Re: [RFC PATCH v2 1/7] statx: add I/O alignment information

From: "Darrick J. Wong" <djwong@kernel.org>
To: Florian Weimer <fweimer@redhat.com>
Cc: Eric Biggers <ebiggers@kernel.org>,
	linux-fsdevel@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 <kbusch@kernel.org>
Subject: Re: [RFC PATCH v2 1/7] statx: add I/O alignment information
Date: Fri, 27 May 2022 09:22:54 -0700	[thread overview]
Message-ID: <YpD63ocQmmgpZVrd@magnolia> (raw)
In-Reply-To: <87r14ffivd.fsf@oldenburg.str.redhat.com>

On Fri, May 27, 2022 at 11:02:46AM +0200, Florian Weimer wrote:
> * Eric Biggers:
> 
> > diff --git a/include/uapi/linux/stat.h b/include/uapi/linux/stat.h
> > index 1500a0f58041a..f822b23e81091 100644
> > --- a/include/uapi/linux/stat.h
> > +++ b/include/uapi/linux/stat.h
> > @@ -124,9 +124,13 @@ struct statx {
> >  	__u32	stx_dev_minor;
> >  	/* 0x90 */
> >  	__u64	stx_mnt_id;
> > -	__u64	__spare2;
> > +	__u32	stx_mem_align_dio;	/* Memory buffer alignment for direct I/O */
> > +	__u32	stx_offset_align_dio;	/* File offset alignment for direct I/O */
> >  	/* 0xa0 */
> > -	__u64	__spare3[12];	/* Spare space for future expansion */
> > +	__u32	stx_offset_align_optimal; /* Optimal file offset alignment for I/O */
> > +	__u32	__spare2;
> > +	/* 0xa8 */
> > +	__u64	__spare3[11];	/* Spare space for future expansion */
> >  	/* 0x100 */
> >  };
> 
> Are 32 bits enough?  Would it make sense to store the base-2 logarithm
> instead?

I don't think a log2 will work here, XFS will want to report things like
raid stripe sizes, which can be any multiple of the fs blocksize.

32 bits is probably enough, seeing as the kernel won't do an IO larger
than 2GB anyway.

--D

> Thanks,
> Florian
> 