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 C9BA218CBE0 for ; Tue, 10 Sep 2024 13:51:39 +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=1725976299; cv=none; b=TFLfAI9naOlFNduXLL6s4Rm+LJLG14cXywdaK6tcL4XO29LV7/dW8rkgwJdSjyp9vFEl9biXCh2b1MgSCB2DGLZNseWVg69zLNyRdwKO42WE/6/jkNsJ8EYjtCGrPAzvB24A1slghbLBoj4iXaBmLHSu5Hm4V5DR20W6dXGsWkY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725976299; c=relaxed/simple; bh=HKRM6pfTQbJNkmKdINLi0KaVoAkaWfUGtxppquLoIKY=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=cRvF4Sy5Vgg3tJMN8P19gppJdpvFU+bnG3a2GOrVBHxD9n0QKHZ2FTPTJKJ5b9V/MtEHsIK+Xdagv+EsXwjjO6N1izGK/ly5/HU4lu2lpZiAOxUbW8snCug8nveMDvuYoyciNQIAEzfmtvcV8+asZsn1oKVjEKJzKwSOXW+eYKs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=qJImo14G; 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="qJImo14G" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B5971C4CEC3; Tue, 10 Sep 2024 13:51:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1725976299; bh=HKRM6pfTQbJNkmKdINLi0KaVoAkaWfUGtxppquLoIKY=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=qJImo14Gd74g6Rv8A+O81RN8wnHLXRuVaZG2mTjaSNOO1O3r8lFLlrMf2torr5ss6 v35tlSf8XQUDyg/fXfoFKwelhuZN+TF3r7h/mMJ/uRyPh+/LP1Dy4VHUGwocwCLuGp K/E7qFxVwctaGhaPRefGu48T1Qv/y/73x18WoKvCV6qvzNmCiQ9n5Dea5dhvYKiiMm fLFC1LgWsgUg0+snKgoTfl3BAV7fhlQRjCBX/kPgHm5MsNZu0ljFcWrklo3FZH6lWk pJlt/NtSll1THTKpMFBK9yn/ssV9IxCi3wWb2Z6IvLYAb1zSy9XsuIWkNRcFKU9NfO mvL3rpOihQ21w== Message-ID: <80c0e982a5a55f6c0a0eeb4e717b9fc32d93de3f.camel@kernel.org> Subject: Re: [PATCH] uidgid: make sure we fit into one cacheline From: Jeff Layton To: Christian Brauner , Arnd Bergmann Cc: linux-fsdevel@vger.kernel.org, Jan Kara , Amir Goldstein , Josef Bacik Date: Tue, 10 Sep 2024 09:51:37 -0400 In-Reply-To: <20240910-work-uid_gid_map-v1-1-e6bc761363ed@kernel.org> References: <20240910-work-uid_gid_map-v1-1-e6bc761363ed@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 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2tAMJUvFTdDbfe3zKlPDTuVsyNsALBGclPLagJ5ZTP+Vp2irAN9uwBuacBOTtmOdz4ZN2tdvNgozz uxp4CHBDVzAslUi2idy+xpsp47DWPxYFIRP3M8QG/aNW052LaPc0cedYxp8+9eiVUNpxF4SiU4i9J DfX/sn9XcfoVZIxMpCRE750zvJvcCUz9HojsrMQ1NFc7MFT1z3MOW2/RlzPcog7xvR5ENPH19ojRD CHqumUHRry+RF0lH00clzX/W8OrQJZtoBPXv9ahka/Vp7kEulcBJr1cH5Wz/WprhsIM7U9pse1f1g Yy9YbXtWctUz8uvDR7shsQxAhX3qO7DilMtuGo1v97I/Kx4gXQ52syh/w6EBny71CZrOgD6kJwPVV 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 3.52.4 (3.52.4-1.fc40) Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, 2024-09-10 at 10:16 +0200, Christian Brauner wrote: > When I expanded uidgid mappings I intended for a struct uid_gid_map to > fit into a single cacheline on x86 as they tend to be pretty > performance sensitive (idmapped mounts etc). But a 4 byte hole was added > that brought it over 64 bytes. Fix that by adding the static extent > array and the extent counter into a substruct. C's type punning for > unions guarantees that we can access ->nr_extents even if the last > written to member wasn't within the same object. This is also what we > rely on in struct_group() and friends. This