Re: [RFC 1/7] iomap: Don't fall back to buffered write if the write is atomic

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Dec 04, 2023 at 09:02:56AM +0000, John Garry wrote:
> On 01/12/2023 22:07, Dave Chinner wrote:
> > > Sure, and I think that we need a better story for supporting buffered IO for
> > > atomic writes.
> > > 
> > > Currently we have:
> > > - man pages tell us RWF_ATOMIC is only supported for direct IO
> > > - statx gives atomic write unit min/max, not explicitly telling us it's for
> > > direct IO
> > > - RWF_ATOMIC is ignored for !O_DIRECT
> > > 
> > > So I am thinking of expanding statx support to enable querying of atomic
> > > write capabilities for buffered IO and direct IO separately.
> > You're over complicating this way too much by trying to restrict the
> > functionality down to just what you want to implement right now.
> > 
> > RWF_ATOMIC is no different to RWF_NOWAIT. The API doesn't decide
> > what can be supported - the filesystems themselves decide what part
> > of the API they can support and implement those pieces.
> 
> Sure, but for RWF_ATOMIC we still have the associated statx call to tell us
> whether atomic writes are supported for a file and the specific range
> capability.
> 
> > 
> > TO go back to RWF_NOWAIT, for a long time we (XFS) only supported
> > RWF_NOWAIT on DIO, and buffered reads and writes were given
> > -EOPNOTSUPP by the filesystem. Then other filesystems started
> > supporting DIO with RWF_NOWAIT. Then buffered read support was added
> > to the page cache and XFS, and as other filesystems were converted
> > they removed the RWF_NOWAIT exclusion check from their read IO
> > paths.
> > 
> > We are now in the same place with buffered write support for
> > RWF_NOWAIT. XFS, the page cache and iomap allow buffered writes w/
> > RWF_NOWAIT, but ext4, btrfs and f2fs still all return -EOPNOTSUPP
> > because they don't support non-blocking buffered writes yet.
> > 
> > This is the same model we should be applying with RWF_ATOMIC - we
> > know that over time we'll be able to expand support for atomic
> > writes across both direct and buffered IO, so we should not be
> > restricting the API or infrastructure to only allow RWF_ATOMIC w/
> > DIO.
> 
> Agreed.
> 
> > Just have the filesystems reject RWF_ATOMIC w/ -EOPNOTSUPP if
> > they don't support it,
> 
> Yes, I was going to add this regardless.
> 
> > and for those that do it is conditional on
> > whther the filesystem supports it for the given type of IO being
> > done.
> > 
> > Seriously - an application can easily probe for RWF_ATOMIC support
> > without needing information to be directly exposed in statx() - just
> > open a O_TMPFILE, issue the type of RWF_ATOMIC IO you require to be
> > supported, and if it returns -EOPNOTSUPP then it you can't use
> > RWF_ATOMIC optimisations in the application....
> 
> ok, if that is the done thing.
> 
> So I can't imagine that atomic write unit range will be different for direct
> IO and buffered IO (ignoring for a moment Christoph's idea for CoW always
> for no HW offload) when supported. But it seems that we may have a scenario
> where statx tells is that atomic writes are supported for a file, and a DIO
> write succeeds and a buffered IO write may return -EOPNOTSUPP. If that's
> acceptable then I'll work towards that.
> 
> If we could just run statx on a file descriptor here then that would be
> simpler...

statx(fd, "", AT_EMPTY_PATH, ...); ?

--D

> Thanks,
> John
> 
> 
>