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[100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 35EF540DA6; Tue, 20 Jan 2026 12:50:40 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0AB6DC19422; Tue, 20 Jan 2026 12:50:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768913440; bh=vYIy05TFBjkJSOZIJnTTiuPvxldNCSA6NdRoiqFyWpc=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=e1zT2vocwF78r7FL+5WzUHXIWy93w1CsZBXidVyUfNwBF1R9uERAYCeSR1I34Y48P fZA1tCB9PdNKfqEm3Ik3seHCB/fan0+KTHSp4ICQcZWb4Gsp7fA/RoVS82lqp79kRd W7mln6Md5hIUxVVb8qiRT771zybKHVixN7sK/3MvHcNmuLW8t/T6Y7VNyfw+KedZu2 MWac278JPjkATMe0Lt9fyxkUXmIhbV7lpHHCqOA3jzbt0OyYU8ZgSAhX24ZaUONwTl Nw2byKXBQA/Ft6+a+bhzFkGe4xozYyiwbpysp6m+qMx0ppPTXrqrWjl6fl5eTnA+Jf bsFdLxD14gb5w== Message-ID: Subject: Re: [PATCH 00/29] fs: require filesystems to explicitly opt-in to nfsd export support From: Jeff Layton To: Christian Brauner , NeilBrown Cc: Christoph Hellwig , Amir Goldstein , Alexander Viro , Chuck Lever , Olga Kornievskaia , Dai Ngo , Tom Talpey , Hugh Dickins , Baolin Wang , Andrew Morton , Theodore Ts'o , Andreas Dilger , Jan Kara , Gao Xiang , Chao Yu , Yue Hu , Jeffle Xu , Sandeep Dhavale , Hongbo Li , Chunhai Guo , Carlos Maiolino , Ilya Dryomov , Alex Markuze , Viacheslav Dubeyko , Chris Mason , David Sterba , Luis de Bethencourt , Salah Triki , Phillip Lougher , Steve French , Paulo Alcantara , Ronnie Sahlberg , Shyam Prasad N , Bharath SM , Miklos Szeredi , Mike Marshall , Martin Brandenburg , Mark Fasheh , Joel Becker , Joseph Qi , Konstantin Komarov , Ryusuke Konishi , Trond Myklebust , Anna Schumaker , Dave Kleikamp , David Woodhouse , Richard Weinberger , Jan Kara , Andreas Gruenbacher , OGAWA Hirofumi , Jaegeuk Kim , linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-ext4@vger.kernel.org, linux-erofs@lists.ozlabs.org, linux-xfs@vger.kernel.org, ceph-devel@vger.kernel.org, linux-btrfs@vger.kernel.org, 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k7976VnrZH3GtrA7drCeoX2BkrQBIyjbJ7sbWXdwlGNOhbK5H/YrFfc2e4zg7X7f+DT0pGFDaQatrYQNWgonQT74RKSUrrDvlUg32GWGAWIhFzQ1elEDqBqOa251YdTXFr8u7zI5I0CH2IGS0N9X8YjtZuqkqlJ21MU7q2neteuOWlswyzcsuAN8892/bsFBVffgUAfwr/TiQshEHKoZhsKaRyvouvXKLZn0mmogIjnQ2xGIpTa4dFd9mMK0xs6b15tsUXsKI28EbR6PFrst/d/MqPQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, 2026-01-20 at 11:31 +0100, Christian Brauner wrote: > On Tue, Jan 20, 2026 at 08:41:50PM +1100, NeilBrown wrote: > > On Tue, 20 Jan 2026, Christian Brauner wrote: > > > On Tue, Jan 20, 2026 at 07:45:35AM +1100, NeilBrown wrote: > > > > On Mon, 19 Jan 2026, Christian Brauner wrote: > > > > > On Mon, Jan 19, 2026 at 06:22:42PM +1100, NeilBrown wrote: > > > > > > On Mon, 19 Jan 2026, Christoph Hellwig wrote: > > > > > > > On Mon, Jan 19, 2026 at 10:23:13AM +1100, NeilBrown wrote: > > > > > > > > > This was Chuck's suggested name. His point was that STABL= E means that > > > > > > > > > the FH's don't change during the lifetime of the file. > > > > > > > > >=20 > > > > > > > > > I don't much care about the flag name, so if everyone lik= es PERSISTENT > > > > > > > > > better I'll roll with that. > > > > > > > >=20 > > > > > > > > I don't like PERSISTENT. > > > > > > > > I'd rather call a spade a spade. > > > > > > > >=20 > > > > > > > > EXPORT_OP_SUPPORTS_NFS_EXPORT > > > > > > > > or > > > > > > > > EXPORT_OP_NOT_NFS_COMPATIBLE > > > > > > > >=20 > > > > > > > > The issue here is NFS export and indirection doesn't bring = any benefits. > > > > > > >=20 > > > > > > > No, it absolutely is not. And the whole concept of calling s= omething > > > > > > > after the initial or main use is a recipe for a mess. > > > > > >=20 > > > > > > We are calling it for it's only use. If there was ever another= use, we > > > > > > could change the name if that made sense. It is not a public n= ame, it > > > > > > is easy to change. > > > > > >=20 > > > > > > >=20 > > > > > > > Pick a name that conveys what the flag is about, and document= those > > > > > > > semantics well. This flag is about the fact that for a given= file, > > > > > > > as long as that file exists in the file system the handle is = stable. > > > > > > > Both stable and persistent are suitable for that, nfs is ever= ything > > > > > > > but. > > > > > >=20 > > > > > > My understanding is that kernfs would not get the flag. > > > > > > kernfs filehandles do not change as long as the file exist. > > > > > > But this is not sufficient for the files to be usefully exporte= d. > > > > > >=20 > > > > > > I suspect kernfs does re-use filehandles relatively soon after = the > > > > > > file/object has been destroyed. Maybe that is the real problem= here: > > > > > > filehandle reuse, not filehandle stability. > > > > > >=20 > > > > > > Jeff: could you please give details (and preserve them in futur= e cover > > > > > > letters) of which filesystems are known to have problems and wh= at > > > > > > exactly those