[Qemu-devel] [PATCH v2 0/3] qcow2: minor compression improvements

[Qemu-devel] [PATCH v2 0/3] qcow2: minor compression improvements

[Eric Blake]

Updates to v1:
- fix whitespace [Berto]
- fix g_try_malloc usage [Berto, Kevin]
- improve comments [Berto, Kevin]
- add a patch to avoid overflow on 512TB images with 2M clusters

Eric Blake (3):
  qcow2: Prefer byte-based calls into bs->file
  qcow2: Don't allow overflow during cluster allocation
  qcow2: Avoid memory over-allocation on compressed images

 block/qcow2.h          |  6 ++++++
 block/qcow2-cluster.c  | 32 ++++++++++++++++++++------------
 block/qcow2-refcount.c | 26 ++++++++++++++++----------
 block/qcow2.c          |  2 +-
 4 files changed, 43 insertions(+), 23 deletions(-)

-- 
2.14.3

[Qemu-devel] [PATCH v2 1/3] qcow2: Prefer byte-based calls into bs->file

[Eric Blake]

We had only three 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>

---
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 e406b0f3b9e..85be7d5e340 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -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;
diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index d46b69d7f34..28afbb1b5ea 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -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

[Qemu-devel] [PATCH v2 2/3] 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.

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.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 block/qcow2.h          |  6 ++++++
 block/qcow2-refcount.c | 20 +++++++++++++-------
 2 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/block/qcow2.h b/block/qcow2.h
index 883802241fb..560008c331d 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 28afbb1b5ea..58c19789a22 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;
     }
@@ -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 +1079,10 @@ 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,
+                                               (1ULL << s->csize_shift) - 1);
             if (new_cluster < 0) {
                 return new_cluster;
             }
-- 
2.14.3

[Qemu-devel] [PATCH v2 3/3] 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.  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 up to 2 full clusters of sectors beyond
the initial offset; and the way decompression works, it really
doesn't matter if we read too much (gzip ignores slop, once it
has decoded a full cluster), so it's feasible to encounter a
third-party image that reports the maximum 'nb_csectors'
possible, even if it no longer has any bearing to the actual
compressed size.  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>

---
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 85be7d5e340..7d5276b5f6b 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -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 2 full clusters beyond the sector
+         * containing offset, and gzip ignores trailing slop, so it's
+         * easier to just allocate that much up front than to reject
+         * third-party images with overlarge csize.
          */
+        assert(!!s->cluster_data == !!s->cluster_cache);
+        assert(csize < 2 * s->cluster_size + 512);
         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 + 512);
             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 288b5299d80..6ad3436e0e5 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -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

Re: [Qemu-devel] [PATCH v2 2/3] qcow2: Don't allow overflow during cluster allocation

[Alberto Garcia]

On Thu 22 Feb 2018 12:39:52 AM CET, Eric Blake wrote:
>      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,
> +                                               (1ULL << s->csize_shift) - 1);

(1ULL << s->csize_shift) - 1) is the same as s->cluster_offset_mask, but
I guess it's confusing to use that here, so your approach looks
appropriate.

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

Berto

Re: [Qemu-devel] [PATCH v2 3/3] qcow2: Avoid memory over-allocation on compressed images

[Alberto Garcia]

On Thu 22 Feb 2018 12:39:53 AM CET, Eric Blake wrote:
> +        assert(!!s->cluster_data == !!s->cluster_cache);
> +        assert(csize < 2 * s->cluster_size + 512);
>          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 + 512);
>              if (!s->cluster_data) {
>                  return -ENOMEM;
>              }

Why the "+ 512" ?

nb_csectors is guaranteed to be at most twice the cluster size, you can
even assert that:

    int max_csize = (s->csize_mask + 1) * 512;
    assert(max_csize == s->cluster_size * 2);
    s->cluster_data = qemu_try_blockalign(bs->file->bs, max_csize);

And csize is at most (max_csize - sector_offset), so you can change your
assertion to this:

   assert(csize <= 2 * s->cluster_size);

Berto

Re: [Qemu-devel] [PATCH v2 2/3] qcow2: Don't allow overflow during cluster allocation

[Eric Blake]

On 02/22/2018 04:29 AM, Alberto Garcia wrote:
> On Thu 22 Feb 2018 12:39:52 AM CET, Eric Blake wrote:
>>       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,
>> +                                               (1ULL << s->csize_shift) - 1);
> 
> (1ULL << s->csize_shift) - 1) is the same as s->cluster_offset_mask, but
> I guess it's confusing to use that here, so your approach looks
> appropriate.

Actually, s->cluster_offset_mask fits better - we want to ensure that 
the allocated cluster fits within the mask!  I'll adjust on respin.

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

Thanks for bearing with me.

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

Re: [Qemu-devel] [PATCH v2 3/3] qcow2: Avoid memory over-allocation on compressed images

[Eric Blake]

On 02/22/2018 04:50 AM, Alberto Garcia wrote:
> On Thu 22 Feb 2018 12:39:53 AM CET, Eric Blake wrote:
>> +        assert(!!s->cluster_data == !!s->cluster_cache);
>> +        assert(csize < 2 * s->cluster_size + 512);
>>           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 + 512);
>>               if (!s->cluster_data) {
>>                   return -ENOMEM;
>>               }
> 
> Why the "+ 512" ?

I was thinking "number of sectors is up to two clusters, and we add one, 
PLUS we must read from the initial sector containing the offset".  But I 
was obviously not careful enough - the maximum value (all 1s) is 512 
bytes short of 2 full clusters, and we add at most 511 more bytes for 
the initial sector containing the offset (our +1 covers the leading 
sector).  So you are right, I can tighten this down for a slightly 
smaller allocation (and a nice power-of-2 allocation may be slightly 
more efficient as well).  v3 coming up.

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