[Qemu-devel] [PATCH v6 0/6] minor qcow2 compression improvements

[Qemu-devel] [PATCH v6 0/6] minor qcow2 compression improvements

[Eric Blake]

Even though v5 was posted earlier today, it was worth a respin:
- 2/6: add R-b [Berto]
- 4/6, 6/6: improve commit messages [Max]
- 5/6: new patch, with an iotests proving that 4/6 is a bug fix [Max]

The new test is rather slow (nearly 90 seconds for me using
tmpfs) unless it skips entirely (such as testing on ext4); ideas
for speeding it up are welcome (translation: maybe qemu should
optimize the search for the next available cluster to allocate,
and/or qemu-img check should be faster at reporting leaked
clusters)

001/6:[----] [--] 'qcow2: Prefer byte-based calls into bs->file'
002/6:[----] [--] 'qcow2: Document some maximum size constraints'
003/6:[----] [--] 'qcow2: Reduce REFT_OFFSET_MASK'
004/6:[----] [--] 'qcow2: Don't allow overflow during cluster allocation'
005/6:[down] 'iotests: Add new test 214 for max compressed cluster offset'
006/6:[----] [--] 'qcow2: Avoid memory over-allocation on compressed images'

Eric Blake (6):
  qcow2: Prefer byte-based calls into bs->file
  qcow2: Document some maximum size constraints
  qcow2: Reduce REFT_OFFSET_MASK
  qcow2: Don't allow overflow during cluster allocation
  iotests: Add new test 214 for max compressed cluster offset
  qcow2: Avoid memory over-allocation on compressed images

 docs/interop/qcow2.txt     | 40 +++++++++++++++++--
 block/qcow2.h              |  8 +++-
 block/qcow2-cluster.c      | 32 +++++++++------
 block/qcow2-refcount.c     | 27 ++++++++-----
 block/qcow2.c              |  2 +-
 tests/qemu-iotests/214     | 97 ++++++++++++++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/214.out | 54 ++++++++++++++++++++++++++
 tests/qemu-iotests/group   |  1 +
 8 files changed, 234 insertions(+), 27 deletions(-)
 create mode 100755 tests/qemu-iotests/214
 create mode 100644 tests/qemu-iotests/214.out

-- 
2.14.3

[Qemu-devel] [PATCH v6 1/6] qcow2: Prefer byte-based calls into bs->file

[Eric Blake]

We had only a few sector-based stragglers left; convert them to use
our preferred byte-based accesses.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>

---
v5: commit message tweak
v2: indentation fix
---
 block/qcow2-cluster.c  | 5 ++---
 block/qcow2-refcount.c | 6 +++---
 2 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 1aee726c6a4..1dc00ff2110 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1616,13 +1616,12 @@ int qcow2_decompress_cluster(BlockDriverState *bs, uint64_t cluster_offset)
         }

         BLKDBG_EVENT(bs->file, BLKDBG_READ_COMPRESSED);
-        ret = bdrv_read(bs->file, coffset >> 9, s->cluster_data,
-                        nb_csectors);
+        ret = bdrv_pread(bs->file, coffset, s->cluster_data, csize);
         if (ret < 0) {
             return ret;
         }
         if (decompress_buffer(s->cluster_cache, s->cluster_size,
-                              s->cluster_data + sector_offset, csize) < 0) {
+                              s->cluster_data, csize) < 0) {
             return -EIO;
         }
         s->cluster_cache_offset = coffset;
diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 6b8b63514af..6bfc11bb48f 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -2359,8 +2359,8 @@ write_refblocks:
         on_disk_refblock = (void *)((char *) *refcount_table +
                                     refblock_index * s->cluster_size);

-        ret = bdrv_write(bs->file, refblock_offset / BDRV_SECTOR_SIZE,
-                         on_disk_refblock, s->cluster_sectors);
+        ret = bdrv_pwrite(bs->file, refblock_offset, on_disk_refblock,
+                          s->cluster_size);
         if (ret < 0) {
             fprintf(stderr, "ERROR writing refblock: %s\n", strerror(-ret));
             goto fail;
@@ -2582,7 +2582,7 @@ fail:
  * - 0 if writing to this offset will not affect the mentioned metadata
  * - a positive QCow2MetadataOverlap value indicating one overlapping section
  * - a negative value (-errno) indicating an error while performing a check,
- *   e.g. when bdrv_read failed on QCOW2_OL_INACTIVE_L2
+ *   e.g. when bdrv_pread failed on QCOW2_OL_INACTIVE_L2
  */
 int qcow2_check_metadata_overlap(BlockDriverState *bs, int ign, int64_t offset,
                                  int64_t size)
-- 
2.14.3