problems are? > > > > > >=20 > > > > > > >=20 > > > > > > > Remember nfs also support volatile file handles, and other ap= plications > > > > > > > might rely on this (I know of quite a few user space applicat= ions that > > > > > > > do, but they are kinda hardwired to xfs anyway). > > > > > >=20 > > > > > > The NFS protocol supports volatile file handles. knfsd does no= t. > > > > > > So maybe > > > > > > EXPORT_OP_NOT_NFSD_COMPATIBLE > > > > > > might be better. or EXPORT_OP_NOT_LINUX_NFSD_COMPATIBLE. > > > > > > (I prefer opt-out rather than opt-in because nfsd export was th= e > > > > > > original purpose of export_operations, but it isn't something > > > > > > I would fight for) > > > > >=20 > > > > > I prefer one of the variants you proposed here but I don't partic= ularly > > > > > care. It's not a hill worth dying on. So if Christoph insists on = the > > > > > other name then I say let's just go with it. > > > > >=20 > > > >=20 > > > > This sounds like you are recommending that we give in to bullying. > > > > I would rather the decision be made based on the facts of the case,= not > > > > the opinions that are stated most bluntly. > > > >=20 > > > > I actually think that what Christoph wants is actually quite differ= ent > > > > from what Jeff wants, and maybe two flags are needed. But I don't = yet > > > > have a clear understanding of what Christoph wants, so I cannot be = sure. > > >=20 > > > I've tried to indirectly ask whether you would be willing to compromi= se > > > here or whether you want to insist on your alternative name. Apparent= ly > > > that didn't come through. > >=20 > > This would be the "not a hill worthy dying on" part of your statement. > > I think I see that implication now. > > But no, I don't think compromise is relevant. I think the problem > > statement as originally given by Jeff is misleading, and people have > > been misled to an incorrect name. > >=20 > > >=20 > > > I'm unclear what your goal is in suggesting that I recommend "we" giv= e > > > into bullying. All it achieved was to further derail this thread. > > >=20 > >=20 > > The "We" is the same as the "us" in "let's just go with it". > >=20 > >=20 > > > I also think it's not very helpful at v6 of the discussion to start > > > figuring out what the actual key rift between Jeff's and Christoph's > > > position is. If you've figured it out and gotten an agreement and thi= s > > > is already in, send a follow-up series. > >=20 > > v6? v2 was posted today. But maybe you are referring the some other > > precursors. > >=20 > > The introductory statement in v2 is > >=20 > > This patchset adds a flag that indicates whether the filesystem supp= orts > > stable filehandles (i.e. that they don't change over the life of the > > file). It then makes any filesystem that doesn't set that flag > > ineligible for nfsd export. > >=20 > > Nobody else questioned the validity of that. I do. > > No evidence was given that there are *any* filesystems that don't > > support stable filehandles. The only filesystem mentioned is cgroups > > and it DOES provide stable filehandles. >=20 Across reboot? Not really. It's quite possible that we may end up with the same "id" numbers in cgroupfs on a new incarnation of the filesystem after a reboot. The files in there are not the same ones as the ones before, but their filehandles may match because kernfs doesn't factor in an i_generation number. Could we fix it by adding a random i_generation value or something? Possibly, but there really isn't a good use-case that I can see for allowing cgroupfs to be exported via nfsd. Best to disallow it until someone comes up with one. > Oh yes we did. And this is a merry-go-round. >=20 > It is very much fine for a filesystems to support file handles without > wanting to support exporting via NFS. That is especially true for > in-kernel pseudo filesystems. >=20 > As I've said before multiple times I want a way to allow filesystems > such as pidfs and nsfs to use file handles without supporting export. > Whatever that fscking flag is called at this point I fundamentally don't > care. And we are reliving the same arguments over and over. >=20 > I will _hard NAK_ anything that starts mandating that export of > filesystems must be allowed simply because their file handles fit export > criteria. I do not care whether pidfs or nsfs file handles fit the bill. > They will not be exported. I don't really care what we call the flag. I do care a little about what its semantics are, but the effect should be to ensure that fs maintainers make a conscious decision about whether nfsd export should be allowed on the filesystem.=C2=A0 At this point, maybe we should just go with Neil's=20 EXPORT_OP_SUPPORTS_NFS_EXPORT or something. It's much more arbitrary, than trying to base this on criteria about filehandle stability, but it would give us the effect we want. --=20 Jeff Layton