Re: The proper handling of failed IO error?

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jun 10, 2024 at 06:50:11AM +0930, Qu Wenruo wrote:
> Hi,
> 
> There is a recent (well a year ago) change in btrfs to remove the usage
> of page/folio error, which gets me wondering what would happen if we got
> a lot of write errors and high memory pressure?
> 
> Yes, all file systems calls mapping_set_error() so that fsync call would
> return error, but I'm wondering what would happen to those folios that
> failed to be written?
> 
> Those folios has their DIRTY flag cleared before submission, and and
> their endio functions, the WRITEBACK flags is also cleared.
> 
> Meaning after such write failure, the page/folio has UPTODATE flag, and
> no DIRTY/ERROR/WRITEBACK flags (at least for btrfs and ext4, meanwhile
> iomap still set the ERROR flag).
> 
> Would any memory pressure just reclaim those pages/folios without them
> really reaching the disk?

Yes.

Core code doesn't (and hasn't in some time) checked the page/folio
error flag.  That's why it's being removed.

Also, btrfs was using it incorrectly to indicate a write error.
It was supposed to be used for read errors, not write errors.
Another good reason to remove it.