[Qemu-devel] [PATCH v6 2/6] qcow2: Document some maximum size constraints

[Eric Blake]

Although off_t permits up to 63 bits (8EB) of file offsets, in
practice, we're going to hit other limits first.  Document some
of those limits in the qcow2 spec, and how choice of cluster size
can influence some of the limits.

While at it, notice that since we cannot map any virtual cluster
to any address higher than 64 PB (56 bits) (due to the current L1/L2
field encoding stopping at bit 55), it makes little sense to require
the refcount table to access host offsets beyond that point.  Mark
the upper bits of the refcount table entries as reserved to match
the L1/L2 table, with no ill effects, since it is unlikely that there
are any existing images larger than 64PB in the first place, and thus
all existing images already have those bits as 0.  If 64PB proves to
be too small in the future, we could enlarge all three uses of bit
55 into the reserved bits at that time.  (For reference, ext4 with
4k blocks caps files at 16PB.)

However, there is one limit that reserved bits don't help with: for
compressed clusters, the L2 layout requires an ever-smaller maximum
host offset as cluster size gets larger, down to a 512 TB maximum
with 2M clusters.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>

--
v5: even more wording tweaks
v4: more wording tweaks
v3: new patch
---
 docs/interop/qcow2.txt | 40 +++++++++++++++++++++++++++++++++++++---
 1 file changed, 37 insertions(+), 3 deletions(-)

diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index feb711fb6a8..f0338f356fe 100644
--- a/docs/interop/qcow2.txt
+++ b/docs/interop/qcow2.txt
@@ -40,7 +40,17 @@ The first cluster of a qcow2 image contains the file header:
                     with larger cluster sizes.

          24 - 31:   size
-                    Virtual disk size in bytes
+                    Virtual disk size in bytes.
+
+                    Note: with a 2 MB cluster size, the maximum
+                    virtual size is 2 EB (61 bits) for a sparse file,
+                    but other sizing limitations in refcount and L1/L2
+                    tables mean that an image cannot have more than 64
+                    PB (56 bits) of populated clusters (assuming the
+                    image does not first hit other limits such as a
+                    file system's maximum size).  With a 512 byte
+                    cluster size, the maximum virtual size drops to
+                    128 GB (37 bits).

          32 - 35:   crypt_method
                     0 for no encryption
@@ -326,6 +336,15 @@ in the image file.
 It contains pointers to the second level structures which are called refcount
 blocks and are exactly one cluster in size.

+All qcow2 metadata, including the refcount table and refcount blocks,
+must currently reside at host offsets below 64 PB (56 bits) (if the
+underlying protocol can even be sized that large); this limit could be
+enlarged by putting reserved bits into use, but only if a similar
+limit on L1/L2 tables is revisited at the same time.  While a larger
+cluster size theoretically allows the refcount table to cover more
+host offsets, in practice, other inherent limits will constrain the
+maximum image size before the refcount table is full.
+
 Given a offset into the image file, the refcount of its cluster can be obtained
 as follows:

@@ -341,7 +360,7 @@ Refcount table entry:

     Bit  0 -  8:    Reserved (set to 0)

-         9 - 63:    Bits 9-63 of the offset into the image file at which the
+         9 - 55:    Bits 9-55 of the offset into the image file at which the
                     refcount block starts. Must be aligned to a cluster
                     boundary.

@@ -349,6 +368,8 @@ Refcount table entry:
                     been allocated. All refcounts managed by this refcount block
                     are 0.

+        56 - 63:    Reserved (set to 0)
+
 Refcount block entry (x = refcount_bits - 1):

     Bit  0 -  x:    Reference count of the cluster. If refcount_bits implies a
@@ -365,6 +386,17 @@ The L1 table has a variable size (stored in the header) and may use multiple
 clusters, however it must be contiguous in the image file. L2 tables are
 exactly one cluster in size.

