[PATCH 5.4 024/158] page_frag: Recover from memory pressure

[Greg Kroah-Hartman]

From: Dongli Zhang <dongli.zhang@xxxxxxxxxx>

[ Upstream commit d8c19014bba8f565d8a2f1f46b4e38d1d97bf1a7 ]

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.

Suggested-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Aruna Ramakrishna <aruna.ramakrishna@xxxxxxxxxx>
Cc: Bert Barbe <bert.barbe@xxxxxxxxxx>
Cc: Rama Nichanamatlu <rama.nichanamatlu@xxxxxxxxxx>
Cc: Venkat Venkatsubra <venkat.x.venkatsubra@xxxxxxxxxx>
Cc: Manjunath Patil <manjunath.b.patil@xxxxxxxxxx>
Cc: Joe Jin <joe.jin@xxxxxxxxxx>
Cc: SRINIVAS <srinivas.eeda@xxxxxxxxxx>
Fixes: 79930f5892e1 ("net: do not deplete pfmemalloc reserve")
Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20201115201029.11903-1-dongli.zhang@xxxxxxxxxx
Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 mm/page_alloc.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4910,6 +4910,11 @@ refill:
 		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;