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(2403-580d-fda1--299.ip6.aussiebb.net. [2403:580d:fda1::299]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-7992d0c349csm16959651b3a.50.2025.10.14.21.18.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Oct 2025 21:18:30 -0700 (PDT) Message-ID: <1de12f07-c7ab-4a8f-8fb4-00cb29145178@suse.com> Date: Wed, 15 Oct 2025 14:48:23 +1030 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 0/1] ovl: brtfs' temp_fsid doesn't work with ovl index=on To: Anand Jain , dsterba@suse.cz, =?UTF-8?Q?Andr=C3=A9_Almeida?= Cc: linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-unionfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, kernel-dev@igalia.com, Miklos Szeredi , Amir Goldstein , Chris Mason , David Sterba , Anand Jain , "Guilherme G . Piccoli" References: <20251014015707.129013-1-andrealmeid@igalia.com> <20251014182414.GD13776@twin.jikos.cz> <6982bc0a-bb12-458a-bb8c-890c363ba807@suse.com> <0791edfb-6985-45d7-bb3e-08ab7a341dab@gmail.com> Content-Language: en-US From: Qu Wenruo Autocrypt: addr=wqu@suse.com; keydata= xsBNBFnVga8BCACyhFP3ExcTIuB73jDIBA/vSoYcTyysFQzPvez64TUSCv1SgXEByR7fju3o 8RfaWuHCnkkea5luuTZMqfgTXrun2dqNVYDNOV6RIVrc4YuG20yhC1epnV55fJCThqij0MRL 1NxPKXIlEdHvN0Kov3CtWA+R1iNN0RCeVun7rmOrrjBK573aWC5sgP7YsBOLK79H3tmUtz6b 9Imuj0ZyEsa76Xg9PX9Hn2myKj1hfWGS+5og9Va4hrwQC8ipjXik6NKR5GDV+hOZkktU81G5 gkQtGB9jOAYRs86QG/b7PtIlbd3+pppT0gaS+wvwMs8cuNG+Pu6KO1oC4jgdseFLu7NpABEB AAHNGFF1IFdlbnJ1byA8d3F1QHN1c2UuY29tPsLAlAQTAQgAPgIbAwULCQgHAgYVCAkKCwIE FgIDAQIeAQIXgBYhBC3fcuWlpVuonapC4cI9kfOhJf6oBQJnEXVgBQkQ/lqxAAoJEMI9kfOh Jf6o+jIH/2KhFmyOw4XWAYbnnijuYqb/obGae8HhcJO2KIGcxbsinK+KQFTSZnkFxnbsQ+VY fvtWBHGt8WfHcNmfjdejmy9si2jyy8smQV2jiB60a8iqQXGmsrkuR+AM2V360oEbMF3gVvim 2VSX2IiW9KERuhifjseNV1HLk0SHw5NnXiWh1THTqtvFFY+CwnLN2GqiMaSLF6gATW05/sEd V17MdI1z4+WSk7D57FlLjp50F3ow2WJtXwG8yG8d6S40dytZpH9iFuk12Sbg7lrtQxPPOIEU rpmZLfCNJJoZj603613w/M8EiZw6MohzikTWcFc55RLYJPBWQ+9puZtx1DopW2jOwE0EWdWB rwEIAKpT62HgSzL9zwGe+WIUCMB+nOEjXAfvoUPUwk+YCEDcOdfkkM5FyBoJs8TCEuPXGXBO Cl5P5B8OYYnkHkGWutAVlUTV8KESOIm/KJIA7jJA+Ss9VhMjtePfgWexw+P8itFRSRrrwyUf E+0WcAevblUi45LjWWZgpg3A80tHP0iToOZ5MbdYk7YFBE29cDSleskfV80ZKxFv6koQocq0 vXzTfHvXNDELAuH7Ms/WJcdUzmPyBf3Oq6mKBBH8J6XZc9LjjNZwNbyvsHSrV5bgmu/THX2n g/3be+iqf6OggCiy3I1NSMJ5KtR0q2H2Nx2Vqb1fYPOID8McMV9Ll6rh8S8AEQEAAcLAfAQY AQgAJgIbDBYhBC3fcuWlpVuonapC4cI9kfOhJf6oBQJnEXWBBQkQ/lrSAAoJEMI9kfOhJf6o cakH+QHwDszsoYvmrNq36MFGgvAHRjdlrHRBa4A1V1kzd4kOUokongcrOOgHY9yfglcvZqlJ qfa4l+1oxs1BvCi29psteQTtw+memmcGruKi+YHD7793zNCMtAtYidDmQ2pWaLfqSaryjlzR /3tBWMyvIeWZKURnZbBzWRREB7iWxEbZ014B3gICqZPDRwwitHpH8Om3eZr7ygZck6bBa4MU o1XgbZcspyCGqu1xF/bMAY2iCDcq6ULKQceuKkbeQ8qxvt9hVxJC2W3lHq8dlK1pkHPDg9wO JoAXek8MF37R8gpLoGWl41FIUb3hFiu3zhDDvslYM4BmzI18QgQTQnotJH8= In-Reply-To: <0791edfb-6985-45d7-bb3e-08ab7a341dab@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 在 2025/10/15 10:35, Anand Jain 写道: > On 15-Oct-25 5:08 AM, Qu Wenruo wrote: >> >> >> 在 2025/10/15 04:54, David Sterba 写道: >>> On Mon, Oct 13, 2025 at 10:57:06PM -0300, André Almeida wrote: >>>> Hi everyone, >>>> >>>> When using overlayfs with the mount option index=on, the first time >>>> a directory is >>>> used as upper dir, overlayfs stores in a xattr "overlay.origin" the >>>> UUID of the >>>> filesystem being used in the