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

[Matthew Wilcox]

On Tue, Jun 24, 2025 at 02:26:18PM -0400, Jeff Layton wrote:
> On Tue, 2025-06-24 at 07:14 -0700, Christoph Hellwig wrote:
> > On Sun, Jun 22, 2025 at 08:32:18PM +0800, ying chen wrote:
> > > Normally, user space returns immediately after writing data to the
> > > buffer cache. However, if an error occurs during the actual disk
> > > write operation, data loss may ensue, and there is no way to report
> > > this error back to user space immediately. Current kernels may report
> > > writeback errors when fsync() is called, but frequent invocations of
> > > fsync() can degrade performance. Therefore, a new sysctl
> > > fs.xfs.report_writeback_error_on_read is introduced, which, when set
> > > to 1, reports writeback errors when read() is called. This allows user
> > > space to be notified of writeback errors more promptly.
> > 
> > That's really kernel wide policy and not something magic done by a
> > single file system.
> 
> ...not to mention that getting an error back on a read for a prior
> writeback error would be completely unexpected by most applications.

Well.  It's somewhat understandable:

	write() (returns success)
	writeback happens, error logged
	memory pressure evicts folio
	read() brings folio into page cache
	attempt to read contents fails, error returned

I'm not sure it's a good solution, but it's plausible.