From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id F3CA1EAC6; Sun, 9 Feb 2025 17:05:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739120724; cv=none; b=upMWbHGUPfR+nktN6lHwfede0QKHUorJ185CI4YSDzntc72NhemQ4HP7tx1mxsqriwXksQoB6U0KhLgLvZsKNHJwzKqF585GChUcZ5FKZ/UWs0KBcfzlFqxpGRNt6Rmqr2p7fRyn+MjEKxGYe+pg8OqCC7JKYpD7h8S3pLqGB6k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1739120724; c=relaxed/simple; bh=2qdYcrcx4kG+29As4PpoSVVefggjkuBEXHtbUQZjYws=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=XQpzXiGbTnZ3SNqwtzF6R6kRSTSIppatUsMzd1HTL2ArPJGDQ4FOlDFRj2ml+Fu434Z5wkNxmD3cHAqYskYSl5Fshyf8Fbi+MHchfYlTa2Z73Cd6+rDDe3XkRlSKVlKJlj0JRquRBc/yrtbrCjQ0bSZ0u/LHAbOpWDitMoiQ5/0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=F8XwX3eY; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="F8XwX3eY" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A37B6C4CEDD; Sun, 9 Feb 2025 17:05:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1739120723; bh=2qdYcrcx4kG+29As4PpoSVVefggjkuBEXHtbUQZjYws=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=F8XwX3eYixTbH/WOwC2Ws8c5DOCFGSSlqKW5iVrYaNcHSVYk/a5rrqekzTadfXlaW H0XTG38EqRzKCP6RtYfHmjlK3hxofi7qItaegAbDYS4+2tEksFABz3rqUJcYQn6J7T gFtMJXlYpQ4mpQQ3vf8JswFIBGt9hkrzONCp3pcS4tVUz2f/o4kuvw3CG6luj4cfWj cm4reBAVqIzz3AJGHBlPBbTW5+LhQagOwgEZXiMMWW/A2VL0x0gZ96vRFbK2EN4/5w HsFTdyuhirzkShiDWeqJr6fhvYzl2XwPz64MrZT1DhVz5XTKtYsfo7cxmL6aX4sOwJ hCUWwyRBIrN2A== Message-ID: <76d23f2ad2d45f04cc60711cdf60dc59e7d0ae77.camel@kernel.org> Subject: Re: [PATCH v5 6/7] nfsd: handle CB_SEQUENCE NFS4ERR_SEQ_MISORDERED error better From: Jeff Layton To: Tom Talpey , Chuck Lever , Neil Brown , Olga Kornievskaia , Dai Ngo , "J. Bruce Fields" Cc: linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org Date: Sun, 09 Feb 2025 12:05:21 -0500 In-Reply-To: References: <20250207-nfsd-6-14-v5-0-f3b54fb60dc0@kernel.org> <20250207-nfsd-6-14-v5-6-f3b54fb60dc0@kernel.org> <28174296-129d-4459-aa23-a94bbf00d257@oracle.com> <3e4d14075482489cd010e4ea621c0bd368700e27.camel@kernel.org> <40970e33-4689-4623-a423-b346e739ba80@talpey.com> <66532654ca25280ffa30168a977601ba4a37aaab.camel@kernel.org> <29e739f1-2d85-40c2-a549-5ab9d71686b0@talpey.com> <35cae0eb73781bb36c49aed2c2bc49a808698635.camel@kernel.org> <2f9fe86f-b49c-460c-bf2e-fed97970952d@oracle.com> <7da740d0-1e4f-4e1b-986f-9516c8286d19@talpey.com> <6606c3bb229513af8a8e1b4cc398aa6e72257666.camel@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv 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linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Sun, 2025-02-09 at 11:58 -0500, Tom Talpey wrote: > On 2/9/2025 11:51 AM, Jeff Layton wrote: > > On Sun, 2025-02-09 at 11:26 -0500, Tom Talpey wrote: > > > On 2/8/2025 9:14 PM, Jeff Layton wrote: > > > > On Sat, 2025-02-08 at 20:24 -0500, Tom Talpey wrote: > > > > > On 2/8/2025 4:07 PM, Chuck Lever wrote: > > > > > > On 2/8/25 3:45 PM, Jeff Layton wrote: > > > > > > > On Sat, 2025-02-08 at 14:18 -0500, Tom Talpey wrote: > > > > > > > > On 2/8/2025 11:08 AM, Jeff Layton wrote: > > > > > > > > > On Sat, 2025-02-08 at 13:40 -0500, Tom Talpey wrote: > > > > > > > > > > On 2/8/2025 10:02 AM, Jeff Layton wrote: > > > > > > > > > > > On Sat, 2025-02-08 at 12:01 -0500, Chuck Lever wrote: > > > > > > > > > > > > On 2/7/25 4:53 PM, Jeff Layton wrote: > > > > > > > > > > > > > For NFS4ERR_SEQ_MISORDERED, do one attempt with a= seqid of 1, and then > > > > > > > > > > > > > fall back to treating it like a BADSLOT if that f= ails. > > > > > > > > > > > > >=20 > > > > > > > > > > > > > Signed-off-by: Jeff Layton > > > > > > > > > > > > > --- > > > > > > > > > > > > > fs/nfsd/nfs4callback.c | 16 ++++++++++-----= - > > > > > > > > > > > > > 1 file changed, 10 insertions(+), 6 deletio= ns(-) > > > > > > > > > > > > >=20 > > > > > > > > > > > > > diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs= 4callback.c > > > > > > > > > > > > > index 10067a34db3afff8d4e4383854ab9abd9767c2d6..d= 6e3e8bb2efabadda9f922318880e12e1cb2c23f 100644 > > > > > > > > > > > > > --- a/fs/nfsd/nfs4callback.c > > > > > > > > > > > > > +++ b/fs/nfsd/nfs4callback.c > > > > > > > > > > > > > @@ -1393,6 +1393,16 @@ static bool nfsd4_cb_seque= nce_done(struct rpc_task *task, struct nfsd4_callback > > > > > > > > > > > > > goto