page_frag: Recover from memory pressure

The ethernet driver may allocate skb (and skb->data) via napi_alloc_skb().
This ends up to page_frag_alloc() to allocate skb->data from
page_frag_cache->va.

During the memory pressure, page_frag_cache->va may be allocated as
pfmemalloc page. As a result, the skb->pfmemalloc is always true as
skb->data is from page_frag_cache->va. The skb will be dropped if the
sock (receiver) does not have SOCK_MEMALLOC. This is expected behaviour
under memory pressure.

However, once kernel is not under memory pressure any longer (suppose large
amount of memory pages are just reclaimed), the page_frag_alloc() may still
re-use the prior pfmemalloc page_frag_cache->va to allocate skb->data. As a
result, the skb->pfmemalloc is always true unless page_frag_cache->va is
re-allocated, even if the kernel is not under memory pressure any longer.

Here is how kernel runs into issue.

1. The kernel is under memory pressure and allocation of
PAGE_FRAG_CACHE_MAX_ORDER in __page_frag_cache_refill() will fail. Instead,
the pfmemalloc page is allocated for page_frag_cache->va.

2: All skb->data from page_frag_cache->va (pfmemalloc) will have
skb->pfmemalloc=true. The skb will always be dropped by sock without
SOCK_MEMALLOC. This is an expected behaviour.

3. Suppose a large amount of pages are reclaimed and kernel is not under
memory pressure any longer. We expect skb->pfmemalloc drop will not happen.

4. Unfortunately, page_frag_alloc() does not proactively re-allocate
page_frag_alloc->va and will always re-use the prior pfmemalloc page. The
skb->pfmemalloc is always true even kernel is not under memory pressure any
longer.

Fix this by freeing and re-allocating the page instead of recycling it.

References: https://lore.kernel.org/lkml/[email protected]/
References: https://lore.kernel.org/linux-mm/[email protected]/
Suggested-by: Matthew Wilcox (Oracle) <[email protected]>
Cc: Aruna Ramakrishna <[email protected]>
Cc: Bert Barbe <[email protected]>
Cc: Rama Nichanamatlu <[email protected]>
Cc: Venkat Venkatsubra <[email protected]>
Cc: Manjunath Patil <[email protected]>
Cc: Joe Jin <[email protected]>
Cc: SRINIVAS <[email protected]>
Fixes: 79930f5 ("net: do not deplete pfmemalloc reserve")
Signed-off-by: Dongli Zhang <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

mm/page_alloc.c

@@ -5103,6 +5103,11 @@ void *page_frag_alloc(struct page_frag_cache *nc,
 		if (!page_ref_sub_and_test(page, nc->pagecnt_bias))
 			goto refill;
 
+		if (unlikely(nc->pfmemalloc)) {
+			free_the_page(page, compound_order(page));
+			goto refill;
+		}
+
 #if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
 		/* if size can vary use size else just use PAGE_SIZE */
 		size = nc->size;