+The L1 and L2 tables have implications on the maximum virtual file
+size; a larger cluster size is required for the guest to have access
+to more space.  Furthermore, a virtual cluster must currently map to a
+host offset below 64 PB (56 bits) (this limit could be enlarged by
+putting reserved bits into use, but only if a similar limit on
+refcount tables is revisited at the same time).  Additionally, with
+larger cluster sizes, compressed clusters have a smaller limit on host
+cluster mappings (a 2M cluster size requires compressed clusters to
+reside below 512 TB (49 bits), where enlarging this would require an
+incompatible layout change).
+
 Given a offset into the virtual disk, the offset into the image file can be
 obtained as follows:

@@ -427,7 +459,9 @@ Standard Cluster Descriptor:
 Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)):

     Bit  0 - x-1:   Host cluster offset. This is usually _not_ aligned to a
-                    cluster or sector boundary!
+                    cluster or sector boundary!  If cluster_bits is
+                    small enough that this field includes bits beyond
+                    55, those upper bits must be set to 0.

          x - 61:    Number of additional 512-byte sectors used for the
                     compressed data, beyond the sector containing the offset
-- 
2.14.3

[Qemu-devel] [PATCH v6 3/6] qcow2: Reduce REFT_OFFSET_MASK

[Eric Blake]

Match our code to the spec change in the previous patch - there's
no reason for the refcount table to allow larger offsets than the
L1/L2 tables. In practice, no image has more than 64PB of
allocated clusters anyways, as anything beyond that can't be
expressed via L2 mappings to host offsets.

Suggested-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>

---
v4: new patch
---
 block/qcow2.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/qcow2.h b/block/qcow2.h
index adf5c3950fd..1df15a18aa1 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -443,7 +443,7 @@ typedef enum QCow2MetadataOverlap {
 #define L2E_OFFSET_MASK 0x00fffffffffffe00ULL
 #define L2E_COMPRESSED_OFFSET_SIZE_MASK 0x3fffffffffffffffULL

-#define REFT_OFFSET_MASK 0xfffffffffffffe00ULL
+#define REFT_OFFSET_MASK 0x00fffffffffffe00ULL

 static inline int64_t start_of_cluster(BDRVQcow2State *s, int64_t offset)
 {
-- 
2.14.3

[Qemu-devel] [PATCH v6 4/6] qcow2: Don't allow overflow during cluster allocation

[Eric Blake]

Our code was already checking that we did not attempt to
allocate more clusters than what would fit in an INT64 (the
physical maximimum if we can access a full off_t's worth of
data).  But this does not catch smaller limits enforced by
various spots in the qcow2 image description: L1 and normal
clusters of L2 are documented as having bits 63-56 reserved
for other purposes, capping our maximum offset at 64PB (bit
55 is the maximum bit set).  And for compressed images with
2M clusters, the cap drops the maximum offset to bit 48, or
a maximum offset of 512TB.  If we overflow that offset, we
would write compressed data into one place, but try to
decompress from another, which won't work.

It's actually possible to prove that overflow can cause image
corruption without this patch; I'll add the iotests separately
in the next commit.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>

---
v6: improve commit message, now that iotests is possible [Max]
v3: use s->cluster_offset_mask instead of open-coding it [Berto],
use MIN() to clamp offset on small cluster size, add spec patch
first to justify clamping even on refcount allocations
---
 block/qcow2.h          |  6 ++++++
 block/qcow2-refcount.c | 21 ++++++++++++++-------
 2 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/block/qcow2.h b/block/qcow2.h
index 1df15a18aa1..a3b9faa9d53 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -41,6 +41,12 @@
 #define QCOW_MAX_CRYPT_CLUSTERS 32
 #define QCOW_MAX_SNAPSHOTS 65536

+/* Field widths in qcow2 mean normal cluster offsets cannot reach
+ * 64PB; depending on cluster size, compressed clusters can have a
+ * smaller limit (64PB for up to 16k clusters, then ramps down to
+ * 512TB for 2M clusters).  */
+#define QCOW_MAX_CLUSTER_OFFSET ((1ULL << 56) - 1)
+
 /* 8 MB refcount table is enough for 2 PB images at 64k cluster size
  * (128 GB for 512 byte clusters, 2 EB for 2 MB clusters) */
 #define QCOW_MAX_REFTABLE_SIZE 0x800000
diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 6bfc11bb48f..fcbea3c9644 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -31,7 +31,8 @@
 #include "qemu/bswap.h"
 #include "qemu/cutils.h"

