[PATCH V2] mm, page_alloc: fix core hung in free_pcppages_bulk()

From: Charan Teja Reddy
Date: Tue Aug 11 2020 - 08:58:48 EST

The following race is observed with the repeated online, offline and a
delay between two successive online of memory blocks of movable zone.

P1 P2

Online the first memory block in
the movable zone. The pcp struct
values are initialized to default
values,i.e., pcp->high = 0 &
pcp->batch = 1.

Allocate the pages from the
movable zone.

Try to Online the second memory
block in the movable zone thus it
entered the online_pages() but yet
to call zone_pcp_update().
This process is entered into
the exit path thus it tries
to release the order-0 pages
to pcp lists through
As pcp->high = 0, pcp->count = 1
proceed to call the function
Update the pcp values thus the
new pcp values are like, say,
pcp->high = 378, pcp->batch = 63.
Read the pcp's batch value using
READ_ONCE() and pass the same to
free_pcppages_bulk(), pcp values
passed here are, batch = 63,
count = 1.

Since num of pages in the pcp
lists are less than ->batch,
then it will stuck in
while(list_empty(list)) loop
with interrupts disabled thus
a core hung.

Avoid this by ensuring free_pcppages_bulk() is called with proper count
of pcp list pages.

The mentioned race is some what easily reproducible without [1] because
pcp's are not updated for the first memory block online and thus there
is a enough race window for P2 between alloc+free and pcp struct values
update through onlining of second memory block.

With [1], the race is still exists but it is very much narrow as we
update the pcp struct values for the first memory block online itself.

[1]: https://patchwork.kernel.org/patch/11696389/

Signed-off-by: Charan Teja Reddy <charante@xxxxxxxxxxxxxx>

v1: https://patchwork.kernel.org/patch/11707637/

mm/page_alloc.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e4896e6..839039f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1304,6 +1304,11 @@ static void free_pcppages_bulk(struct zone *zone, int count,
struct page *page, *tmp;

+ /*
+ * Ensure proper count is passed which otherwise would stuck in the
+ * below while (list_empty(list)) loop.
+ */
+ count = min(pcp->count, count);
while (count) {
struct list_head *list;

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