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 6868136B058; Tue, 11 Nov 2025 15:45:29 +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=1762875929; cv=none; b=Kg7Y9pgY/i/g+qZciV6G9WnpC43YCRnZ71Kv40FrRL8x2lc4fq1758T2ALf1OoxcFK89l/baa/Q8ajSrK3/TwnFl+OVkTYjycEXLN4hCPoSbU4/rIPJBgr6hw1xeiAoZQJp6EXANTlfzJaGxin+MKvRMTgskJvnI9MqMcJCOJXg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762875929; c=relaxed/simple; bh=Ui1o3y6I3y4gMn3T7HSz/k5nLl8SJqOoJXFt0V4dVKI=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=eE/UoyX9InAIAYx92rnve9kkPMMDJgK4UUjTnxxZG5PaVLTR693c5uGhZSTPcjqbYb33kYh+kly7c5g7zDkUL3kOtQN1yPfWq0OdKjgLPRlzku9RinnKHHxnd06pewXTx1gBtDMhreMKYi9MeszEipMD4H3xEyuBSlzmmmUPX+o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=MNaeiGZp; 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="MNaeiGZp" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6DDB7C4CEF7; Tue, 11 Nov 2025 15:45:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1762875928; bh=Ui1o3y6I3y4gMn3T7HSz/k5nLl8SJqOoJXFt0V4dVKI=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=MNaeiGZpBCwvOZAlzKKnohYtt+WNQ6NyBPOjF8iS36M6PGV8PsRyXX/q9JxeR3fJK fxneH3DDvuRsPtlhwHr3zb4IdvwgYCp/ZfOYvurhbwsehwU8fETXXjAg7BQq+RXZU9 fC3HxnYEJ0y6Y8f324iG1O6fY2zj2fgHGG2prAWGJv5zbhFdLxKBEizesTrRFR1+Al BFWEo7ZxI1481aKWe9DoWQHbQSOzSYVIwoWJkNOWqglyDiX8mrXNvNwGoidoCAKlTe QUCc4m5/KBp2P8I61VjjFFO/np+8m/xKDU41H5AkQ7EGRm+C+fLzxrpZZ/63IIKmtK eCS0hwHL3cYgw== Message-ID: Subject: Re: [Patch 0/2] NFSD: Fix server hang when there are multiple layout conflicts From: Jeff Layton To: Dai Ngo , Benjamin Coddington Cc: chuck.lever@oracle.com, neilb@ownmail.net, okorniev@redhat.com, tom@talpey.com, hch@lst.de, alex.aring@gmail.com, viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org Date: Tue, 11 Nov 2025 10:45:26 -0500 In-Reply-To: References: <20251106170729.310683-1-dai.ngo@oracle.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Tue, 2025-11-11 at 07:24 -0800, Dai Ngo wrote: > Hi Ben, >=20 > On 11/9/25 10:34 AM, Benjamin Coddington wrote: > > On 6 Nov 2025, at 12:05, Dai Ngo wrote: > >=20 > > > When a layout conflict triggers a call to __break_lease, the function > > > nfsd4_layout_lm_break clears the fl_break_time timeout before sending > > > the CB_LAYOUTRECALL. As a result, __break_lease repeatedly restarts > > > its loop, waiting indefinitely for the conflicting file lease to be > > > released. > > >=20 > > > If the number of lease conflicts matches the number of NFSD threads (= which > > > defaults to 8), all available NFSD threads become occupied. Consequen= tly, > > > there are no threads left to handle incoming requests or callback rep= lies, > > > leading to a total hang of the NFS server. > > >=20 > > > This issue is reliably reproducible by running the Git test suite on = a > > > configuration using SCSI layout. > > >=20 > > > This patchset fixes this problem by introducing the new lm_breaker_ti= medout > > > operation to lease_manager_operations and using timeout for layout > > > lease break. > > Hey Dai, > >=20 > > I like your solution here, but I worry it can cause unexpected or > > unnecessary client fencing when the problem is server-side (not enough > > threads). Clients might be dutifully sending LAYOUTRETURN, but the ser= ver > > can't service them >=20 > I agreed. This is a server problem and we penalize the client. We need > a long term solution for dealing resource shortage (server threads) > problem. >=20 > Fortunately, the client can detect reservation conflict errors and appear= s > to retry the I/O. Also, the client will ask for new layout and in the > process it re-registers its reservation key so I/O will continue. >=20 > > - and this change will cause some potentially unexpected > > fencing in environments where things could be fixed (by adding more knf= sd > > threads). > > Also, I think we significantly bumped default thread counts > > recently in nfs-utils: > > eb5abb5c60ab (tag: nfs-utils-2-8-2-rc3) nfsd: dump default number of th= reads to 16 >=20 > This helps a bit but if there is always a chance that there is a load > that requires more than the number of server threads. >=20 > >=20 > > You probably have already seen previous discussions about this: > > https://urldefense.com/v3/__https://lore.kernel.org/linux-nfs/1CC82EC5-= 6120-4EE4-A7F0-019CF7BC762C@redhat.com/__;!!ACWV5N9M2RV99hQ!Pq4vHQs-qk71XjZ= 0vOkONTD7nxkuyUUEKTBsJJ0L_OrFWudokphCyc2V0q0_OrNoGD3KnsgoHKp7rb_lDcs$ > >=20 > > This also changes the behavior for all layouts, I haven't thought throu= gh > > the implications of that - but I wish we could have knob for this behav= ior, > > or perhaps a knfsd-specific fl_break_time tuneable. >=20 > There is already a knob to tune the fl_break_time: > # cat /proc/sys/fs/lease-break-time >=20 > but currently lease-break-time is in seconds so the minimum we can set > is 1 which I think is still too long to tight up a server thread. >=20 > >=20 > > Last thought (for now): I think Neil has some work for dynamic knfsd th= read > > count.. or Jeff? (I am having trouble finding it) Would that work arou= nd > > this problem? >=20 It would help, up to a point, but so does increasing the static thread count. Even if we get dynamically sized threadpool, we'll likely still have a hard cap on the number of threads. We'll always going to be subject to this if VFS operations are going to be blocking on delegation breaks. > This would help, and I prefer this route rather than rework __break_lease > to return EAGAIN/jukebox while the server recalling the layout. >=20 One way I can see to address this properly is to allow for non-blocking lease breaks in some fashion. Basically, have the fs return -EAGAIN (maybe after a short wait) at some point so that maybe LAYOUTRETURN can get through once the client retries). Plumbing that intent down to the actual break_layout() calls is a problem though -- that's a lot of layers. I wonder if we need some per- task flag that tells the layout engine "always do non-blocking lease breaks"? That sounds pretty ugly too. The only other way I could see to fix this is to move to an asynchronous model of some sort. IOW, have at least some operations (anything that could conceivably cause a layout break) done asynchronously. Then you could dispatch the operation and put the rqstp on a some sort of waitqueue, and then let the thread do more work instead of blocking. When the work is done, just requeue the rqstp to send the reply. Just thinking out loud, but maybe we could use io_uring's underlying infrastructure for this? Basically, set up an io_uring but do it all in kernel space in nfsd thread context? --=20 Jeff Layton