On 8/12/2020 3:30 PM, David Hildenbrand wrote: > On 12.08.20 11:46, Charan Teja Kalla wrote: >> >> Thanks David for the inputs. >> >> On 8/12/2020 2:35 AM, David Hildenbrand wrote: >>> 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). >> >> Yes, this is my understanding too. I explained the above race in terms >> of just movable zone for which it is observed. We can add the below line >> in the end in patch commit message: >> "This is not limited to the movable zone, it could also happen in cases >> with the normal zone (e.g., hotplug to a node that only has DMA memory, >> or no other memory yet)." > > Yeah, that makes sense! > >> >> Just curious, there exists such systems where just a dma zone present >> and we hot add the normal zone? I am not aware such thing in the >> embedded world. > > You can easily create such setups using QEMU. > > IIRC, just specify a QEMU guest with 2G initial memory and a single NUMA > node, or 4G initial memory and two NUMA nodes. Then hotplug memory. > > (IIRC kata containers always start a VM with 2G and then hotplug memory) > I see. Thanks for letting me know this. >>> >>>> 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? >> >> Fixes: 5f8dcc21211a ("page-allocator: split per-cpu list into >> one-list-per-migrate-type") >> Cc: <stable@xxxxxxxxxxxxxxx> [2.6+] > > Did we have memory hotplug support then already? Yes, it exist. https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/mm/memory_hotplug.c?h=v2.6.39 > >> >> I am not sure If I should have to raise V3 including these? > > > Maybe Andrew can fixup when applying. Okay, let Andrew decide on this. Meanwhile If you find that this patch looks correct, ACK from you helps here. > > -- The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project
