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Thu, 16 Jan 2025 21:52:13 +0100 Message-ID: <46387df4-91d9-426d-8881-e0ae97bd86cf@gmx.com> Date: Fri, 17 Jan 2025 07:22:09 +1030 Precedence: bulk X-Mailing-List: linux-btrfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] btrfs-progs: remove loopback device resolution To: dsterba@suse.cz, Qu Wenruo Cc: linux-btrfs@vger.kernel.org References: <6094201431aa981c6e0d149b6d528bc4b7a5af91.1737020580.git.wqu@suse.com> <20250116114219.GC5777@twin.jikos.cz> Content-Language: en-US From: Qu Wenruo Autocrypt: addr=quwenruo.btrfs@gmx.com; keydata= xsBNBFnVga8BCACyhFP3ExcTIuB73jDIBA/vSoYcTyysFQzPvez64TUSCv1SgXEByR7fju3o 8RfaWuHCnkkea5luuTZMqfgTXrun2dqNVYDNOV6RIVrc4YuG20yhC1epnV55fJCThqij0MRL 1NxPKXIlEdHvN0Kov3CtWA+R1iNN0RCeVun7rmOrrjBK573aWC5sgP7YsBOLK79H3tmUtz6b 9Imuj0ZyEsa76Xg9PX9Hn2myKj1hfWGS+5og9Va4hrwQC8ipjXik6NKR5GDV+hOZkktU81G5 gkQtGB9jOAYRs86QG/b7PtIlbd3+pppT0gaS+wvwMs8cuNG+Pu6KO1oC4jgdseFLu7NpABEB AAHNIlF1IFdlbnJ1byA8cXV3ZW5ydW8uYnRyZnNAZ214LmNvbT7CwJQEEwEIAD4CGwMFCwkI BwIGFQgJCgsCBBYCAwECHgECF4AWIQQt33LlpaVbqJ2qQuHCPZHzoSX+qAUCZxF1YAUJEP5a sQAKCRDCPZHzoSX+qF+mB/9gXu9C3BV0omDZBDWevJHxpWpOwQ8DxZEbk9b9LcrQlWdhFhyn xi+l5lRziV9ZGyYXp7N35a9t7GQJndMCFUWYoEa+1NCuxDs6bslfrCaGEGG/+wd6oIPb85xo naxnQ+SQtYLUFbU77WkUPaaIU8hH2BAfn9ZSDX9lIxheQE8ZYGGmo4wYpnN7/hSXALD7+oun tZljjGNT1o+/B8WVZtw/YZuCuHgZeaFdhcV2jsz7+iGb+LsqzHuznrXqbyUQgQT9kn8ZYFNW 7tf+LNxXuwedzRag4fxtR+5GVvJ41Oh/eygp8VqiMAtnFYaSlb9sjia1Mh+m+OBFeuXjgGlG VvQFzsBNBFnVga8BCACqU+th4Esy/c8BnvliFAjAfpzhI1wH76FD1MJPmAhA3DnX5JDORcga CbPEwhLj1xlwTgpeT+QfDmGJ5B5BlrrQFZVE1fChEjiJvyiSAO4yQPkrPVYTI7Xj34FnscPj /IrRUUka68MlHxPtFnAHr25VIuOS41lmYKYNwPNLRz9Ik6DmeTG3WJO2BQRNvXA0pXrJH1fN GSsRb+pKEKHKtL1803x71zQxCwLh+zLP1iXHVM5j8gX9zqupigQR/Cel2XPS44zWcDW8r7B0 q1eW4Jrv0x19p4P923voqn+joIAostyNTUjCeSrUdKth9jcdlam9X2DziA/DHDFfS5eq4fEv ABEBAAHCwHwEGAEIACYCGwwWIQQt33LlpaVbqJ2qQuHCPZHzoSX+qAUCZxF1gQUJEP5a0gAK CRDCPZHzoSX+qHGpB/kB8A7M7KGL5qzat+jBRoLwB0Y3Zax0QWuANVdZM3eJDlKJKJ4HKzjo B2Pcn4JXL2apSan2uJftaMbNQbwotvabLXkE7cPpnppnBq7iovmBw++/d8zQjLQLWInQ5kNq Vmi36kmq8o5c0f97QVjMryHlmSlEZ2Wwc1kURAe4lsRG2dNeAd4CAqmTw0cMIrR6R/Dpt3ma +8oGXJOmwWuDFKNV4G2XLKcghqrtcRf2zAGNogg3KulCykHHripG3kPKsb7fYVcSQtlt5R6v HZStaZBzw4PcDiaAF3pPDBd+0fIKS6BlpeNRSFG94RYrt84Qw77JWDOAZsyNfEIEE0J6LSR/ In-Reply-To: <20250116114219.GC5777@twin.jikos.cz> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: quoted-printable X-Provags-ID: V03:K1:22XGGJEmOUDy4Cm8fQIIVgPDVCVcZuSuYf7z89irVueSU+Zel1d Lrxy9sCNUJGlG/4yHyYrFnRZrGSRqkdByTUGV4xnfy5kTELBS6E/GBGL6bndNHCXYdbqYAE cIQ4Bi9mE3U7/Y66tAPdAy+cSk/LIU5JDty2e/3ANthsdcXipjwsgYCUIJtOxWxAPChPUx6 hRI8QBFSI18D8bdF92s5Q== X-Spam-Flag: NO UI-OutboundReport: notjunk:1;M01:P0:Crc3qUwcJH4=;QcszwlDlUsAt/Yd0V2caEnvgmZy 16PocTNasqcdcMT+7uO13WWEqiPfeu/pB3/uy0Ahu3YzTBixhp8t7BO/iw1fKHhu3cvKFjVHR 0JG5IBrtWUdarcMPGyvF4TCIHXPPy4ix6AHXHzf08VrwQ7qPxfFMfXa3jhFdq4C87dL5PUgkD Z3QRorBObE6CJ7WqiR48WRt8HcP+wivXnWzbRL+gC4egDXoXEgqN/0+JsRflB3RbIK3y73X06 oXxbWWT/rPwnjekfX9Rzwch/rYH6HGRLJAfDRMA04NzQtcp5NXuDkeneiRA5v4BM3OUNQT79h A4obI+/ksqpxWxJzbW8VMt4JRpjGqhgWnzjmGHyZQN6QRYj03kn+IpXSq03wLAX/OGwsdcgSJ 6wCBdakzLrgZbTc3zb3M9sqjt3ogfjfyxR2m0VnHcrmYdB2StwDnSOjGkAI/SMUPjB6+bNH1z erZcpbkq4bxcBE9LZ/xrdblCmDVvW+e2ga6G8knYu0hA11HD/0V6bhst37EZrnCQCx8LFgYXI 