-static int64_t alloc_clusters_noref(BlockDriverState *bs, uint64_t size);
+static int64_t alloc_clusters_noref(BlockDriverState *bs, uint64_t size,
+                                    uint64_t max);
 static int QEMU_WARN_UNUSED_RESULT update_refcount(BlockDriverState *bs,
                             int64_t offset, int64_t length, uint64_t addend,
                             bool decrease, enum qcow2_discard_type type);
@@ -362,7 +363,8 @@ static int alloc_refcount_block(BlockDriverState *bs,
     }

     /* Allocate the refcount block itself and mark it as used */
-    int64_t new_block = alloc_clusters_noref(bs, s->cluster_size);
+    int64_t new_block = alloc_clusters_noref(bs, s->cluster_size,
+                                             QCOW_MAX_CLUSTER_OFFSET);
     if (new_block < 0) {
         return new_block;
     }
@@ -954,7 +956,8 @@ int qcow2_update_cluster_refcount(BlockDriverState *bs,


 /* return < 0 if error */
-static int64_t alloc_clusters_noref(BlockDriverState *bs, uint64_t size)
+static int64_t alloc_clusters_noref(BlockDriverState *bs, uint64_t size,
+                                    uint64_t max)
 {
     BDRVQcow2State *s = bs->opaque;
     uint64_t i, nb_clusters, refcount;
@@ -979,9 +982,9 @@ retry:
     }

     /* Make sure that all offsets in the "allocated" range are representable
-     * in an int64_t */
+     * in the requested max */
     if (s->free_cluster_index > 0 &&
-        s->free_cluster_index - 1 > (INT64_MAX >> s->cluster_bits))
+        s->free_cluster_index - 1 > (max >> s->cluster_bits))
     {
         return -EFBIG;
     }
@@ -1001,7 +1004,7 @@ int64_t qcow2_alloc_clusters(BlockDriverState *bs, uint64_t size)

     BLKDBG_EVENT(bs->file, BLKDBG_CLUSTER_ALLOC);
     do {
-        offset = alloc_clusters_noref(bs, size);
+        offset = alloc_clusters_noref(bs, size, QCOW_MAX_CLUSTER_OFFSET);
         if (offset < 0) {
             return offset;
         }
@@ -1083,7 +1086,11 @@ int64_t qcow2_alloc_bytes(BlockDriverState *bs, int size)
     free_in_cluster = s->cluster_size - offset_into_cluster(s, offset);
     do {
         if (!offset || free_in_cluster < size) {
-            int64_t new_cluster = alloc_clusters_noref(bs, s->cluster_size);
+            int64_t new_cluster;
+
+            new_cluster = alloc_clusters_noref(bs, s->cluster_size,
+                                               MIN(s->cluster_offset_mask,
+                                                   QCOW_MAX_CLUSTER_OFFSET));
             if (new_cluster < 0) {
                 return new_cluster;
             }
-- 
2.14.3

[Qemu-devel] [PATCH v6 5/6] iotests: Add new test 214 for max compressed cluster offset

[Eric Blake]

If you have a capable file system (tmpfs is good, ext4 not so much;
run ./check with TEST_DIR pointing to a good location so as not
to skip the test), it's actually possible to create a qcow2 file
that expands to a sparse 512T image with just over 38M of content.
The test is not the world's fastest (qemu crawling through 256M
bits of refcount table to find the next cluster to allocate takes
several seconds, as does qemu-img check reporting millions of
leaked clusters); but it DOES catch the problem that the previous
patch just fixed where writing a compressed cluster to a full
image ended up overwriting the wrong cluster.

Suggested-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

---
v6: new patch; took over 90 seconds to run on my setup, using tmpfs
---
 tests/qemu-iotests/214     | 97 ++++++++++++++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/214.out | 54 ++++++++++++++++++++++++++
 tests/qemu-iotests/group   |  1 +
 3 files changed, 152 insertions(+)
 create mode 100755 tests/qemu-iotests/214
 create mode 100644 tests/qemu-iotests/214.out

