qcow2: minor compression improvements

Since v2:
- new patch 2, spec changes based on list followup
- tweak patch 3 to use MIN() and stop open-coding a calculation [Berto]
- tweak patch 4 to stop overallocating by a sector [Berto]

Eric Blake (4):
qcow2: Prefer byte-based calls into bs->file
qcow2: Document some maximum size constraints
qcow2: Don't allow overflow during cluster allocation
qcow2: Avoid memory over-allocation on compressed images

 docs/interop/qcow2.txt | 29 ++++++++++++++++++++++++++---
 block/qcow2.h          |  6 ++++++
 block/qcow2-cluster.c  | 32 ++++++++++++++++++++------------
 block/qcow2-refcount.c | 27 +++++++++++++++++----------
 block/qcow2.c          |  2 +-
 5 files changed, 70 insertions(+), 26 deletions(-)

2.14.3

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

---
v2: indentation fix

---
 block/qcow2-cluster.c  | 5 ++---
 block/qcow2-refcount.c | 6 +++---
 2 files changed, 5 insertions(+), 6 deletions(-)

index e406b0f3b9e..85be7d5e340 100644
@@ -1615,13 +1615,12 @@ 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;

index d46b69d7f34..28afbb1b5ea 100644
@@ -2310,8 +2310,8 @@ 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;

@@ -2533,7 +2533,7 @@ 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

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 L1/L2 field encoding), 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, 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.

---
 docs/interop/qcow2.txt | 29 ++++++++++++++++++++++++++---
 1 file changed, 26 insertions(+), 3 deletions(-)

index feb711fb6a8..2417522ca9b 100644
@@ -40,7 +40,16 @@ 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 mean that an image
+                    cannot have more than 64 PB of populated clusters
+                    (and may hit other sizing limitations as well,
+                    such as underlying protocol limits).  With a 512
+                    byte cluster size, the maximum virtual size drops
+                    to 128 GB (37 bits).
 
          32 - 35:   crypt_method
                     0 for no encryption

@@ -318,6 +327,11 @@ for each host cluster. A refcount of 0 means that the cluster is free, 1 means
 that it is used, and >= 2 means that it is used and any write access must
 perform a COW (copy on write) operation.
 
+The refcount table has implications on the maximum host file size; a
+larger cluster size is required for the refcount table to cover larger
+offsets.  Furthermore, all qcow2 metadata must reside at offsets below
+64 PB (56 bits).
+
 The refcounts are managed in a two-level table. The first level is called
 refcount table and has a variable size (which is stored in the header). The
 refcount table can cover multiple clusters, however it needs to be contiguous

@@ -341,7 +355,7 @@ 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 +363,8 @@ 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 +381,11 @@ 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 guest to have access to a
+larger size.  Furthermore, a virtual cluster must map to a host offset
+below 64 PB (56 bits).
+
 Given a offset into the virtual disk, the offset into the image file can be
 obtained as follows:
 

@@ -427,7 +448,9 @@ 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

But refcount blocks are not addressed by L2 tables, so in principle it should be possible to have refcount blocks after the first 64PB.

But I agree that it's a good idea to set that as a maximum possible physical size of the qcow2 image.

But (if we don't make this change) that's about all you can usefully have (and it would be a self-referencing refcount block).

We could, but that should be a separate patch from the spec change. We could also add some validation that any offsets in the header point to less than the 64PB limit.

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

Yeah, true.

Ok, let's leave that for another patch then.

Berto

Am 22.02.2018 um 16:59 hat Eric Blake geschrieben:

I think it would be good to mention the exact reason for the 56 bits in the spec. Even this commit message is rather vague ('L1/L2 field encoding'), so if at some point someone wonders, if we couldn't simply extend the allowed range, they won't easily see that it's related to compressed clusters.

Kevin

Note that L1 and L2 fields both stop at bit 55 currently, but do have room for expansion up to bit 61; so all three limits (if we include refcount table in the set capped at bit 55 for now) could be raised simultaneously if we find 64P too small in the future.

Compressed clusters are also related, but there, the limit is even smaller - with 2M clusters, a compressed cluster must reside within the first 512T of host offsets, and there are no free bits available for allowing additional compressed cluster storage without making an incompatible change of how compressed clusters are represented.

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

64 petabytes ought to be enough for anybody :-)

Berto

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.

I don't have 512TB handy to prove whether things break if we compress so much data that we overflow that limit, and don't think that iotests can (quickly) test it either. Test 138 comes close (it corrupts an image into thinking something lives at 32PB, which is half the maximum for L1 sizing - although it relies on 512-byte clusters). But that test points out that we will generally hit other limits first (such as running out of memory for the refcount table, or exceeding file system limits like 16TB on ext4, etc), so this is more a theoretical safety valve than something likely to be hit.

---
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(-)

index 883802241fb..560008c331d 100644
@@ -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

index 28afbb1b5ea..c46a2257a9e 100644
@@ -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;
     }

@@ -947,7 +949,8 @@ 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;

@@ -972,9 +975,9 @@ 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;
     }

@@ -994,7 +997,7 @@ 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;
         }

@@ -1076,7 +1079,11 @@ 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

Berto

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. But 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% (or 62 sectors larger for an unfortunate 2M compression). In fact, the qcow2 spec permits an all-ones sector count, plus 511 bytes from the sector containing the initial offset, for a maximum read of nearly 2 full clusters; and thanks to the way decompression works, even though such a value is probably too large for the actual compressed data, 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.

---
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(-)

index 85be7d5e340..af51d71d012 100644
@@ -1598,20 +1598,29 @@ 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);

index 288b5299d80..6ad3436e0e5 100644
@@ -2103,7 +2103,7 @@ 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

           [...]

I think it should be <=

If sector_offset is 0 and nb_csector is the maximum allowed value then csize is exactly 2 * s->cluster_size bytes.

Berto

Sigh, yes you're right. I was thinking that "qemu sets csize to a maximum of s->cluster_size, but only when sector_offset is not 0" - but as long as we're dealing with externally-produced images, sector_offset can be 0 at the same time as providing all 1s to the field. So I did indeed have an off-by-one.

Perhaps the maintainer can fix it up, instead of me spinning a v4?

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

That would work for me, but note that this part of the commit message also needs to be updated:

In fact, the qcow2 spec permits an all-ones sector count, plus 511 bytes from the sector containing the initial offset, for a maximum read of nearly 2 full clusters;

With those two things corrected,

Berto