Re: [PATCH] xfs: report a writeback error on a read() call

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jun 26, 2025 at 01:57:59PM +1000, Dave Chinner wrote:
> writeback errors. Because scientists and data analysts that wrote
> programs to chew through large amounts of data didn't care about
> persistence of their data mid-processing. They just wanted what they
> wrote to be there the next time the processing pipeline read it.

That's only going to work if your RAM is as large as your permanent
storage :)

> IOWs, checking for a past writeback IO error is as simple as:
> 
> 	if (sync_file_range(fd, 0, 0, SYNC_FILE_RANGE_WAIT_BEFORE) < 0) {
> 		/* An unreported writeback error was pending on the file */
> 		wb_err = -errno;
> 		......
> 	}
> 
> This does not cause new IO to be issued, it only blocks on writeback
> that is currently in progress, and it has no data integrity
> requirements at all. If the writeback has already been done, all it
> will do is sweep residual errors out to userspace.....

Not quite.

This will still wait for all I/O on the range, and given that
sync_file_range treats a 0 length as the entire file that might actually
do a significant amount of waiting.  But yes, it's the closest we get
right now.