diff --git a/tests/qemu-iotests/214 b/tests/qemu-iotests/214
new file mode 100755
index 00000000000..b4127facd12
--- /dev/null
+++ b/tests/qemu-iotests/214
@@ -0,0 +1,97 @@
+#!/bin/bash
+#
+# max limits on compression in huge qcow2 files
+#
+# Copyright (C) 2018 Red Hat, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+seq=`basename $0`
+echo "QA output created by $seq"
+
+here=`pwd`
+status=1    # failure is the default!
+
+_cleanup()
+{
+    _cleanup_test_img
+}
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+# get standard environment, filters and checks
+. ./common.rc
+. ./common.filter
+. ./common.pattern
+
+_supported_fmt qcow2
+_supported_proto file
+_supported_os Linux
+
+echo "== Creating huge file =="
+
+# Sanity check: We require a file system that permits the creation
+# of a HUGE (but very sparse) file.  tmpfs works, ext4 does not.
+if ! truncate --size=513T "$TEST_IMG"; then
+    _notrun "file system on $TEST_DIR does not support large enough files"
+fi
+rm "$TEST_IMG"
+IMGOPTS='cluster_size=2M,refcount_bits=1' _make_test_img 513T
+
+echo "== Populating refcounts =="
+# We want an image with 256M refcounts * 2M clusters = 512T referenced.
+# Each 2M cluster holds 16M refcounts; the refcount table initially uses
+# 1 refblock, so we need to add 15 more.  The refcount table lives at 2M,
+# first refblock at 4M, L2 at 6M, so our remaining additions start at 8M.
+# Then, for each refblock, mark it as fully populated.
+to_hex() {
+    printf %016x\\n $1 | sed 's/\(..\)/\\x\1/g'
+}
+truncate --size=38m "$TEST_IMG"
+entry=$((0x200000))
+$QEMU_IO_PROG -f raw -c "w -P 0xff 4m 2m" "$TEST_IMG" | _filter_qemu_io
+for i in {1..15}; do
+    offs=$((0x600000 + i*0x200000))
+    poke_file "$TEST_IMG" $((i*8 + entry)) $(to_hex $offs)
+    $QEMU_IO_PROG -f raw -c "w -P 0xff $offs 2m" "$TEST_IMG" | _filter_qemu_io
+done
+
+echo "== Checking file before =="
+# FIXME: 'qemu-img check' doesn't diagnose refcounts beyond the end of
+# the file as leaked clusters
+_check_test_img 2>&1 | sed '/^Leaked cluster/d'
+stat -c 'image size %s' "$TEST_IMG"
+
+echo "== Trying to write compressed cluster =="
+# Given our file size, the next available cluster at 512T lies beyond the
+# maximum offset that a compressed 2M cluster can reside in
+$QEMU_IO_PROG -c 'w -c 0 2m' "$TEST_IMG" | _filter_qemu_io
+# The attempt failed, but ended up allocating a new refblock
+stat -c 'image size %s' "$TEST_IMG"
+
+echo "== Writing normal cluster =="
+# The failed write should not corrupt the image, so a normal write succeeds
+$QEMU_IO_PROG -c 'w 0 2m' "$TEST_IMG" | _filter_qemu_io
+
+echo "== Checking file after =="
+# qemu-img now sees the millions of leaked clusters, thanks to the allocations
+# at 512T.  Undo many of our faked references to speed up the check.
+$QEMU_IO_PROG -f raw -c "w -z 5m 1m" -c "w -z 8m 30m" "$TEST_IMG" |
+    _filter_qemu_io
+_check_test_img 2>&1 | sed '/^Leaked cluster/d'
+
+# success, all done
+echo "*** done"
+rm -f $seq.full
+status=0
diff --git a/tests/qemu-iotests/214.out b/tests/qemu-iotests/214.out
new file mode 100644
index 00000000000..38aafe1b961
--- /dev/null
+++ b/tests/qemu-iotests/214.out
@@ -0,0 +1,54 @@
+QA output created by 214
+== Creating huge file ==
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=564049465049088
+== Populating refcounts ==
+wrote 2097152/2097152 bytes at offset 4194304
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 8388608
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 10485760
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 12582912
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 14680064
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 16777216
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 18874368
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 20971520
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 23068672
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 25165824
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 27262976
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 29360128
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 31457280
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 33554432
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 35651584
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 2097152/2097152 bytes at offset 37748736
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+== Checking file before ==
+No errors were found on the image.
+image size 39845888
+== Trying to write compressed cluster ==
+write failed: Input/output error
+image size 562949957615616
+== Writing normal cluster ==
+wrote 2097152/2097152 bytes at offset 0
+2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+== Checking file after ==
+wrote 1048576/1048576 bytes at offset 5242880
+1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 31457280/31457280 bytes at offset 8388608
+30 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+
+8388589 leaked clusters were found on the image.
+This means waste of disk space, but no harm to data.
+*** done
diff --git a/tests/qemu-iotests/group b/tests/qemu-iotests/group
index 52a80f3f9e9..3e2dcdfa33d 100644
--- a/tests/qemu-iotests/group
+++ b/tests/qemu-iotests/group
@@ -212,3 +212,4 @@
 211 rw auto quick
 212 rw auto quick
 213 rw auto quick
+214 rw auto
-- 
2.14.3

