Re: [PATCH] xfs: Remove i_rwsem lock in buffered read

[Christoph Hellwig <hch@infradead.org>]

On Tue, Jan 28, 2025 at 07:49:17AM +1100, Dave Chinner wrote:
> > As for why an exclusive lock is needed for append writes, it's because
> > we don't want the EOF to be modified during the append write.
> 
> We don't care if the EOF moves during the append write at the
> filesystem level. We set kiocb->ki_pos = i_size_read() from
> generic_write_checks() under shared locking, and if we then race
> with another extending append write there are two cases:
> 
> 	1. the other task has already extended i_size; or
> 	2. we have two IOs at the same offset (i.e. at i_size).
> 
> In either case, we don't need exclusive locking for the IO because
> the worst thing that happens is that two IOs hit the same file
> offset. IOWs, it has always been left up to the application
> serialise RWF_APPEND writes on XFS, not the filesystem.

I disagree.  O_APPEND (RWF_APPEND is just the Linux-specific
per-I/O version of that) is extensively used for things like
multi-thread loggers where you have multiple threads doing O_APPEND
writes to a single log file, and they expect to not lose data
that way.  The fact that we currently don't do that for O_DIRECT
is a bug, which is just papered over that barely anyone uses
O_DIRECT | O_APPEND as that's not a very natural use case for
most applications (in fact NFS got away with never allowing it
at all).  But extending racy O_APPEND to buffered writes would
break a lot of applications.