Re: 6.6.8 stable: crash in folio_mark_dirty

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jan 03, 2024 at 06:49:07PM +0800, Hillf Danton wrote:
> On Mon, 1 Jan 2024 14:11:02 +0000 Matthew Wilcox
> > 
> > From an mm point of view, what is implicit is that truncate calls
> > unmap_mapping_folio -> unmap_mapping_range_tree ->
> > unmap_mapping_range_vma -> zap_page_range_single -> unmap_single_vma ->
> > unmap_page_range -> zap_p4d_range -> zap_pud_range -> zap_pmd_range ->
> > zap_pte_range -> pte_offset_map_lock()
> > 
> > So a truncate will take the page lock, then spin on the pte lock
> > until the racing munmap() has finished (ok, this was an exit(), not
> > a munmap(), but exit() does an implicit munmap()).
> > 
> But ptl fails to explain the warning reported, while the sequence in
> __block_commit_write()
> 
> 	mark_buffer_dirty();
> 	folio_mark_uptodate();
> 
> hints the warning is bogus.

The folio is locked when filesystems call __block_commit_write().

Nothing explains the reported warning, IMO.  Other than data corruption,
and I'm not sure that we've found the last data corrupter.