Re: [PATCH] shmem: Update folio if shmem_replace_page() updates the page

[Matthew Wilcox <willy@infradead.org>]

On Tue, Aug 02, 2022 at 05:46:37PM -0700, Andrew Morton wrote:
> On Sat, 30 Jul 2022 05:25:18 +0100 "Matthew Wilcox (Oracle)" <willy@infradead.org> wrote:
> 
> > If we allocate a new page, we need to make sure that our folio matches
> > that new page.  This will be solved by changing shmem_replace_page()
> > to shmem_replace_folio(), but this is the minimal fix.
> > 
> > ...
> >
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -1771,6 +1771,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
> >  
> >  	if (shmem_should_replace_folio(folio, gfp)) {
> >  		error = shmem_replace_page(&page, gfp, info, index);
> > +		folio = page_folio(page);
> >  		if (error)
> >  			goto failed;
> >  	}
> 
> What are the user-visible runtime effects of the bug?
> 
> Should we backport this into 5.19.X?

Definitely should be backported.  The next line not visible in this
patch context says:

        error = shmem_add_to_page_cache(folio, mapping, index,
                                        swp_to_radix_entry(swap), gfp,
                                        charge_mm);

so if we do end up in this path, we store the wrong page in the
shmem inode's page cache, and I would rather imagine that data
corruption ensues.