Re: [PATCH] page_frag: Recover from memory pressure

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

On Thu, Nov 05, 2020 at 12:56:43PM +0100, Vlastimil Babka wrote:
> > +++ b/mm/page_alloc.c
> > @@ -5139,6 +5139,10 @@ void *page_frag_alloc(struct page_frag_cache *nc,
> >   		if (!page_ref_sub_and_test(page, nc->pagecnt_bias))
> >   			goto refill;
> > +		if (nc->pfmemalloc) {
> > +			free_the_page(page, compound_order(page));
> > +			goto refill;
> 
> Theoretically the refill can fail and we return NULL while leaving nc->va
> pointing to a freed page, so I think you should set nc->va to NULL.
> 
> Geez, can't the same thing already happen after we sub the nc->pagecnt_bias
> from page ref, and last users of the page fragments then return them and dec
> the ref to zero and the page gets freed?

I don't think you read __page_frag_cache_refill() closely enough ...

        if (unlikely(!page))
                page = alloc_pages_node(NUMA_NO_NODE, gfp, 0);

        nc->va = page ? page_address(page) : NULL;