[Qemu-devel] [PATCH v6 6/6] qcow2: Avoid memory over-allocation on compressed images

[Eric Blake]

When reading a compressed image, we were allocating s->cluster_data
to 32*cluster_size + 512 (possibly over 64 megabytes, for an image
with 2M clusters).  Let's check out the history:

Back when qcow2 was first written, we used s->cluster_data for
everything, including copy_sectors() and encryption, where we want
to operate on more than one cluster at once.  Obviously, at that
point, the buffer had to be aligned for other users, even though
compression itself doesn't require any alignment (the fact that
the compressed data generally starts mid-sector means that aligning
our buffer buys us nothing - either the protocol already supports
byte-based access into whatever offset we want, or we are already
using a bounce buffer to read a full sector, and copying into
our destination no longer requires alignment).

But commit 1b9f1491 (v1.1!) changed things to allocate parallel
buffers on demand rather than sharing a single buffer, for encryption
and COW, leaving compression as the final client of s->cluster_data.
That use was still preserved, because if a single compressed cluster
is read more than once, we reuse the cache instead of decompressing
it a second time (someday, we may come up with better caching to
avoid wasting repeated decompressions while still being more parallel,
but that is a task for another patch; the XXX comment in
qcow2_co_preadv for QCOW2_CLUSTER_COMPRESSED is telling).

Much later, in commit de82815d (v2.2), we noticed that a 64M
allocation is prone to failure, so we switched over to a graceful
memory allocation error message.  Elsewhere in the code, we do
g_malloc(2 * cluster_size) without ever checking for failure, but
even 4M starts to be large enough that trying to be nice is worth
the effort, so we want to keep that aspect.

Then even later, in 3e4c7052 (2.11), we realized that allocating
a large buffer up front for every qcow2 image is expensive, and
switched to lazy allocation only for images that actually had
compressed clusters.  But in the process, we never even bothered
to check whether what we were allocating still made sense in its
new context!

So, it's time to cut back on the waste.  A compressed cluster
written by qemu will NEVER occupy more than an uncompressed
cluster, but based on mid-sector alignment, we may still need
to read 1 cluster + 1 sector in order to recover enough bytes
for the decompression.  Meanwhile, third-party producers of
qcow2 may not be as smart, and gzip DOES document that because
the compression stream adds metadata, and because of the
pigeonhole principle, there are worst case scenarios where
attempts to compress will actually inflate an image, by up to
0.015% (at most 1 sector larger for an unfortunate 2M
compression), meaning we should realistically expect to
sometimes need to read 1 cluster + 2 sectors.

However, the qcow2 spec permits an all-ones sector count, plus
a full sector containing the initial offset, for a maximum read
of 2 full clusters.  Thanks to the way decompression works, even
though such a value is too large for the actual compressed data
(for all but 512-byte clusters), it really doesn't matter if we
read too much (gzip ignores slop, once it has decoded a full
cluster).  So it's easier to just allocate cluster_data to be as
large as we can ever possibly see; even if it still wastes up to
2M on any image created by qcow2, that's still an improvment of
60M less waste than pre-patch.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>

---
v5: fix math error in commit message [Max]
v4: fix off-by-one in assertion and commit message [Berto]
v3: tighten allocation [Berto]
v2: actually check allocation failure (previous version meant
to use g_malloc, but ended up posted with g_try_malloc without
checking); add assertions outside of conditional, improve
commit message to better match reality now that qcow2 spec bug
has been fixed
---
 block/qcow2-cluster.c | 27 ++++++++++++++++++---------
 block/qcow2.c         |  2 +-
 2 files changed, 19 insertions(+), 10 deletions(-)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 1dc00ff2110..6e3eb88a37a 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1599,20 +1599,29 @@ int qcow2_decompress_cluster(BlockDriverState *bs, uint64_t cluster_offset)
         sector_offset = coffset & 511;
         csize = nb_csectors * 512 - sector_offset;

