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 26B8B192D91; Mon, 30 Sep 2024 19:27:13 +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=1727724434; cv=none; b=K90MnHf0vFOgboOC4rMeoVkddNpWxid13MKEIdP2AsIUzoP6SNS/h1qIhZO8BeZaY4+CvL5u1ZKZ7yxb3ER00ijfbTDcBGXNEfM7UwzPq12Uk19Vp98/acxSr8oIwDRVJ24sTgMaw2sHbhS+XWu09IRnUBckFC9g5I9qgKthtIM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727724434; c=relaxed/simple; bh=/5wXr1oh9gufFfSYG1R2hwOhkyVnx3BomvZ9/lVLzIo=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=eSfTvdvdGTfuhrVzTn/gQ6oiHfyX3h8XB+Alf+E9sponCIOa/3Zu+WvDjs8v548zqj9aHJNdL/AKjQSZcuBHu837M+C/sn+aSEvprC1kLaHW8fxnq5axCYTHos5e4bxQ3y0ZyG5GfBqq1hbFmGLtdQI353B7ZJ8ZVKEgGWHytKM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=AK0TAJUJ; 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="AK0TAJUJ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EB838C4CED4; Mon, 30 Sep 2024 19:27:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1727724433; bh=/5wXr1oh9gufFfSYG1R2hwOhkyVnx3BomvZ9/lVLzIo=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=AK0TAJUJM3Xv49IK6XGCyH92xhnwOY2zorCgMuKqi+LttdoItINQPDXlkzdwHxTTF CLLSCLIMEFRGMUXv9UYBGiBlmrL8MbgXPcxfP5IDpuguKrlrFxv1SKZ1fzRn5zOEY5 6UlP24HTWIqyztmoECd5Z9pVkRSUdK/gSGRoPtvtJeHiyOkMQtsXkugqw001yq2Hmo 1rtFHzZfZta+4ulA1KQzQCfkRIylvZQdE2xUJb1YDXeCpGvZO0Uo6E0HeHQhS2Ilg2 IYLG8kBdyBQuhxHl8oP28QoJIK5BNZqQD+D8GxQ/64pdwvtLjigDbEsUJ+bSr2L1/B uYtSEDnDhz2pQ== Message-ID: <753938ef8e46e9f3d9ea7d977537cd8f5a6533b2.camel@kernel.org> Subject: Re: [PATCH v8 01/11] timekeeping: move multigrain timestamp floor handling into timekeeper From: Jeff Layton To: Thomas Gleixner , John Stultz , Stephen Boyd , Alexander Viro , Christian Brauner , Jan Kara , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Jonathan Corbet , Chandan Babu R , "Darrick J. Wong" , Theodore Ts'o , Andreas Dilger , Chris Mason , Josef Bacik , David Sterba , Hugh Dickins , Andrew Morton , Chuck Lever , Vadim Fedorenko Cc: Randy Dunlap , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org Date: Mon, 30 Sep 2024 15:27:09 -0400 In-Reply-To: <87bk050xb9.ffs@tglx> References: <20240914-mgtime-v8-0-5bd872330bed@kernel.org> <20240914-mgtime-v8-1-5bd872330bed@kernel.org> <87bk050xb9.ffs@tglx> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 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at 13:07, Jeff Layton wrote: >=20 > > For multigrain timestamps, we must keep track of the latest timestamp > > that has ever been handed out, and never hand out a coarse time below > > that value. >=20 > How is that correct when the clock is off by an hour and then set back > to the correct value? Then you'd get the same stale timestamp for an > hour unless something invokes ktime_get_real_ts64_mg() which will set > the "latest" timestamp back to a time before the previous one. > > > Add a static singleton atomic64_t into timekeeper.c that we can use to > > keep track of the latest fine-grained time ever handed out. This is > > tracked as a monotonic ktime_t value to ensure that it isn't affected b= y > > clock jumps. >=20 > That's just wishful thinking. >=20 > ktime_get_real_ts64_mg(ts) > ts =3D Tmono_1 + offset_1; // TReal_1 > floor =3D Tmono_1; >=20 > // newtime < TReal_1 = =20 > clock_settime(REALTIME, newtime); > xtime =3D newtime; // TReal_2 > offset_2 =3D offset_1 + Treal_2 - TRea= l(now); > --> offset_2 < offset_1 >=20 > ktime_get_coarse_real_ts64_mg(ts) > ts =3D tk_xtime(); // TReal_2 > offs =3D offset_2; >=20 > if (Tmono_1 + offset_2 > ts) > ts =3D Tmono_1 + offset_2; // Not taken >=20 > So this returns T_Real_2 because >=20 > offset_2 < offset_1 >=20 > and therefore >=20 > Tmono_1 + offset_2 < TReal_2 >=20 > so the returned time will jump backwards vs. TReal_1 as it should > because that's the actual time, no? >=20 > So if that's the intended behaviour then the changelog is misleading at > best. >=20 > If the intention is to never return a value < TReal_1 then this does not > work. You can make it work by using the Realtime timestamp as floor, but > that'd be more than questionable vs. clock_settime() making the clock go > backwards. >=20 That is the intended behavior and I'll plan to fix the changelog to clarify this point: If someone jumps the realtime clock backward by a large value, then the realtime timestamp _can_ appear to go backward. This is a problem today even without this patchset. If two files get stamped and a realtime clock jump backward happens in between them, all bets are off as to which one will appear to have been modified first. I don't think that is something we can reasonably prevent, since we must stamp files according to the realtime clock. The main thing I'm trying to prevent is the timestamps being misordered in the absence of such a clock jump. Without tracking the floor as I am here, that's a possibility. --=20 Jeff Layton