linux-kernel - Re: [man-pages RFC PATCH] statx.2, open.2: document STATX

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Message-Id: <622BA3BB-03EA-4271-8A2E-2ADAFB574155@dilger.ca>
Date:   Thu, 23 Jun 2022 10:27:19 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     Eric Biggers <ebiggers@...nel.org>, linux-fsdevel@...r.kernel.org,
        linux-man@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-xfs@...r.kernel.org,
        linux-api@...r.kernel.org, linux-fscrypt@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Keith Busch <kbusch@...nel.org>
Subject: Re: [man-pages RFC PATCH] statx.2, open.2: document STATX_DIOALIGN

On Jun 23, 2022, at 10:02 AM, Darrick J. Wong <djwong@...nel.org> wrote:
> 
> On Thu, Jun 16, 2022 at 01:21:41PM -0700, Eric Biggers wrote:
>> From: Eric Biggers <ebiggers@...gle.com>
>> 
>> @@ -244,8 +249,11 @@ STATX_SIZE	Want stx_size
>> STATX_BLOCKS	Want stx_blocks
>> STATX_BASIC_STATS	[All of the above]
>> STATX_BTIME	Want stx_btime
>> +STATX_ALL	The same as STATX_BASIC_STATS | STATX_BTIME.
>> +         	This is deprecated and should not be used.
> 
> STATX_ALL is deprecated??  I was under the impression that _ALL meant
> all the known bits for that kernel release, but...

For userspace STATX_ALL doesn't make sense, and it isn't used by the kernel.

Firstly, that would be a compile-time value for an application, so it
may be incorrect for the kernel the code is actually run on (either too
many or too few bits could be set).

Secondly, it isn't really useful for an app to request "all attributes"
if it doesn't know what they all mean, as that potentially adds useless
overhead.  Better for it to explicitly request the attributes that it
needs.  If that is fewer than the kernel could return it is irrelevant,
since the app would ignore them anyway.

The kernel will already ignore and mask attributes that *it* doesn't
understand, so requesting more is fine and STATX_ALL doesn't help this.

Cheers, Andreas

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