need to be exact; if it is a
* bit off, either some bytes will be "leaked" (which is fine) or we
* will need to increase the file size by some bytes (which is fine,
* too, as long as the bulk is allocated here). Therefore, using
* floating point arithmetic is fine. */
int64_t meta_size = 0;
uint64_t nreftablee, nrefblocke, nl1e, nl2e, refblock_count;
uint64_t nl1e, nl2e;
int64_t aligned_total_size = align_offset(total_size, cluster_size);
int cluster_bits = ctz32(cluster_size);
int refblock_bits, refblock_size;
/* refcount entry size in bytes */
double rces = (1 << refcount_order) / 8.;
/* see qcow2_open() */
refblock_bits = cluster_bits - (refcount_order - 3);
refblock_size = 1 << refblock_bits;
/* header: 1 cluster */
meta_size += cluster_size;
@ -2580,33 +2604,10 @@ static int64_t qcow2_calc_prealloc_size(int64_t total_size,
nl1e = align_offset(nl1e, cluster_size / sizeof(uint64_t));
meta_size += nl1e * sizeof(uint64_t);
/* total size of refcount blocks
*
* note: every host cluster is reference-counted, including metadata
* (even refcount blocks are recursively included).
* Let:
* a = total_size (this is the guest disk size)
* m = meta size not including refcount blocks and refcount tables
* c = cluster size
* y1 = number of refcount blocks entries
* y2 = meta size including everything
* rces = refcount entry size in bytes
* then,
* y1 = (y2 + a)/c
* y2 = y1 * rces + y1 * rces * sizeof(u64) / c + m
* we can get y1:
* y1 = (a + m) / (c - rces - rces * sizeof(u64) / c)
*/
nrefblocke = (aligned_total_size + meta_size + cluster_size)
/ (cluster_size - rces - rces * sizeof(uint64_t)
/ cluster_size);
refblock_count = DIV_ROUND_UP(nrefblocke, refblock_size);
meta_size += refblock_count * cluster_size;
/* total size of refcount tables */
nreftablee = align_offset(refblock_count,
cluster_size / sizeof(uint64_t));
meta_size += nreftablee * sizeof(uint64_t);
/* total size of refcount table and blocks */
meta_size += qcow2_refcount_metadata_size(
(meta_size + aligned_total_size) / cluster_size,
cluster_size, refcount_order);
return meta_size + aligned_total_size;
}