Re: [PATCH 1/2 V2] memory-hotplug: fix BUG_ON in move_freepages()

[Yasuaki Ishimatsu]

On Mon, 20 Apr 2015 11:42:10 +0800
Xishi Qiu <qiuxishi@xxxxxxxxxx> wrote:

> On 2015/4/20 11:29, Yasuaki Ishimatsu wrote:
> 
> > 
> > On Mon, 20 Apr 2015 10:45:45 +0800
> > Xishi Qiu <qiuxishi@xxxxxxxxxx> wrote:
> > 
> >> On 2015/4/20 9:42, Gu Zheng wrote:
> >>
> >>> Hi Xishi,
> >>> On 04/18/2015 04:05 AM, Yasuaki Ishimatsu wrote:
> >>>
> >>>>
> >>>> Your patches will fix your issue.
> >>>> But, if BIOS reports memory first at node hot add, pgdat can
> >>>> not be initialized.
> >>>>
> >>>> Memory hot add flows are as follows:
> >>>>
> >>>> add_memory
> >>>>   ...
> >>>>   -> hotadd_new_pgdat()
> >>>>   ...
> >>>>   -> node_set_online(nid)
> >>>>
> >>>> When calling hotadd_new_pgdat() for a hot added node, the node is
> >>>> offline because node_set_online() is not called yet. So if applying
> >>>> your patches, the pgdat is not initialized in this case.
> >>>
> >>> Ishimtasu's worry is reasonable. And I am afraid the fix here is a bit
> >>> over-kill. 
> >>>
> >>>>
> >>>> Thanks,
> >>>> Yasuaki Ishimatsu
> >>>>
> >>>> On Fri, 17 Apr 2015 18:50:32 +0800
> >>>> Xishi Qiu <qiuxishi@xxxxxxxxxx> wrote:
> >>>>
> >>>>> Hot remove nodeXX, then hot add nodeXX. If BIOS report cpu first, it will call
> >>>>> hotadd_new_pgdat(nid, 0), this will set pgdat->node_start_pfn to 0. As nodeXX
> >>>>> exists at boot time, so pgdat->node_spanned_pages is the same as original. Then
> >>>>> free_area_init_core()->memmap_init() will pass a wrong start and a nonzero size.
> >>>
> >>> As your analysis said the root cause here is passing a *0* as the node_start_pfn,
> >>> then the chaos occurred when init the zones. And this only happens to the re-hotadd
> >>> node, so how about using the saved *node_start_pfn* (via get_pfn_range_for_nid(nid, &start_pfn, &end_pfn))
> >>> instead if we find "pgdat->node_start_pfn == 0 && !node_online(XXX)"?
> >>>
> >>> Thanks,
> >>> Gu
> >>>
> >>
> >> Hi Gu,
> >>
> >> I first considered this method, but if the hot added node's start and size are different
> >> from before, it makes the chaos.
> >>
> > 
> >> e.g.
> >> nodeXX (8-16G)
> >> remove nodeXX 
> >> BIOS report cpu first and online it
> >> hotadd nodeXX
> >> use the original value, so pgdat->node_start_pfn is set to 8G, and size is 8G
> >> BIOS report mem(10-12G)
> >> call add_memory()->__add_zone()->grow_zone_span()/grow_pgdat_span()
> >> the start is still 8G, not 10G, this is chaos!
> > 
> > If you set CONFIG_HAVE_MEMBLOCK_NODE_MAP, kernel shows the following
> > pr_info()'s message.
> > 
> > void __paginginit free_area_init_node(int nid, unsigned long *zones_size,
> >                 unsigned long node_start_pfn, unsigned long *zholes_size)
> > {
> > ...
> > #ifdef CONFIG_HAVE_MEMBLOCK_NODE_MAP
> >         get_pfn_range_for_nid(nid, &start_pfn, &end_pfn);
> >         pr_info("Initmem setup node %d [mem %#018Lx-%#018Lx]\n", nid,
> >                 (u64)start_pfn << PAGE_SHIFT, ((u64)end_pfn << PAGE_SHIFT) - 1);
> > #endif
> > }
> > 
> > Is the memory range of the message "8G - 16G"?
> > If so, the reason is that memblk is not deleted at memory hot remove.
> > 
> > Thanks,
> > Yasuaki Ishimatsu
> > 
> 
> Hi Yasuaki,
> 

> By reading the code, I find memblk is not deleted at memory hot remove.
> I am not sure whether we should remove it. If remove it, we should also reset
> "arch_zone_lowest_possible_pfn", right? It seems a little complicated.

I think memblk should be added/removed by hot adding/removing memory.
But, arch_zone_lowest_possible_pfn should not be changed.

Thanks,
Yasuaki Ishimatsu

> 
> Thanks,
> Xishi Qiu
> 
> > 
> > 
> >>
> >> Thanks,
> >> Xishi Qiu
> >>
> > 
> > .
> > 
> 
> 
> 
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