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Date: Wed, 12 Aug 2020 14:43:32 +0200 From: Miklos Szeredi <miklos@...redi.hu> To: Karel Zak <kzak@...hat.com> Cc: Steven Whitehouse <swhiteho@...hat.com>, David Howells <dhowells@...hat.com>, Linus Torvalds <torvalds@...ux-foundation.org>, linux-fsdevel <linux-fsdevel@...r.kernel.org>, Al Viro <viro@...iv.linux.org.uk>, Jeff Layton <jlayton@...hat.com>, Miklos Szeredi <mszeredi@...hat.com>, Nicolas Dichtel <nicolas.dichtel@...nd.com>, Christian Brauner <christian@...uner.io>, Lennart Poettering <lennart@...ttering.net>, Linux API <linux-api@...r.kernel.org>, Ian Kent <raven@...maw.net>, LSM <linux-security-module@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: file metadata via fs API On Wed, Aug 12, 2020 at 1:28 PM Karel Zak <kzak@...hat.com> wrote: > The proposal is based on paths and open(), how do you plan to deal > with mount IDs? David's fsinfo() allows to ask for mount info by mount > ID and it works well with mount notification where you get the ID. The > collaboration with notification interface is critical for our use-cases. One would use the notification to keep an up to date set of attributes for each watched mount, right? That presumably means the mount ID <-> mount path mapping already exists, which means it's just possible to use the open(mount_path, O_PATH) to obtain the base fd. If that assumption is not true, we could add a new interface for opening the root of the mount by ID. Fsinfo uses the dfd as a root for checking connectivity and the filename as the mount ID + a special flag indicating that it's not "dfd + path" we are dealing with but "rootfd + mntid". That sort of semantic multiplexing is quite ugly and I wouldn't suggest doing that with openat(2). A new syscall that returns an fd pointing to the root of the mount might be the best solution: int open_mount(int root_fd, u64 mntid, int flags); Yeah, yeah this is adding just another syscall interface, but notice how: a) it does one simple thing, no multiplexing at all b) is general purpose, and could be used for example in conjunction with open_by_handle_at(2), that also requires an fd pointing to a mount. Thanks, Miklos
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