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Date:   Tue, 5 Jan 2021 09:16:54 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Dan Williams <dan.j.williams@intel.com>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>,
        Oscar Salvador <osalvador@suse.de>
Subject: Re: uninitialized pmem struct pages
Message-ID: <20210105081654.GU13207@dhcp22.suse.cz>
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On Tue 05-01-21 09:01:00, Michal Hocko wrote:
> On Mon 04-01-21 16:44:52, David Hildenbrand wrote:
> > On 04.01.21 16:43, David Hildenbrand wrote:
> > > On 04.01.21 16:33, Michal Hocko wrote:
> > >> On Mon 04-01-21 16:15:23, David Hildenbrand wrote:
> > >>> On 04.01.21 16:10, Michal Hocko wrote:
> > >> [...]
> > >>> Do the physical addresses you see fall into the same section as boot
> > >>> memory? Or what's around these addresses?
> > >>
> > >> Yes I am getting a garbage for the first struct page belonging to the
> > >> pmem section [1]
> > >> [    0.020161] ACPI: SRAT: Node 0 PXM 0 [mem 0x100000000-0x603fffffff]
> > >> [    0.020163] ACPI: SRAT: Node 4 PXM 4 [mem 0x6060000000-0x11d5fffffff] non-volatile
> > >>
> > >> The pfn without the initialized struct page is 0x6060000. This is a
> > >> first pfn in a section.
> > > 
> > > Okay, so we're not dealing with the "early section" mess I described,
> > > different story.
> > > 
> > > Due to [1], is_mem_section_removable() called
> > > pfn_to_page(PHYS_PFN(0x6060000)). page_zone(page) made it crash, as not
> > > initialized.
> > > 
> > > Let's assume this is indeed a reserved pfn in the altmap. What's the
> > > actual address of the memmap?
> > > 
> > > I do wonder what hosts pfn_to_page(PHYS_PFN(0x6060000)) - is it actually
> > > part of the actual altmap (i.e. > 0x6060000) or maybe even self-hosted?
> > > 
> > > If it's not self-hosted, initializing the relevant memmaps should work
> > > just fine I guess. Otherwise things get more complicated.
> > 
> > Oh, I forgot: pfn_to_online_page() should at least in your example make
> > sure other pfn walkers are safe. It was just an issue of
> > is_mem_section_removable().
> 
> Hmm, I suspect you are right. I haven't put this together, thanks! The memory
> section is indeed marked offline so pfn_to_online_page would indeed bail
> out:
> crash> p (0x6060000>>15)
> $3 = 3084
> crash> p mem_section[3084/128][3084 & 127]
> $4 = {
>   section_mem_map = 18446736128020054019,
>   usage = 0xffff902dcf956680,
>   page_ext = 0x0,
>   pad = 0
> }
> crash> p 18446736128020054019 & (1UL<<2)
> $5 = 0
> 
> That makes it considerably less of a problem than I thought!

Forgot to add that those who are running kernels without 53cdc1cb29e8
("drivers/base/memory.c: indicate all memory blocks as removable") for
some reason can fix the crash by the following simple patch.

Index: linux-5.3-users_mhocko_SLE15-SP2_for-next/drivers/base/memory.c
===================================================================
--- linux-5.3-users_mhocko_SLE15-SP2_for-next.orig/drivers/base/memory.c
+++ linux-5.3-users_mhocko_SLE15-SP2_for-next/drivers/base/memory.c
@@ -152,9 +152,14 @@ static ssize_t removable_show(struct dev
 		goto out;
 
 	for (i = 0; i < sections_per_block; i++) {
-		if (!present_section_nr(mem->start_section_nr + i))
+		unsigned long nr = mem->start_section_nr + i;
+		if (!present_section_nr(nr))
 			continue;
-		pfn = section_nr_to_pfn(mem->start_section_nr + i);
+		if (!online_section_nr()) {
+			ret = 0;
+			break;
+		}
+		pfn = section_nr_to_pfn(nr);
 		ret &= is_mem_section_removable(pfn, PAGES_PER_SECTION);
 	}
 

-- 
Michal Hocko
SUSE Labs