requeue; > > > > > > > > > > > > > rpc_delay(task, 2 * HZ); > > > > > > > > > > > > > return false; > > > > > > > > > > > > > + case -NFS4ERR_SEQ_MISORDERED: > > > > > > > > > > > > > + /* > > > > > > > > > > > > > + * Reattempt once with seq_nr 1. If that fails= , treat this > > > > > > > > > > > > > + * like BADSLOT. > > > > > > > > > > > > > + */ > > > > > > > > > > > >=20 > > > > > > > > > > > > Nit: this comment says exactly what the code says. = If it were me, I'd > > > > > > > > > > > > remove it. Is there a "why" statement that could be= made here? Like, > > > > > > > > > > > > why retry with a seq_nr of 1 instead of just failin= g immediately? > > > > > > > > > > > >=20 > > > > > > > > > > >=20 > > > > > > > > > > > There isn't one that I know of. It looks like Kinglon= g Mee added it in > > > > > > > > > > > 7ba6cad6c88f, but there is no real mention of that in= the changelog. > > > > > > > > > > >=20 > > > > > > > > > > > TBH, I'm not enamored with this remedy either. What i= f the seq_nr was 2 > > > > > > > > > > > when we got this error, and we then retry with a seq_= nr of 1? Does the > > > > > > > > > > > server then treat that as a retransmission? > > > > > > > > > >=20 > > > > > > > > > > So I assume you mean the requester sent seq_nr 1, saw a= reply and sent a > > > > > > > > > > subsequent seq_nr 2, to which it gets SEQ_MISORDERED. > > > > > > > > > >=20 > > > > > > > > > > If so, yes definitely backing up the seq_nr to 1 will r= esult in the > > > > > > > > > > peer considering it to be a retransmission, which will = be bad. > > > > > > > > > >=20 > > > > > > > > >=20 > > > > > > > > > Yes, that's what I meant. > > > > > > > > >=20 > > > > > > > > > > > We might be best off > > > > > > > > > > > dropping this and just always treating it like BADSLO= T. > > > > > > > > > >=20 > > > > > > > > > > But, why would this happen? Usually I'd think the peer = sent seq_nr X > > > > > > > > > > before it received a reply to seq_nr X-1, which would b= e a peer bug. > > > > > > > > > >=20 > > > > > > > > > > OTOH, SEQ_MISORDERED is a valid response to an in-progr= ess retry. So, > > > > > > > > > > how does the requester know the difference? > > > > > > > > > >=20 > > > > > > > > > > If treating it as BADSLOT completely resets the sequenc= e, then sure, > > > > > > > > > > but either a) the request is still in-progress, or b) i= f a bug is > > > > > > > > > > causing the situation, well it's not going to converge = on a functional > > > > > > > > > > session. > > > > > > > > > >=20 > > > > > > > > >=20 > > > > > > > > > With this patchset, on BADSLOT, we'll set SEQ4_STATUS_BAC= KCHANNEL_FAULT > > > > > > > > > in the next forechannel SEQUENCE on the session. That sho= uld cause the > > > > > > > > > client to (eventually) send a DESTROY_SESSION and create = a new one. > > > > > > > > >=20 > > > > > > > > > Unfortunately, in the meantime, because of the way the ca= llback channel > > > > > > > > > update works, the server can end up trying to send the ca= llback again > > > > > > > > > on the same session (and maybe more than once). I'm not s= ure that > > > > > > > > > that's a real problem per-se, but it's less than ideal. > > > > > > > > >=20 > > > > > > > > > > Not sure I have a solid suggestion right now. Whatever = the fix, it > > > > > > > > > > should capture any subtlety in a comment. > > > > > > > > > >=20 > > > > > > > > >=20 > > > > > > > > > At this point, I'm leaning toward just treating it like B= ADSLOT. > > > > > > > > > Basically, mark the backchannel faulty, and leak the slot= so that > > > > > > > > > nothing else uses it. That allows us to send backchannel = requests on > > > > > > > > > the other slots until the session gets recreated. > > > > > > > >=20 > > > > > > > > Hmm, leaking the slot is a workable approach, as long as it= doesn't > > > > > > > > cascade more than a time or two. Some sort of trigger shoul= d be armed > > > > > > > > to prevent runaway retries. > > > > > > > >=20 > > > > > > > > It's maybe worth considering what state the peer might be i= n when this > > > > > > > > happens. It too may effectively leak a slot, and if is reta= ining some > > > > > > > > bogus state either as a result of or because of the previou= s exchange(s) > > > > > > > > then this may lead to future