Re: [PATCH] hv_balloon: avoid touching uninitialized struct page during tail onlining

[Sasha Levin]

On Mon, Jan 07, 2019 at 02:44:30PM +0100, 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.
>
> 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.

This should probably be marked for stable then :)

--
Thanks,
Sasha