-        /* Allocate buffers on first decompress operation, most images are
-         * uncompressed and the memory overhead can be avoided.  The buffers
-         * are freed in .bdrv_close().
+        /* Allocate buffers on the first decompress operation; most
+         * images are uncompressed and the memory overhead can be
+         * avoided.  The buffers are freed in .bdrv_close().  qemu
+         * never writes an inflated cluster, and gzip itself never
+         * inflates a problematic cluster by more than 0.015%, but the
+         * qcow2 format allows up to 1 byte short of 2 full clusters
+         * when including the sector containing offset.  gzip ignores
+         * trailing slop, so just allocate that much up front rather
+         * than reject third-party images with overlarge csize.
          */
+        assert(!!s->cluster_data == !!s->cluster_cache);
+        assert(csize <= 2 * s->cluster_size);
         if (!s->cluster_data) {
-            /* one more sector for decompressed data alignment */
-            s->cluster_data = qemu_try_blockalign(bs->file->bs,
-                    QCOW_MAX_CRYPT_CLUSTERS * s->cluster_size + 512);
+            s->cluster_data = g_try_malloc(2 * s->cluster_size);
             if (!s->cluster_data) {
                 return -ENOMEM;
             }
-        }
-        if (!s->cluster_cache) {
-            s->cluster_cache = g_malloc(s->cluster_size);
+            s->cluster_cache = g_try_malloc(s->cluster_size);
+            if (!s->cluster_cache) {
+                g_free(s->cluster_data);
+                s->cluster_data = NULL;
+                return -ENOMEM;
+            }
         }

         BLKDBG_EVENT(bs->file, BLKDBG_READ_COMPRESSED);
diff --git a/block/qcow2.c b/block/qcow2.c
index ef68772acad..a8301371ccc 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2151,7 +2151,7 @@ static void qcow2_close(BlockDriverState *bs)
     g_free(s->image_backing_format);

     g_free(s->cluster_cache);
-    qemu_vfree(s->cluster_data);
+    g_free(s->cluster_data);
     qcow2_refcount_close(bs);
     qcow2_free_snapshots(bs);
 }
-- 
2.14.3

Re: [Qemu-devel] [PATCH v6 5/6] iotests: Add new test 214 for max compressed cluster offset

[Alberto Garcia]

On Thu 26 Apr 2018 04:51:28 AM CEST, Eric Blake wrote:
> If you have a capable file system (tmpfs is good, ext4 not so much;
> run ./check with TEST_DIR pointing to a good location so as not
> to skip the test), it's actually possible to create a qcow2 file
> that expands to a sparse 512T image with just over 38M of content.
> The test is not the world's fastest (qemu crawling through 256M
> bits of refcount table to find the next cluster to allocate takes
> several seconds, as does qemu-img check reporting millions of
> leaked clusters); but it DOES catch the problem that the previous
> patch just fixed where writing a compressed cluster to a full
> image ended up overwriting the wrong cluster.
>
> Suggested-by: Max Reitz <mreitz@redhat.com>
> Signed-off-by: Eric Blake <eblake@redhat.com>

Nice test :-)

Reviewed-by: Alberto Garcia <berto@igalia.com>

Berto

Re: [Qemu-devel] [PATCH v6 0/6] minor qcow2 compression improvements

[Eric Blake]

