linux-ext4 - Re: [PATCH v10 0/5] add support for direct I/O with fscrypt using blk-crypto

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Date:   Thu, 20 Jan 2022 23:12:56 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     Dave Chinner <david@...morbit.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fscrypt@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-xfs@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>
Subject: Re: [PATCH v10 0/5] add support for direct I/O with fscrypt using
 blk-crypto

On Thu, Jan 20, 2022 at 06:36:03PM -0800, Darrick J. Wong wrote:
> Sure.  How's this?  I couldn't think of a real case of directio
> requiring different alignments for pos and bytecount, so the only real
> addition here is the alignment requirements for best performance.

While I see some benefits of adding the information to a catchall like
statx we really need to be careful to not bloat the structure like
crazy.

> struct statx {
> ...
> 	/* 0x90 */
> 	__u64	stx_mnt_id;
> 
> 	/* Memory buffer alignment required for directio, in bytes. */
> 	__u32	stx_dio_mem_align;
> 
> 	/* File range alignment required for directio, in bytes. */
> 	__u32	stx_dio_fpos_align_min;

So this really needs a good explanation why we need both iven that we
had no real use case for this.

> 	/* File range alignment needed for best performance, in bytes. */
> 	__u32	stx_dio_fpos_align_opt;

And why we really care about this.  I guess you want to allow sector
size dio in reflink setups, but discourage it.  But is this really as
important?

> 	/* Maximum size of a directio request, in bytes. */
> 	__u32	stx_dio_max_iosize;

I know XFS_IOC_DIOINFO had this, but does it really make much sense?
Why do we need it for direct I/O and not buffered I/O?