layers. If the upper dir is reused, >>>> overlayfs >>>> refuses to mount for a different filesystem, by comparing the UUID >>>> with what's >>>> stored at overlay.origin, and it fails with "failed to verify upper >>>> root origin" >>>> on dmesg. Remounting with the very same fs is supported and works fine. >>>> >>>> However, btrfs mounts may have volatiles UUIDs. When mounting the >>>> exact same >>>> disk image with btrfs, a random UUID is assigned for the following >>>> disks each >>>> time they are mounted, stored at temp_fsid and used across the >>>> kernel as the >>>> disk UUID. `btrfs filesystem show` presents that. Calling statfs() >>>> however shows >>>> the original (and duplicated) UUID for all disks. >>>> >>>> This feature doesn't work well with overlayfs with index=on, as when >>>> the image >>>> is mounted a second time, will get a different UUID and ovl will >>>> refuse to >>>> mount, breaking the user expectation that using the same image >>>> should work. A >>>> small script can be find in the end of this cover letter that >>>> illustrates this. >>>> >>>> >From this, I can think of some options: >>>> >>>> - Use statfs() internally to always get the fsid, that is >>>> persistent. The patch >>>> here illustrates that approach, but doesn't fully implement it. >>>> - Create a new sb op, called get_uuid() so the filesystem returns >>>> what's >>>> appropriated. >>>> - Have a workaround in ovl for btrfs. >>>> - Document this as unsupported, and userland needs to erase >>>> overlay.origin each >>>> time it wants to remount. >>>> - If ovl detects that temp_fsid and index are being used at the same >>>> time, >>>> refuses to mount. >>>> >>>> I'm not sure which one would be better here, so I would like to hear >>>> some ideas >>>> on this. >>> >>> I haven't looked deeper if there's a workable solution, but the feature >>> combination should be refused. I don't think this will affect many >>> users. >>> >> >> I believe the root problem is that we're not fully implementing the >> proper handling just like other single-device fses. >> >> We do not use on-disk flags which means at least one fsid is >> registered into btrfs, thus we have to use different temp-fsid. >> >> If fully single-device feature flag is properly implemented, we should >> be able to return the same uuid without extra hacks thus solve the >> problem. > > I had looked into this some time ago. Some libs, like libblkid, > don't handle multi-device filesystems or cloned devices with > temp FSIDs very well. I'm aware of it. > > I've been making some progress on fixing those cases, but it's > a bit extensive since we first need enough test coverage, > and recent reappear-device inline with that. > > Let's see how we can support use cases with identical devices > (where changing the UUID isn't an option) and keep things > compatible with systemd and library tools. > My current idea is to introduce a new ro compat flag, so that mounting that device will not go through the fsid lookup procedure completely. But go through the common get_tree_bdev() routine, which will check if the fs is already mounted using bdev holder. (And of course, no fsid recorded inside btrfs module) This will make single device btrfs with that special flag to behave exactly like all the other filesystems.