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. 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, -- Vitaly _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