990J4FYkeTL3Mixk+BMTk3aqSSB6AQOzsdP5TqVRMIcfcOWSCKV9pdIDioiFLcmXeorkAUXEN bLBpDvmStw207XF680q5dRMeVMZkVQ9pV2RZ8pKeFd9mgNPXcYMh6Wq+7clLMZfvMjXOf0Shm OSuzuogPkDIowrlHTC8Y0VdBC5MK4KeRRwU7wZu3iuk8puSunq+n53kbQixeQ9GLgbDBO82IW SBrq5T67z9M9OfELYxT13qbPKZ0RiHVSvnrjA1xumQvBvNeNDGT4YQ+8yX/t4vv1dMrCQbi9q MLbIqv95f9z/ueKg2c1IxhMkmBMfWIzUrPnQtFmd2ztlsoFMKS2Aa/KI/PDXzlHI5vCWp5plF 8NTE6/gqME/1CVHYh5b/UpdzdGfdeMuXmTZZiML/xVRIUycWItewUl5QpgDl0uI4fwUQKUwbv xaQB/fjKW5Tlu+yy7APKjZi8B3gORquXSUHG3IqbWzVRKka7j9ja2eoR7VF3MBOfVIKl9njUc gYABdlTW3O0NXXItVwfazhNC4JWGtG815hJoJWBIg40tvuBhwZqCGzjiBD9gDRiVTOKWMjNaH Y7jok7W0PPaUA++Yy2sZk8p0XXwmP8AmlCIi5/A7oJOA6dTKz9nVdG83nYGQ7MuPJhc4T8DIp qndzg2AXC+osU9fv5/vWUTCBRR3/Yl/VcG0cNp/IWuQ== =E5=9C=A8 2025/1/16 22:12, David Sterba =E5=86=99=E9=81=93: > On Thu, Jan 16, 2025 at 08:13:01PM +1030, Qu Wenruo wrote: >> [BUG] >> mkfs.btrfs has a built-in loopback device resolution, to avoid the same >> file being added to the same fs, using loopback device and the file >> itself. >> >> But it has one big bug: >> >> - It doesn't detect partition on loopback devices correctly >> The function is_loop_device() only utilize major number to detect a >> loopback device. >> But partitions on loopback devices doesn't use the same major number >> as the loopback device: >> >> NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS >> loop0 7:0 0 5G 0 loop >> |-loop0p1 259:3 0 128M 0 part >> `-loop0p2 259:4 0 4.9G 0 part >> >> Thus `/dev/loop0p1` will not be treated as a loopback device, thus i= t >> will not even resolve the source file. >> >> And this can not even be fixed, as if we do extra "/dev/loop*" based >> file lookup, `/dev/loop0p1` and `/dev/loop0p2` will resolve to the >> same source file, and refuse to mkfs on two different partitions. >> >> [FIX] >> The loopback file detection is the baby sitting that no one asks for. >> >> Just as I explained, it only brings new bugs, and we will never fix all >> ways that an experienced user can come up with. >> And I didn't see any other mkfs tool doing such baby sitting. >> >> So remove the loopback file resolution, just regular is_same_blk_file() >> is good enough. > > The loop device resolution had some bugs in the past and was added for a > reason that's not mentioned in the changelogs. > > The commits have some details why it's done, they also mention that > partitioned devices are resolved so it's not that there's no support for > that. > > 0cf3b78f404b01 ("btrfs-progs: Fix partitioned loop devices resolving") > abdb0ced0123d4 ("Btrfs-progs: fix resolving of loop devices") The problem of those two fixes are pretty simple, they just do not work in the first place. resolve_loop_device() is only triggered if is_loop_dev() returns >0 values= . But as I explained, resolve_loop_device() won't return >0 if a partitioned loopback device is passed. So why those patches are introduced in the first place? Thanks, Qu > 09559bfe7bcd43 ("multidevice support for check_mounted") > > and some fixup commits. > > So before removing the code completely I'd like to see if there's a use > case that can be broken, but I don't have anything in particular. > There's only mkfs-tests/006-partitioned-loopdev that's quite simple and > does not try to do any tricks with symlinks or partitions on the > devices. >