On 12/30/2012 09:58 AM, Wen Congyang wrote: > At 12/25/2012 04:35 PM, Glauber Costa Wrote: >> On 12/24/2012 04:09 PM, Tang Chen wrote: >>> From: Wen Congyang <wency@xxxxxxxxxxxxxx> >>> >>> memory can't be offlined when CONFIG_MEMCG is selected. >>> For example: there is a memory device on node 1. The address range >>> is [1G, 1.5G). You will find 4 new directories memory8, memory9, memory10, >>> and memory11 under the directory /sys/devices/system/memory/. >>> >>> If CONFIG_MEMCG is selected, we will allocate memory to store page cgroup >>> when we online pages. When we online memory8, the memory stored page cgroup >>> is not provided by this memory device. But when we online memory9, the memory >>> stored page cgroup may be provided by memory8. So we can't offline memory8 >>> now. We should offline the memory in the reversed order. >>> >>> When the memory device is hotremoved, we will auto offline memory provided >>> by this memory device. But we don't know which memory is onlined first, so >>> offlining memory may fail. In such case, iterate twice to offline the memory. >>> 1st iterate: offline every non primary memory block. >>> 2nd iterate: offline primary (i.e. first added) memory block. >>> >>> This idea is suggested by KOSAKI Motohiro. >>> >>> Signed-off-by: Wen Congyang <wency@xxxxxxxxxxxxxx> >> >> Maybe there is something here that I am missing - I admit that I came >> late to this one, but this really sounds like a very ugly hack, that >> really has no place in here. >> >> Retrying, of course, may make sense, if we have reasonable belief that >> we may now succeed. If this is the case, you need to document - in the >> code - while is that. >> >> The memcg argument, however, doesn't really cut it. Why can't we make >> all page_cgroup allocations local to the node they are describing? If >> memcg is the culprit here, we should fix it, and not retry. If there is >> still any benefit in retrying, then we retry being very specific about why. > > We try to make all page_cgroup allocations local to the node they are describing > now. If the memory is the first memory onlined in this node, we will allocate > it from the other node. > > For example, node1 has 4 memory blocks: 8-11, and we online it from 8 to 11 > 1. memory block 8, page_cgroup allocations are in the other nodes > 2. memory block 9, page_cgroup allocations are in memory block 8 > > So we should offline memory block 9 first. But we don't know in which order > the user online the memory block. > > I think we can modify memcg like this: > allocate the memory from the memory block they are describing > > I am not sure it is OK to do so. I don't see a reason why not. You would have to tweak a bit the lookup function for page_cgroup, but assuming you will always have the pfns and limits, it should be easy to do. I think the only tricky part is that today we have a single node_page_cgroup, and we would of course have to have one per memory block. My assumption is that the number of memory blocks is limited and likely not very big. So even a static array would do. Kamezawa, do you have any input in here? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>