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Wed, 21 May 2025 10:27:32 +0000 Received: from AM8PR08MB5732.eurprd08.prod.outlook.com ([fe80::e543:a83c:a260:602b]) by AM8PR08MB5732.eurprd08.prod.outlook.com ([fe80::e543:a83c:a260:602b%5]) with mapi id 15.20.8769.019; Wed, 21 May 2025 10:27:32 +0000 Message-ID: Date: Wed, 21 May 2025 12:27:31 +0200 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 3/4] block/copy-before-write: reverse access bitmap To: Eric Blake Cc: qemu-block@nongnu.org, vsementsov@yandex-team.ru, jsnow@redhat.com, kwolf@redhat.com, hreitz@redhat.com, qemu-devel@nongnu.org, andrey.drobyshev@virtuozzo.com, den@virtuozzo.com References: <20250513013238.1213539-1-andrey.zhadchenko@virtuozzo.com> <20250513013238.1213539-4-andrey.zhadchenko@virtuozzo.com> Content-Language: en-US From: Andrey Zhadchenko In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: FR4P281CA0293.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:e7::6) To AM8PR08MB5732.eurprd08.prod.outlook.com (2603:10a6:20b:1d4::16) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: AM8PR08MB5732:EE_|DB3PR08MB8795:EE_ X-MS-Office365-Filtering-Correlation-Id: 26518884-4f2b-4533-80c1-08dd9852187d X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|366016|7053199007; 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envelope-from=andrey.zhadchenko@virtuozzo.com; helo=EUR05-DB8-obe.outbound.protection.outlook.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 5/20/25 19:26, Eric Blake wrote: > [?? ??????? ????????? ?????? ?? eblake@redhat.com. ???????, ?????? ??? ?????, ?? ?????? https://aka.ms/LearnAboutSenderIdentification ] > > On Tue, May 13, 2025 at 03:32:37AM +0200, Andrey Zhadchenko wrote: >> HBitmaps allow us to search set bits pretty fast. On the contrary, >> when searching zeroes, we may be forced to fully traverse the lower >> level. >> When we run blockdev-backup with mode=full on top of snapshot filter >> + cbw filter, the job fills copy bitmap by calling block_status() >> with range (X, virtual_size). The problem is that we check for zeroes >> in this whole range. We also hit the worst case here, as access >> bitmap is fully set and we need to scan the entire lowest level. >> After scanning the full bitmap we actually ask the block status of >> original image, which may return significantly lower amount of empty >> clusters. >> Beacuse of this, the backup job 'hangs' on block copy initializaiton >> for a long time with 100% CPU. >> >> Example copy bitmap buildup time for image with clu_size=65536 and >> preallocated metadata >> size 10T 11T >> blockdev-backup 52s 57s >> cbw + snap 325s 413s >> cbw + snap + patch 55s 61s >> >> To fix it, reverse the access bitmap in cbw filter: rather set it > > s/reverse/invert/ > >> when the user is not allowed to read the cluster. >> >> Update qemu-iotest 257: now access bitmap have count 0 instead of >> the image size 67108864 >> >> Signed-off-by: Andrey Zhadchenko >> --- >> @@ -501,9 +501,12 @@ static int cbw_open(BlockDriverState *bs, QDict *options, int flags, >> return -EINVAL; >> } >> bdrv_disable_dirty_bitmap(s->access_bitmap); >> - bdrv_dirty_bitmap_merge_internal(s->access_bitmap, >> - block_copy_dirty_bitmap(s->bcs), NULL, >> - true); >> + if (bitmap) { >> + bdrv_dirty_bitmap_merge_internal(s->access_bitmap, >> + block_copy_dirty_bitmap(s->bcs), NULL, >> + true); >> + bdrv_dirty_bitmap_reverse(s->access_bitmap); > > Is this setting the bits correctly? Inverting a bitmap is a > reversible operation, but it looks odd that you are only reversing > once around the merge. Either the two sources of the merge have the > same sense (whether that be 0 for dirty 1 for clean, or 0 for readable > 1 for avoid) and no inverting is needed before or after the merge, or > the two sources have opposite sense (in which case, I would have > expected inverting one of the bitmaps before the merge to get them to > agree on sense, then merging, then inverting back to the desired > sense). Am I missing something? We have just created access bitmap a few lines above. So it is empty: everything is accessible. - If cbw did not have any bitmap, we don't need to to do anything - If someone set bitmap on cbw, we used it to init block-copy dirty bitmap. So to get the access bitmap, we inverse the bitmap that marks to-be-copied blocks: access bitmap now has all of them as 0 (allowed) and not-to-be-copied as 1 (disallowed) This case is also covered with iotests/image-fleecing with filter snapshot + cbw bitmap case: some not-in-a-bitmap blocks are accessed and we expect to get errors: --- Sanity Check --- read -P0x5d 0 64k read -P0xd5 1M 64k read -P0xdc 32M 64k read -P0xcd 0x3ff0000 64k read -P0 0x00f8000 32k read failed: Invalid argument read -P0 0x2010000 32k read failed: Invalid argument read -P0 0x3fe0000 64k read failed: Invalid argument > > -- > Eric Blake, Principal Software Engineer > Red Hat, Inc. > Virtualization: qemu.org | libguestfs.org >