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

[Christoph Hellwig <hch@infradead.org>]

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?