Re: [PATCH stable] mm/khugepaged: write all dirty file folios when collapsing

[David Hildenbrand (Arm)]

On 7/2/26 18:54, Pedro Falcato wrote:
> As-is, khugepaged and writable-file opening exclude each other. A file
> cannot be open writeable and have THPs (because the filesystem is not aware
> of them). khugepaged will never collapse file pages for files that are
> opened writeable. On an open(O_RDWR/O_WRONLY), the page cache for that
> particular file is dropped. This is fine because nothing could've been
> dirtied.
> 
> However, there is an edge-case: collapse_file() might not be able to
> coexist with concurrent writers, but it can coexist with dirty folios
> (from previous writers). Therefore, the following can happen:
> 
> open(file, O_RDWR)
> write(file)
> close(file)

Okay, folios are dirty.

> madvise(file_mapping, MADV_COLLAPSE, some non-dirty range)

collapse_file() has

	if (!is_shmem && (folio_test_dirty(folio) ||
			  folio_test_writeback(folio))) {
		...
		result = SCAN_PAGE_DIRTY_OR_WRITEBACK;
		goto out_unlock;
	}

Making us abort collapse.

What am I missing?

-- 
Cheers,

David