On 11.08.20 14:58, Charan Teja Reddy wrote: > 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 > free_unref_page_commit(). > As pcp->high = 0, pcp->count = 1 > proceed to call the function > free_pcppages_bulk(). > 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/ > IIUC, this is not limited to the movable zone, it could also happen in corner cases with the normal zone (e.g., hotplug to a node that only has DMA memory, or no other memory yet). > 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; > LIST_HEAD(head); > > + /* > + * 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; > > Fixes: and Cc: stable... tags? -- Thanks, David / dhildenb
