From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:59322) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Y8v3l-0000s6-Ro for qemu-devel@nongnu.org; Wed, 07 Jan 2015 13:10:23 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Y8v3g-0007Bv-TS for qemu-devel@nongnu.org; Wed, 07 Jan 2015 13:10:21 -0500 Received: from mx1.redhat.com ([209.132.183.28]:48176) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Y8v3g-0007Br-MZ for qemu-devel@nongnu.org; Wed, 07 Jan 2015 13:10:16 -0500 Message-ID: <54AD7683.3030602@redhat.com> Date: Wed, 07 Jan 2015 13:10:11 -0500 From: John Snow MIME-Version: 1.0 References: <1420566495-13284-1-git-send-email-peter@lekensteyn.nl> <1420566495-13284-13-git-send-email-peter@lekensteyn.nl> In-Reply-To: <1420566495-13284-13-git-send-email-peter@lekensteyn.nl> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v2 12/12] block/dmg: improve zeroes handling List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Wu , qemu-devel@nongnu.org Cc: Kevin Wolf , Stefan Hajnoczi On 01/06/2015 12:48 PM, Peter Wu wrote: > Disk images may contain large all-zeroes gaps (1.66k sectors or 812 MiB > is seen in the real world). These blocks (type 2) do not need to be > extracted into a temporary buffer, there is no need to allocate memory > for these blocks nor to check its length. > > (For the test image, the maximum uncompressed size is 1054371 bytes, > probably for a bzip2-compressed block.) > > Signed-off-by: Peter Wu > --- > v2: no changes (did not receive comments last time) > --- > block/dmg.c | 18 +++++++++++++++--- > 1 file changed, 15 insertions(+), 3 deletions(-) > > diff --git a/block/dmg.c b/block/dmg.c > index 8239221..4e24076 100644 > --- a/block/dmg.c > +++ b/block/dmg.c > @@ -137,7 +137,9 @@ static void update_max_chunk_size(BDRVDMGState *s, uint32_t chunk, > uncompressed_sectors = (s->lengths[chunk] + 511) / 512; > break; > case 2: /* zero */ > - uncompressed_sectors = s->sectorcounts[chunk]; > + /* as the all-zeroes block may be large, it is treated specially: the > + * sector is not copied from a large buffer, a simple memset is used > + * instead. Therefore uncompressed_sectors does not need to be set. */ > break; > } > > @@ -269,7 +271,9 @@ static int dmg_read_mish_block(BDRVDMGState *s, DmgHeaderState *ds, > /* sector count */ > s->sectorcounts[i] = buff_read_uint64(buffer, offset + 0x10); > > - if (s->sectorcounts[i] > DMG_SECTORCOUNTS_MAX) { > + /* all-zeroes sector (type 2) does not need to be "uncompressed" and can > + * therefore be unbounded. */ > + if (s->types[i] != 2 && s->sectorcounts[i] > DMG_SECTORCOUNTS_MAX) { > error_report("sector count %" PRIu64 " for chunk %" PRIu32 > " is larger than max (%u)", > s->sectorcounts[i], i, DMG_SECTORCOUNTS_MAX); > @@ -644,7 +648,8 @@ static inline int dmg_read_chunk(BlockDriverState *bs, uint64_t sector_num) > } > break; > case 2: /* zero */ > - memset(s->uncompressed_chunk, 0, 512 * s->sectorcounts[chunk]); > + /* see dmg_read, it is treated specially. No buffer needs to be > + * pre-filled, the zeroes can be set directly. */ > break; > } > s->current_chunk = chunk; > @@ -663,6 +668,13 @@ static int dmg_read(BlockDriverState *bs, int64_t sector_num, > if (dmg_read_chunk(bs, sector_num + i) != 0) { > return -1; > } > + /* Special case: current chunk is all zeroes. Do not perform a memcpy as > + * s->uncompressed_chunk may be too small to cover the large all-zeroes > + * section. dmg_read_chunk is called to find s->current_chunk */ > + if (s->types[s->current_chunk] == 2) { /* all zeroes block entry */ > + memset(buf + i * 512, 0, 512); > + continue; > + } > sector_offset_in_chunk = sector_num + i - s->sectors[s->current_chunk]; > memcpy(buf + i * 512, > s->uncompressed_chunk + sector_offset_in_chunk * 512, 512); > Reviewed-by: John Snow