Re: [PATCH] page_frag: Recover from memory pressure

[Vlastimil Babka <vbabka@suse.cz>]

On 11/5/20 1:05 PM, Matthew Wilcox wrote:
> 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 ...

Or rather not at all, sorry :) somehow I just saw "ah here we call the page 
allocator".

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> 
>          if (unlikely(!page))
>                  page = alloc_pages_node(NUMA_NO_NODE, gfp, 0);
> 
>          nc->va = page ? page_address(page) : NULL;
> 
>