Re: [PATCH vfs.tmpfs 4/5] tmpfs: trivial support for direct IO

[Hugh Dickins <hughd@google.com>]

On Thu, 10 Aug 2023, Darrick J. Wong wrote:
> On Wed, Aug 09, 2023 at 06:41:17AM -0700, Christoph Hellwig wrote:
> > Please do not add a new ->direct_IO method.  I'm currently working hard
> > on removing it, just set FMODE_CAN_ODIRECT and handle the fallback in
> > your read_iter/write_iter methods.
> > 
> > But if we just start claiming direct I/O support for file systems that
> > don't actually support it, I'm starting to seriously wonder why we
> > bother with the flag at all and don't just allow O_DIRECT opens
> > to always succeed..
> 
> I see it differently -- you can do byte-aligned directio to S_DAX files
> on persistent memory, so I don't see why you can't do that for tmpfs
> files too.

Helpful support, thanks.  But I didn't read Christoph as unhappy with
the granularity issue: just giving me directIOn to FMODE_CAN_ODIRECT,
and rightly wondering why we ever fail O_DIRECTs.

Hugh

> 
> (I'm not advocating for letting *disk* based filesystems allow O_DIRECT
> even if read and writes are always going to go through the page cache
> and get flushed to disk.  If programs wanted that, they'd use O_SYNC.)
> 
> /mnt is a pmem filesystem, /mnt/on/file has S_DAX set, and /mnt/off/file
> does not:
> 
> # xfs_io -c statx /mnt/{on,off}/file
> fd.path = "/mnt/on/file"
> fd.flags = non-sync,non-direct,read-write
> stat.ino = 132
> stat.type = regular file
> stat.size = 1048576
> stat.blocks = 2048
> fsxattr.xflags = 0x8002 [-p------------x--]
> fsxattr.projid = 0
> fsxattr.extsize = 0
> fsxattr.cowextsize = 0
> fsxattr.nextents = 1
> fsxattr.naextents = 0
> dioattr.mem = 0x200
> dioattr.miniosz = 512
> dioattr.maxiosz = 2147483136
> fd.path = "/mnt/off/file"
> fd.flags = non-sync,non-direct,read-write
> stat.ino = 8388737
> stat.type = regular file
> stat.size = 1048576
> stat.blocks = 2048
> fsxattr.xflags = 0x2 [-p---------------]
> fsxattr.projid = 0
> fsxattr.extsize = 0
> fsxattr.cowextsize = 0
> fsxattr.nextents = 1
> fsxattr.naextents = 0
> dioattr.mem = 0x200
> dioattr.miniosz = 512
> dioattr.maxiosz = 2147483136
> 
> And now we try a byte-aligned direct write:
> 
> # xfs_io -d -c 'pwrite -S 0x58 47 1' /mnt/off/file
> pwrite: Invalid argument
> # xfs_io -d -c 'pwrite -S 0x58 47 1' /mnt/on/file
> wrote 1/1 bytes at offset 47
> 1.000000 bytes, 1 ops; 0.0001 sec (5.194 KiB/sec and 5319.1489 ops/sec)
> 
> --D