From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 98D99C433EF for ; Fri, 22 Jul 2022 16:31:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229850AbiGVQb4 (ORCPT ); Fri, 22 Jul 2022 12:31:56 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60004 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229567AbiGVQbz (ORCPT ); Fri, 22 Jul 2022 12:31:55 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E805C89A9B; Fri, 22 Jul 2022 09:31:53 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id C2190B82970; Fri, 22 Jul 2022 16:31:51 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 682F5C341C6; Fri, 22 Jul 2022 16:31:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1658507510; bh=2zJKttnRhj1+bfpQ1neKDFu0IlRxRblBoec0bp8Sdok=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=R6uxrMTGkXqjhGQr3zA8qNa8GSPQYWhhd4mQ3yYrU74pGY1LX1IaIR5hc0nNP92LQ q9wz+Ac3IrLVRoZlFBxpRSiQx2lr3niXhLmKlEKd0WaePq36/63ztLdxUHKVP91cna iTiD0U+7tuK8NQRrlnfsoyqCoOVisiWxoj95uBcTC6TpEN1dPCJvAwk4AG+OLFbipY Ev1zg3asT7LDP81fPvnsbpQUSyN9RHHDGbrlF5ehxKVjBYDOL18hUhi2yOduvU0hdK zkACGw2vJii36YKdGWHobF9F/E/t5jlN1+H172Y6t7CaMxCzV8c30PiPug7zXwMxcb 86mtOuQB7uKRA== Date: Fri, 22 Jul 2022 09:31:49 -0700 From: "Darrick J. Wong" To: Eric Biggers Cc: linux-fsdevel@vger.kernel.org, linux-man@vger.kernel.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-xfs@vger.kernel.org, linux-api@vger.kernel.org, linux-fscrypt@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, Keith Busch Subject: Re: [man-pages RFC PATCH v2] statx.2, open.2: document STATX_DIOALIGN Message-ID: References: <20220722074229.148925-1-ebiggers@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220722074229.148925-1-ebiggers@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On Fri, Jul 22, 2022 at 12:42:28AM -0700, Eric Biggers wrote: > From: Eric Biggers > > Document the proposed STATX_DIOALIGN support for statx() > (https://lore.kernel.org/linux-fsdevel/20220722071228.146690-1-ebiggers@kernel.org/T/#u). > > Signed-off-by: Eric Biggers > --- > > v2: rebased onto man-pages master branch, mentioned xfs, and updated > link to patchset > > man2/open.2 | 43 ++++++++++++++++++++++++++++++++----------- > man2/statx.2 | 29 +++++++++++++++++++++++++++++ > 2 files changed, 61 insertions(+), 11 deletions(-) > > diff --git a/man2/open.2 b/man2/open.2 > index d1485999f..ef29847c3 100644 > --- a/man2/open.2 > +++ b/man2/open.2 > @@ -1732,21 +1732,42 @@ of user-space buffers and the file offset of I/Os. > In Linux alignment > restrictions vary by filesystem and kernel version and might be > absent entirely. > -However there is currently no filesystem\-independent > -interface for an application to discover these restrictions for a given > -file or filesystem. > -Some filesystems provide their own interfaces > -for doing so, for example the > +The handling of misaligned > +.B O_DIRECT > +I/Os also varies; they can either fail with > +.B EINVAL > +or fall back to buffered I/O. > +.PP > +Since Linux 5.20, > +.B O_DIRECT > +support and alignment restrictions for a file can be queried using > +.BR statx (2), > +using the > +.B STATX_DIOALIGN > +flag. > +Support for > +.B STATX_DIOALIGN > +varies by filesystem; see > +.BR statx (2). > +.PP > +Some filesystems provide their own interfaces for querying > +.B O_DIRECT > +alignment restrictions, for example the > .B XFS_IOC_DIOINFO > operation in > .BR xfsctl (3). > +.B STATX_DIOALIGN > +should be used instead when it is available. > .PP > -Under Linux 2.4, transfer sizes, the alignment of the user buffer, > -and the file offset must all be multiples of the logical block size > -of the filesystem. > -Since Linux 2.6.0, alignment to the logical block size of the > -underlying storage (typically 512 bytes) suffices. > -The logical block size can be determined using the > +If none of the above is available, then direct I/O support and alignment > +restrictions can only be assumed from known characteristics of the filesystem, > +the individual file, the underlying storage device(s), and the kernel version. > +In Linux 2.4, most block device based filesystems require that the file offset > +and the length and memory address of all I/O segments be multiples of the > +filesystem block size (typically 4096 bytes). > +In Linux 2.6.0, this was relaxed to the logical block size of the block device > +(typically 512 bytes). > +A block device's logical block size can be determined using the > .BR ioctl (2) > .B BLKSSZGET > operation or from the shell using the command: > diff --git a/man2/statx.2 b/man2/statx.2 > index 0326e9af0..ea38ec829 100644 > --- a/man2/statx.2 > +++ b/man2/statx.2 > @@ -61,7 +61,12 @@ struct statx { > containing the filesystem where the file resides */ > __u32 stx_dev_major; /* Major ID */ > __u32 stx_dev_minor; /* Minor ID */ > + > __u64 stx_mnt_id; /* Mount ID */ > + > + /* Direct I/O alignment restrictions */ > + __u32 stx_dio_mem_align; > + __u32 stx_dio_offset_align; > }; > .EE > .in > @@ -247,6 +252,8 @@ STATX_BTIME Want stx_btime > STATX_ALL The same as STATX_BASIC_STATS | STATX_BTIME. > It is deprecated and should not be used. > STATX_MNT_ID Want stx_mnt_id (since Linux 5.8) > +STATX_DIOALIGN Want stx_dio_mem_align and stx_dio_offset_align > + (since Linux 5.20; support varies by filesystem) > .TE > .in > .PP > @@ -407,6 +414,28 @@ This is the same number reported by > .BR name_to_handle_at (2) > and corresponds to the number in the first field in one of the records in > .IR /proc/self/mountinfo . > +.TP > +.I stx_dio_mem_align > +The alignment (in bytes) required for user memory buffers for direct I/O > +.BR "" ( O_DIRECT ) > +on this file. or 0 if direct I/O is not supported on this file. Nit: "..on this file, or 0 if direct..." > +.IP > +.B STATX_DIOALIGN > +.IR "" ( stx_dio_mem_align > +and > +.IR stx_dio_offset_align ) > +is supported on block devices since Linux 5.20. > +The support on regular files varies by filesystem; it is supported by ext4, > +f2fs, and xfs since Linux 5.20. > +.TP > +.I stx_dio_offset_align > +The alignment (in bytes) required for file offsets and I/O segment lengths for > +direct I/O > +.BR "" ( O_DIRECT ) > +on this file, or 0 if direct I/O is not supported on this file. On this last part -- userspace can only conclude that directio is not supported on the file if ((STATX_DIOALIGN & stx_mask) && stx_dio_offset_align == 0), right? IOWs, if (STATX_DIOALIGN & stx_mask)==0 then userspace can't draw any conclusions from stx_dio_offset_align, correct? If the answers are yes and yes, then I think I've understood all this and can say Reviewed-by: Darrick J. Wong --D > +This will only be nonzero if > +.I stx_dio_mem_align > +is nonzero, and vice versa. > .PP > For further information on the above fields, see > .BR inode (7). > > base-commit: f9f25914e4ed393ac284ab921876e8a78722c504 > -- > 2.37.0 >