Re: [PATCH v3 18/25] mm: Allow large pages to be added to the page cache

[Matthew Wilcox]

On Sun, May 03, 2020 at 08:10:36PM -0700, Matthew Wilcox wrote:
> On Wed, Apr 29, 2020 at 06:36:50AM -0700, Matthew Wilcox wrote:
> > @@ -886,7 +906,7 @@ static int __add_to_page_cache_locked(struct page *page,
> >  	/* Leave page->index set: truncation relies upon it */
> >  	if (!huge)
> >  		mem_cgroup_cancel_charge(page, memcg, false);
> > -	put_page(page);
> > +	page_ref_sub(page, nr);
> >  	return xas_error(&xas);
> >  }
> >  ALLOW_ERROR_INJECTION(__add_to_page_cache_locked, ERRNO);
> 
> This is wrong.  page_ref_sub() will not call __put_page() if the refcount
> gets to zero.  What do people prefer?

*sigh*.  It's not wrong.  The caller holds a reference on the page
already, so calling page_ref_sub() will never reduce the refcount to 0.
The latest version looks like this:

+       page_ref_sub(page, nr);
+       VM_BUG_ON_PAGE(page_count(page) <= 0, page);