Re: [PATCH 13/13] mm: Convert swap_cluster_readahead and swap_vma_readahead to return a folio

[Matthew Wilcox <willy@infradead.org>]

On Sat, Dec 16, 2023 at 09:58:03PM +0800, Kairui Song wrote:
> > @@ -888,14 +887,14 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
> >  {
> >         struct mempolicy *mpol;
> >         pgoff_t ilx;
> > -       struct page *page;
> > +       struct folio *folio;
> >
> >         mpol = get_vma_policy(vmf->vma, vmf->address, 0, &ilx);
> > -       page = swap_use_vma_readahead() ?
> > +       folio = swap_use_vma_readahead() ?
> >                 swap_vma_readahead(entry, gfp_mask, mpol, ilx, vmf) :
> >                 swap_cluster_readahead(entry, gfp_mask, mpol, ilx);
> >         mpol_cond_put(mpol);
> > -       return page;
> > +       return folio_file_page(folio, swp_offset(entry));
> 
> Hi Matthew,
> 
> There is a bug here, folio could be NULL, and cause NULL dereference.

Andrew, syzbot has also picked up on this.  Please add this -fix patch?

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 793b5b9e4f96..8a3a8f1ab20a 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -894,6 +894,9 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
 		swap_vma_readahead(entry, gfp_mask, mpol, ilx, vmf) :
 		swap_cluster_readahead(entry, gfp_mask, mpol, ilx);
 	mpol_cond_put(mpol);
+
+	if (!folio)
+		return NULL;
 	return folio_file_page(folio, swp_offset(entry));
 }