ping

On 04/25/2018 09:51 PM, Eric Blake wrote:
> Even though v5 was posted earlier today, it was worth a respin:
> - 2/6: add R-b [Berto]
> - 4/6, 6/6: improve commit messages [Max]
> - 5/6: new patch, with an iotests proving that 4/6 is a bug fix [Max]
> 
> The new test is rather slow (nearly 90 seconds for me using
> tmpfs) unless it skips entirely (such as testing on ext4); ideas
> for speeding it up are welcome (translation: maybe qemu should
> optimize the search for the next available cluster to allocate,
> and/or qemu-img check should be faster at reporting leaked
> clusters)
> 
> 001/6:[----] [--] 'qcow2: Prefer byte-based calls into bs->file'
> 002/6:[----] [--] 'qcow2: Document some maximum size constraints'
> 003/6:[----] [--] 'qcow2: Reduce REFT_OFFSET_MASK'
> 004/6:[----] [--] 'qcow2: Don't allow overflow during cluster allocation'
> 005/6:[down] 'iotests: Add new test 214 for max compressed cluster offset'
> 006/6:[----] [--] 'qcow2: Avoid memory over-allocation on compressed images'
> 
> Eric Blake (6):
>    qcow2: Prefer byte-based calls into bs->file
>    qcow2: Document some maximum size constraints
>    qcow2: Reduce REFT_OFFSET_MASK
>    qcow2: Don't allow overflow during cluster allocation
>    iotests: Add new test 214 for max compressed cluster offset
>    qcow2: Avoid memory over-allocation on compressed images
> 
>   docs/interop/qcow2.txt     | 40 +++++++++++++++++--
>   block/qcow2.h              |  8 +++-
>   block/qcow2-cluster.c      | 32 +++++++++------
>   block/qcow2-refcount.c     | 27 ++++++++-----
>   block/qcow2.c              |  2 +-
>   tests/qemu-iotests/214     | 97 ++++++++++++++++++++++++++++++++++++++++++++++
>   tests/qemu-iotests/214.out | 54 ++++++++++++++++++++++++++
>   tests/qemu-iotests/group   |  1 +
>   8 files changed, 234 insertions(+), 27 deletions(-)
>   create mode 100755 tests/qemu-iotests/214
>   create mode 100644 tests/qemu-iotests/214.out
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

Re: [Qemu-devel] [PATCH v6 5/6] iotests: Add new test 214 for max compressed cluster offset

[Eric Blake]

On 04/26/2018 07:10 AM, Alberto Garcia wrote:
> On Thu 26 Apr 2018 04:51:28 AM CEST, Eric Blake wrote:
>> If you have a capable file system (tmpfs is good, ext4 not so much;
>> run ./check with TEST_DIR pointing to a good location so as not
>> to skip the test), it's actually possible to create a qcow2 file
>> that expands to a sparse 512T image with just over 38M of content.
>> The test is not the world's fastest (qemu crawling through 256M
>> bits of refcount table to find the next cluster to allocate takes
>> several seconds, as does qemu-img check reporting millions of
>> leaked clusters); but it DOES catch the problem that the previous
>> patch just fixed where writing a compressed cluster to a full
>> image ended up overwriting the wrong cluster.
>>
>> Suggested-by: Max Reitz <mreitz@redhat.com>
>> Signed-off-by: Eric Blake <eblake@redhat.com>
> 
> Nice test :-)
> 
> Reviewed-by: Alberto Garcia <berto@igalia.com>

214 is already in the tree in the meantime; this will need a rebase to 
pick the next available test number (220 might be claimed, so 222?)

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

Re: [Qemu-devel] [PATCH v6 5/6] iotests: Add new test 214 for max compressed cluster offset

[Alberto Garcia]

On Tue 19 Jun 2018 08:51:36 PM CEST, Eric Blake wrote:
> On 04/26/2018 07:10 AM, Alberto Garcia wrote:
>> On Thu 26 Apr 2018 04:51:28 AM CEST, Eric Blake wrote:
>>> If you have a capable file system (tmpfs is good, ext4 not so much;
>>> run ./check with TEST_DIR pointing to a good location so as not
>>> to skip the test), it's actually possible to create a qcow2 file
>>> that expands to a sparse 512T image with just over 38M of content.
>>> The test is not the world's fastest (qemu crawling through 256M
>>> bits of refcount table to find the next cluster to allocate takes
>>> several seconds, as does qemu-img check reporting millions of
>>> leaked clusters); but it DOES catch the problem that the previous
>>> patch just fixed where writing a compressed cluster to a full
>>> image ended up overwriting the wrong cluster.
>>>
>>> Suggested-by: Max Reitz <mreitz@redhat.com>
>>> Signed-off-by: Eric Blake <eblake@redhat.com>
>> 
>> Nice test :-)
>> 
>> Reviewed-by: Alberto Garcia <berto@igalia.com>
>
> 214 is already in the tree in the meantime; this will need a rebase to
> pick the next available test number (220 might be claimed, so 222?)

You can also take 220 if you want, I suspect that it will take a bit
longer to merge the blockdev-reopen patches. Whatever is easier for you.

Berto