Re: Subject: [BUG/RFC] write-open file THP cache purge can discard dirty page cache

[Pedro Falcato <pfalcato@suse.de>]

On Wed, Jul 01, 2026 at 12:54:52PM +0100, Matthew Wilcox wrote:
> On Tue, Jun 30, 2026 at 08:55:12PM +0100, Pedro Falcato wrote:
> > @@ -2043,6 +2044,17 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> >  	 */
> >  	try_to_unmap_flush();
> >  
> > +	/*
> > +	 * If collapse looks to be successful, flush any dirty pages
> > +	 * out the page cache. With the nr_thps incremented, there won't be
> > +	 * any new writers (nor new dirties).
> > +	 */
> > +	if (result == SCAN_SUCCEED && !is_shmem) {
> > +		err = filemap_write_and_wait(mapping);
> > +		if (err)
> > +			result = SCAN_WRITEBACK_FAIL;
> > +	}
> 
> Oh, the other thing is that this should be conditioned on
> !mapping_large_folio_support().

Thinking more about this, I don't think it works. On the file open side, we
always truncate the full page cache, even if large folios are supported (due
to possibly max folio < PMD_ORDER). So if we skip write-and-wait on such cases,
it will be possible to fully reproduce this issue.

-- 
Pedro