On 07.01.19 14:44, Vitaly Kuznetsov wrote: > David Hildenbrand <david@xxxxxxxxxx> writes: > >> On 04.01.19 15:19, Vitaly Kuznetsov wrote: >>> Hyper-V memory hotplug protocol has 2M granularity and in Linux x86 we use >>> 128M. To deal with it we implement partial section onlining by registering >>> custom page onlining callback (hv_online_page()). Later, when more memory >>> arrives we try to online the 'tail' (see hv_bring_pgs_online()). >>> >>> It was found that in some cases this 'tail' onlining causes issues: >>> >>> BUG: Bad page state in process kworker/0:2 pfn:109e3a >>> page:ffffe08344278e80 count:0 mapcount:1 mapping:0000000000000000 index:0x0 >>> flags: 0xfffff80000000() >>> raw: 000fffff80000000 dead000000000100 dead000000000200 0000000000000000 >>> raw: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 >>> page dumped because: nonzero mapcount >>> ... >>> Workqueue: events hot_add_req [hv_balloon] >>> Call Trace: >>> dump_stack+0x5c/0x80 >>> bad_page.cold.112+0x7f/0xb2 >>> free_pcppages_bulk+0x4b8/0x690 >>> free_unref_page+0x54/0x70 >>> hv_page_online_one+0x5c/0x80 [hv_balloon] >>> hot_add_req.cold.24+0x182/0x835 [hv_balloon] >>> ... >>> >>> Turns out that we now have deferred struct page initialization for memory >>> hotplug so e.g. memory_block_action() in drivers/base/memory.c does >>> pages_correctly_probed() check and in that check it avoids inspecting >>> struct pages and checks sections instead. But in Hyper-V balloon driver we >>> do PageReserved(pfn_to_page()) check and this is now wrong. >>> >>> Switch to checking online_section_nr() instead. >>> >>> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> >>> --- >>> drivers/hv/hv_balloon.c | 10 ++++++---- >>> 1 file changed, 6 insertions(+), 4 deletions(-) >>> >>> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c >>> index 5301fef16c31..7c6349a50ef1 100644 >>> --- a/drivers/hv/hv_balloon.c >>> +++ b/drivers/hv/hv_balloon.c >>> @@ -888,12 +888,14 @@ static unsigned long handle_pg_range(unsigned long pg_start, >>> pfn_cnt -= pgs_ol; >>> /* >>> * Check if the corresponding memory block is already >>> - * online by checking its last previously backed page. >>> - * In case it is we need to bring rest (which was not >>> - * backed previously) online too. >>> + * online. It is possible to observe struct pages still >>> + * being uninitialized here so check section instead. >>> + * In case the section is online we need to bring the >>> + * rest of pfns (which were not backed previously) >>> + * online too. >>> */ >>> if (start_pfn > has->start_pfn && >>> - !PageReserved(pfn_to_page(start_pfn - 1))) >>> + online_section_nr(pfn_to_section_nr(start_pfn))) >>> hv_bring_pgs_online(has, start_pfn, pgs_ol); >>> >>> } >>> >> >> I wonder if you should use pfn_to_online_page() and check for PageOffline(). >> >> (I guess online_section_nr() should also do the trick) > > I'm worried a bit about racing with mm code here as we're not doing > mem_hotplug_begin()/done() so I'd slightly prefer keeping > online_section_nr() (pfn_to_online_page() also uses it but then it gets > to the particular struct page). Moreover, with pfn_to_online_page() we > will be looking at some other pfn - because the start_pfn is definitelly > offline (pre-patch we were looking at start_pfn-1). Just looking at the > whole section seems cleaner. Fine with me. I guess the section can never be offlined as it still contains reserved pages if not fully "fake-onlined" by HV code already. But we could still consider mem_hotplug_begin()/done() as we could have a online section although online_pages() has not completed yet. So we could actually touch some "semi onlined section". Acked-by: David Hildenbrand <david@xxxxxxxxxx> > > P.S. I still think about bringing mem_hotplug_begin()/done() to > hv_balloon but that's going to be a separate discussion, here I want to > have a small fix backportable to stable. > > Thanks, > -- Thanks, David / dhildenb _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
