From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 24F424DC557 for ; Thu, 17 Sep 2026 13:00:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789650062; cv=none; b=MYOwjkSCjso/0IRPZrAQz40KcAr3dfuFUJunkF44VXrL0sz2Ih5dQHGVlSj41G0d0hJhITGEAXNxol80hN0RUwOqJHTxieHs5vZv42NosbbuBEFZOkqd3sncRFDclh//WawAKAoyKV/5FqFD03rdZcYizaj6KLDh8CokVj/a6/A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789650062; c=relaxed/simple; bh=wiqvEsnUyt/DLgmxdRvqAgHoOncVuQ0RrPXIs2+RiBk=; h=Message-ID:Subject:From:To:Date:In-Reply-To:References: Content-Type:MIME-Version; b=nn4sBqhI+vi6rUIteUUcOe0I2drD3ENBajCLJXJBBQQznvXBEKQd3dhzeaB3+lhuHtxsrEmKJp9pxXpBhwE6Ot7UV5kJp85A/aHq1ovS3RL5/SifjZcwvGz60M75tC2HknN1jbDpFDye+5yZfCyvKaUbk4EbkWuZAj/7ZMQSlwc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=IVYAa9cF; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="IVYAa9cF" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 602EB1F00893; Thu, 17 Sep 2026 13:00:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789650050; bh=6hpj2jS1l7NAIErG+xuvBKbF8zFZvAfmF8FZvZuYm04=; h=Subject:From:To:Date:In-Reply-To:References; b=IVYAa9cF1m6PhhWvOfMu7nvXnFxVL4YsM8aAFSQdY+50y93hcw1dptOliY7IXb4Yo tsWMvLvCmZSOkavUZT3rQFcpW/lZZHymTXakJ+rQ4J97in09UPMP2JaRnCo+2K6UqW aNDRJ2vOhB66EamG9qwfF8+eiQqaB90+sSpwzckZccle1/T/7RecomqJNslrfMYVH7 I0bSur5MLHpIafDSUuquVUsuPItPKfMgt5UwiAizo+5hh+Ht3EGBXpB/fj0jZd1TG8 +4z1P2Nb/f1uxmrheN4yX03n0wn8hHdUM1PKr99Vfy8XjCRaHMzzHapI982wWXRlYp pMLGto1Arforg== Message-ID: <4b64ead0ecd81596f8c3fbca5a7ac385c07cc8c7.camel@kernel.org> Subject: Re: Need NFSv4.1 "WANT_DELEGATION" op ... From: Jeff Layton To: Roland Mainz , ms-nfs41-client-devel@lists.sourceforge.net, Linux NFS Mailing List Date: Thu, 17 Sep 2026 09:00:49 -0400 In-Reply-To: References: Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn VluJH0/RcxEeFbnSaswVChNSGa7mXJrTR22lRL6ZPjdMgS2Km90haWPRc8Wolcz07Y2se0xpGVLEQ cDEsvv5IMmeMe1/qLZ6NaVkNuL3WOXvxaVT9USW1+/SGipO2IpKJjeDZfehlB/kpfF24+RrK+seQf CBYyUE8QJpvTZyfUHNYldXlrjO6n5MdOempLqWpfOmcGkwnyNRBR46g/jf8KnPRwXs509yAqDB6sE 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AaM1LRC28muq91WCFhs/nzHozpbzcheyGtMUI2Ao4K6mnY+3zIuXPygZMFr9KXE6fF7HzKxKuZMJO aEZCiDOq0anx6FmOzs5E6Jqdpo/mtI8beK+BE7Va6ni7YrQlnT0i3vaTVMTiCThbqsB20VrbMjlhp f8lfK1XVNbRq/R7GZ9zHESlsa35ha60yd/j3pu5hT2xyy8krV8vGhHvnJ1XRMJBAB/UYb6FyC7S+m QZIQXVeAA+smfTT0tDrisj1U5x6ZB9b3nBg65kc= Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.2 (3.60.2-1.fc44) Precedence: bulk X-Mailing-List: linux-nfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Thu, 2026-09-17 at 01:57 +0200, Roland Mainz wrote: > Hi! >=20 > ---- >=20 > We're experimenting with a delegation-driven client-side > consitency+caching model for the ms-nfs41-client Windows NFSV4.2 > client a while now. >=20 > The original idea was to have two modes: > - Mode 1: Get delegations (read delegations for accessing data > read-only, write delegations for accessing data [a]) at OPEN time, and > return the delegation when the file handle gets closed ([b]). The > client only does buffering+caching if it has a delegation. > - Model 2: Do not request a delegation at file open/creation, instead > translate Win32 oplocks into WANT_DELEGATION+DELEGRETURN. The client > only does buffering+caching if it has a delegation. Server-side > revocation of the delegation triggers an oplock break, releasing the > oplock from the Win32 side just returns the delegation. This is > basically how Win32 oplocks work on the SMB level. >=20 > But in real life there are problems: > - "Mode 1" only worked for small testcases. Real Windows applications > typically change filehandle attributes or change ACLs after creating a > file, which results in server-side delegation recall triggered by a > NFS SETATTR, and after that we do not have a delegation. And there is > no WANT_DELEGATION op support in Linux+FreeBSD nfsd to get a > delegation back. Which typically means write delegations not being > enabled and therefore no client-side buffering/caching. > - "Mode 2" does not work because Linux+FreeBSD nfsd does not implement > WANT_DELEGATION to implement this "on-demand delegation" model. >=20 > Or short: For "mode 1" or "mode 2" to work we need the NFSV4.1 > operation "WANT_DELEGATION" to work, at least in syncronous mode (e.g. > no CB_PUSH_DELEG support needed for now [c]). >=20 > @Chuck Lever What do you think ? >=20 > [a] Win32 also supports opening a file for attributes only, which > means no delegation gets requested > [b] Note that WIndows supports close-delayed files, which means that > calling |CloseHandle()| will flush outstanding dirty data, but keeps > the file "active" in the driver for re-use for some time (except on > delete/rename/etc.). For the NFS driver this means that locks are > returned, but the delegation+buffering stays untouched. > [c] Technically requesting WANT_DELEGATION async via CB_PUSH_DELEG > would be a welcome optimisation, because it could help dealing with > files with high contention between machines. >=20 I think we could probably do a trivial WANT_DELEGATION implementation (with no callbacks) in knfsd without too much trouble. We have the infrastructure to handle granting delegations at OPEN, so we could just factor that out for WANT_DELEGATION. Registering a callback to acquire and grant the delegation however is quite a bit more tricky to handle, and opens a lot of questions about how to handle competing WANT_DELEGATIONs from different clients. --=20 Jeff Layton