Re: [PATCH v2] ext4: check folio uptodate state in ext4_page_mkwrite()

["Theodore Tso" <tytso@mit.edu>]

On Fri, Dec 05, 2025 at 03:33:22AM +0000, Matthew Wilcox wrote:
> It sounds like I was confused -- I thought the folios being
> invalidated in mpage_release_unused_pages() belonged to the block
> device, but from what you're saying, they belong to a user-visible
> file?

Yes, correct.  I'm guessing that we were marking the page !uptodate
back when that was the only way to indicate that there had been any
kind of I/O error (either on the read or write side).  Obviously we
have much better ways of doing it in the 21st century.  :-)

> Now, is the folio necessarily dirty at this point?  I guess so if
> we're in the writeback path.  Darrick got rid of similar code in
> iomap a few years ago; see commit e9c3a8e820ed.  So it'd probably be
> good to have ext4 behave the same way.

Hmm, yes.   Agreed.

    commit e9c3a8e820ed0eeb2be05072f29f80d1b79f053b
    Author: Darrick J. Wong <djwong@kernel.org>
    Date:   Mon May 16 15:27:38 2022 -0700

    iomap: don't invalidate folios after writeback errors
    
    XFS has the unique behavior (as compared to the other Linux
    filesystems) that on writeback errors it will completely
    invalidate the affected folio and force the page cache to reread
    the contents from disk.  All other filesystems leave the page
    mapped and up to date.
    
    This is a rude awakening for user programs, since (in the case
    where write fails but reread doesn't) file contents will appear to
    revert old disk contents with no notification other than an EIO on
    fsync.  This might have been annoying back in the days when iomap
    dealt with one page at a time, but with multipage folios, we can
    now throw away *megabytes* worth of data for a single write error...

As Darrick pointed out we could potentially append a *single* byte to
a file, and if there was some kind of writeback error, we could
potentially throw away *vast* amounts of data for no good reason.

     	  	     	       	     - Ted