hangs/failures. Not pretty, an= d maybe not > > > > > > > > worth trying to guess. > > > > > > > >=20 > > > > > > > > Tom. > > > > > > > >=20 > > > > > > >=20 > > > > > > >=20 > > > > > > > The idea here is that eventually the client should figure out= that > > > > > > > something is wrong and reestablish the session. Currently we = don't > > > > > > > limit the number of retries on a callback. > > > > > > >=20 > > > > > > > Maybe they should time out after a while? If we've retried a = callback > > > > > > > for more than two lease periods, give up and log something? > > > > > > >=20 > > > > > > > Either way, I'd consider that to be follow-on work to this se= t. > > > > > >=20 > > > > > > As a general comment, I think making a heroic effort to recover= in any > > > > > > of these cases is probably not worth the additional complexity.= Where it > > > > > > is required or where we believe it is worth the trouble, that's= where we > > > > > > want a detailed comment. > > > > > >=20 > > > > > > What we want to do is ensure forward progress. I'm guessing tha= t error > > > > > > conditions are going to be rare, so leaking the slot until a ce= rtain > > > > > > portion of them are gone, and then indicating a session fault t= o force > > > > > > the client to start over from scratch, is probably the most > > > > > > straightforward approach. > > > > > >=20 > > > > > > So, is there a good reason to retry? There doesn't appear to be= any > > > > > > reasoning mentioned in the commit log or in nearby comments. > > > > >=20 > > > > > Agreed on the general comment. > > > > >=20 > > > > > As for the "any reason to retry" - maybe. If it's a transient err= or we > > > > > don't want to give up early. Unfortunately that appears to be an > > > > > ambiguous situation, because SEQ_MISORDERED is allowed in place o= f > > > > > ERR_DELAY. I don't have any great suggestion however. > > > > >=20 > > > >=20 > > > > IMO, we should retry callbacks (basically) indefinitely, unless the > > > > NFSv4 client is being torn down (i.e. lease expires or an unmount > > > > happened, etc). > > > >=20 > > > > > Jeff, to your point that the "client should figure out something = is > > > > > wrong", I'm not sure how you think that will happen. If the serve= r is > > > > > making a delegation recall and the client receive code chooses to= reject > > > > > it at the sequence check, how would that eventually cause the cli= ent to > > > > > reestablish the session (on the forechannel)? > > > > >=20 > > > > >=20 > > > >=20 > > > > In the BADSLOT case, it calls nfsd4_mark_cb_fault(cb->cb_clp), whic= h > > > > sets a flag in the client that makes it set > > > > SEQ4_STATUS_BACKCHANNEL_FAULT in the next SEQUENCE call. > > >=20 > > > Aha, that's good. RFC8881 only mentions it twice, but it's normative: > > >=20 > > > SEQ4_STATUS_BACKCHANNEL_FAULT > > > The server has encountered an unrecoverable fault with the > > > backchannel (e.g., it has lost track of the sequence ID for a s= lot > > > in the backchannel). The client MUST stop sending more requests= on > > > the session's fore channel, wait for all outstanding requests t= o > > > complete on the fore and back channel, and then destroy the ses= sion. > > >=20 > > > I guess my question is, what if the client ignores it anyway? What > > > server code actually forces the recovery? > > >=20 > > > Tom. > > >=20 > >=20 > > I don't think there is anything that does this right now. Does the RFC > > mention what the server should do if that happens? I suppose the server > > could just unilaterally destroy the session at some point, and force > > the client to reestablish it. >=20 > Nope. :( Like many other requirements, it's unenforced normatively. >=20 > Perhaps we could consider this as an erratum, but it's more of an > omission. Because of that, it may need IETF discussion ("at some point" > needs a MUST). I'll volunteer to open an issue, if you agree to discuss! Sure. I don't have a whole lot to add, but my proposal would be: If the server sends a SEQUENCE reply with SEQ4_STATUS_BACKCHANNEL_FAULT set, then the client has another lease period in which to reestablish the session. After that, the server may unilaterally drop the session and start returning NFS4ERR_BADSESSION to attempts to use it. --=20 Jeff Layton