From mboxrd@z Thu Jan 1 00:00:00 1970 From: Miklos Szeredi Date: Tue, 31 Mar 2020 15:10:34 +0000 Subject: Re: Upcoming: Notifications, FS notifications and fsinfo() Message-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit List-Id: References: <1445647.1585576702@warthog.procyon.org.uk> <20200330211700.g7evnuvvjenq3fzm@wittgenstein> <20200331083430.kserp35qabnxvths@ws.net.home> <20200331122554.GA27469@gardel-login> In-Reply-To: <20200331122554.GA27469@gardel-login> To: Lennart Poettering Cc: Karel Zak , Christian Brauner , David Howells , Linus Torvalds , Al Viro , dray@redhat.com, Miklos Szeredi , Steven Whitehouse , Jeff Layton , Ian Kent , andres@anarazel.de, keyrings@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Aleksa Sarai On Tue, Mar 31, 2020 at 2:25 PM Lennart Poettering wrote: > > On Di, 31.03.20 10:56, Miklos Szeredi (miklos@szeredi.hu) wrote: > > > On Tue, Mar 31, 2020 at 10:34 AM Karel Zak wrote: > > > > > > On Tue, Mar 31, 2020 at 07:11:11AM +0200, Miklos Szeredi wrote: > > > > On Mon, Mar 30, 2020 at 11:17 PM Christian Brauner > > > > wrote: > > > > > > > > > Fwiw, putting down my kernel hat and speaking as someone who maintains > > > > > two container runtimes and various other low-level bits and pieces in > > > > > userspace who'd make heavy use of this stuff I would prefer the fd-based > > > > > fsinfo() approach especially in the light of across namespace > > > > > operations, querying all properties of a mount atomically all-at-once, > > > > > > > > fsinfo(2) doesn't meet the atomically all-at-once requirement. > > > > > > I guess your /proc based idea have exactly the same problem... > > > > Yes, that's exactly what I wanted to demonstrate: there's no > > fundamental difference between the two API's in this respect. > > > > > I see two possible ways: > > > > > > - after open("/mnt", O_PATH) create copy-on-write object in kernel to > > > represent mount node -- kernel will able to modify it, but userspace > > > will get unchanged data from the FD until to close() > > > > > > - improve fsinfo() to provide set (list) of the attributes by one call > > > > I think we are approaching this from the wrong end. Let's just > > ignore all of the proposed interfaces for now and only concentrate on > > what this will be used for. > > > > Start with a set of use cases by all interested parties. E.g. > > > > - systemd wants to keep track attached mounts in a namespace, as well > > as new detached mounts created by fsmount() > > > > - systemd need to keep information (such as parent, children, mount > > flags, fs options, etc) up to date on any change of topology or > > attributes. > > - We also have code that recursively remounts r/o or unmounts some > directory tree (with filters), Recursive remount-ro is clear. What is not clear is whether you need to do this for hidden mounts (not possible from userspace without a way to disable mount following on path lookup). Would it make sense to add a kernel API for recursive setting of mount flags? What exactly is this unmount with filters? Can you give examples? > - We also have code that needs to check if /dev/ is plain tmpfs or > devtmpfs. We cannot use statfs for that, since in both cases > TMPFS_MAGIC is reported, hence we currently parse > /proc/self/mountinfo for that to find the fstype string there, which > is different for both cases. Okay. Thanks, Miklos