Re: [PATCH] pagemap: Add alert to mapping_set_release_always() for mapping with no release_folio

[Matthew Wilcox]

On Thu, Dec 11, 2025 at 10:23:13AM +0100, Jan Kara wrote:
> On Wed 10-12-25 21:36:43, Matthew Wilcox wrote:
> > On Thu, Dec 11, 2025 at 01:31:04AM +0530, Deepakkumar Karn wrote:
> > >  static inline void mapping_set_release_always(struct address_space *mapping)
> > >  {
> > > +	/* Alert while setting the flag with no release_folio callback */
> > 
> > The comment is superfluous.
> 
> Agreed.
> 
> > > +	VM_WARN_ONCE(!mapping->a_ops->release_folio,
> > > +		     "Setting AS_RELEASE_ALWAYS with no release_folio");
> > 
> > But you haven't said why we need to do this.  Surely the NULL pointer
> > splat is enough to tell you that you did something stupid?
> 
> Well, but this will tell it much earlier and it will directly point to the
> place were you've done the mistake (instead of having to figure out why
> drop_buffers() is crashing on you). So I think this assert makes sense to
> ease debugging and as kind of self-reminding documentation :).

Oh.  So the real problem here is this:

        if (mapping && mapping->a_ops->release_folio)
                return mapping->a_ops->release_folio(folio, gfp);
        return try_to_free_buffers(folio);

We should have a block_release_folio(), change all the BH-based
filesystems to add it to their aops, and then change
filemap_release_folio() to do:

	if (mapping)
		return mapping->a_ops->release_folio(folio, gfp);
	return true;

(actually, can the !mapping case be hit?  surely this can't be called
for folios which have already been truncated?)

Then you get a nice NULL pointer dereference instead of calling into
try_to_free_buffers() which is as confusing as hell.