of course relies on > non-strict aliasing which we don't do. >=20 > 99) If the member used to read the contents of a union object is not the > same as the member last used to store a value in the object, the > appropriate part of the object representation of the value is > reinterpreted as an object representation in the new type as > described in 6.2.6 (a process sometimes called "type punning"). >=20 > Link: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf > Signed-off-by: Christian Brauner > --- > Before: >=20 > struct uid_gid_map { > u32 nr_extents; /* 0 4 *= / >=20 > /* XXX 4 bytes hole, try to pack */ >=20 > union { > struct uid_gid_extent extent[5]; /* 8 60 *= / > struct { > struct uid_gid_extent * forward; /* 8 8 *= / > struct uid_gid_extent * reverse; /* 16 8 *= / > }; /* 8 16 *= / > }; /* 8 64 *= / >=20 > /* size: 72, cachelines: 2, members: 2 */ > /* sum members: 68, holes: 1, sum holes: 4 */ > /* last cacheline: 8 bytes */ > }; >=20 > After: >=20 > struct uid_gid_map { > union { > struct { > struct uid_gid_extent extent[5]; /* 0 60 *= / > u32 nr_extents; /* 60 4 *= / > }; /* 0 64 *= / > struct { > struct uid_gid_extent * forward; /* 0 8 *= / > struct uid_gid_extent * reverse; /* 8 8 *= / > }; /* 0 16 *= / > }; /* 0 64 *= / >=20 > /* size: 64, cachelines: 1, members: 1 */ > }; > --- > include/linux/user_namespace.h | 6 ++++-- > kernel/user.c | 6 +++--- > 2 files changed, 7 insertions(+), 5 deletions(-) >=20 > diff --git a/include/linux/user_namespace.h b/include/linux/user_namespac= e.h > index 6030a8235617..3625096d5f85 100644 > --- a/include/linux/user_namespace.h > +++ b/include/linux/user_namespace.h > @@ -21,9 +21,11 @@ struct uid_gid_extent { > }; > =20 > struct uid_gid_map { /* 64 bytes -- 1 cache line */ > - u32 nr_extents; > union { > - struct uid_gid_extent extent[UID_GID_MAP_MAX_BASE_EXTENTS]; > + struct { > + struct uid_gid_extent extent[UID_GID_MAP_MAX_BASE_EXTENTS]; > + u32 nr_extents; > + }; Is this any different from just moving nr_extents to the end of struct_uid_gid_map? I don't quite get how moving it into the union improves things. > struct { > struct uid_gid_extent *forward; > struct uid_gid_extent *reverse; > diff --git a/kernel/user.c b/kernel/user.c > index aa1162deafe4..f46b1d41163b 100644 > --- a/kernel/user.c > +++ b/kernel/user.c > @@ -36,33 +36,33 @@ EXPORT_SYMBOL_GPL(init_binfmt_misc); > */ > struct user_namespace init_user_ns =3D { > .uid_map =3D { > - .nr_extents =3D 1, > { > .extent[0] =3D { > .first =3D 0, > .lower_first =3D 0, > .count =3D 4294967295U, > }, > + .nr_extents =3D 1, > }, > }, > .gid_map =3D { > - .nr_extents =3D 1, > { > .extent[0] =3D { > .first =3D 0, > .lower_first =3D 0, > .count =3D 4294967295U, > }, > + .nr_extents =3D 1, > }, > }, > .projid_map =3D { > - .nr_extents =3D 1, > { > .extent[0] =3D { > .first =3D 0, > .lower_first =3D 0, > .count =3D 4294967295U, > }, > + .nr_extents =3D 1, > }, > }, > .ns.count =3D REFCOUNT_INIT(3), >=20 > --- > base-commit: 698e7d1680544ef114203b0cf656faa0c1216ebc > change-id: 20240910-work-uid_gid_map-cce46aee1b76 >=20 Jeff Layton