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

[Matthew Wilcox]

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().