LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Michal Hocko <email@example.com>
To: Mark Rutland <firstname.lastname@example.org>
Cc: Anshuman Khandual <email@example.com>,
firstname.lastname@example.org, email@example.com, firstname.lastname@example.org,
Subject: Re: [PATCH V3 2/4] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump
Date: Thu, 16 May 2019 13:05:29 +0200 [thread overview]
Message-ID: <20190516110529.GQ16651@dhcp22.suse.cz> (raw)
On Thu 16-05-19 11:23:54, Mark Rutland wrote:
> Hi Michal,
> On Wed, May 15, 2019 at 06:58:47PM +0200, Michal Hocko wrote:
> > On Tue 14-05-19 14:30:05, Anshuman Khandual wrote:
> > > The arm64 pagetable dump code can race with concurrent modification of the
> > > kernel page tables. When a leaf entries are modified concurrently, the dump
> > > code may log stale or inconsistent information for a VA range, but this is
> > > otherwise not harmful.
> > >
> > > When intermediate levels of table are freed, the dump code will continue to
> > > use memory which has been freed and potentially reallocated for another
> > > purpose. In such cases, the dump code may dereference bogus addressses,
> > > leading to a number of potential problems.
> > >
> > > Intermediate levels of table may by freed during memory hot-remove, or when
> > > installing a huge mapping in the vmalloc region. To avoid racing with these
> > > cases, take the memory hotplug lock when walking the kernel page table.
> > Why is this a problem only on arm64
> It looks like it's not -- I think we're just the first to realise this.
> AFAICT x86's debugfs ptdump has the same issue if run conccurently with
> memory hot remove. If 32-bit arm supported hot-remove, its ptdump code
> would have the same issue.
> > and why do we even care for debugfs? Does anybody rely on this thing
> > to be reliable? Do we even need it? Who is using the file?
> The debugfs part is used intermittently by a few people working on the
> arm64 kernel page tables. We use that both to sanity-check that kernel
> page tables are created/updated correctly after changes to the arm64 mmu
> code, and also to debug issues if/when we encounter issues that appear
> to be the result of kernel page table corruption.
OK, I see. Thanks for the clarification.
> So while it's rare to need it, it's really useful to have when we do
> need it, and I'd rather not remove it. I'd also rather that it didn't
> have latent issues where we can accidentally crash the kernel when using
> it, which is what this patch is addressing.
While I agree, do we rather want to document that you shouldn't be using
the debugging tool while the hotplug is ongoing because you might get a
garbage or crash the kernel in the worst case? In other words is the
absolute correctness worth the additional maint. burden wrt. to future
> > I am asking because I would really love to make mem hotplug locking less
> > scattered outside of the core MM than more. Most users simply shouldn't
> > care. Pfn walkers should rely on pfn_to_online_page.
> I'm not sure if that would help us here; IIUC pfn_to_online_page() alone
> doesn't ensure that the page remains online. Is there a way to achieve
> that other than get_online_mems()?
You have to pin the page to make sure the hotplug is not going to
> The big problem for the ptdump code is when tables are freed, since the
> pages can be reused elsewhere (or hot-removed), causing the ptdump code
> to explode.
Yes, I see the danger. I am just wondering whether living with that is
reasonable considering this is a debugfs code.
next prev parent reply other threads:[~2019-05-16 11:05 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-14 9:00 [PATCH V3 0/4] arm64/mm: Enable memory hot remove Anshuman Khandual
2019-05-14 9:00 ` [PATCH V3 1/4] mm/hotplug: Reorder arch_remove_memory() call in __remove_memory() Anshuman Khandual
2019-05-14 9:05 ` David Hildenbrand
2019-05-14 9:00 ` [PATCH V3 2/4] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump Anshuman Khandual
2019-05-14 9:16 ` David Hildenbrand
2019-05-14 15:40 ` Mark Rutland
2019-05-15 1:56 ` Anshuman Khandual
2019-05-15 16:58 ` Michal Hocko
2019-05-16 10:23 ` Mark Rutland
2019-05-16 11:05 ` Michal Hocko [this message]
2019-05-22 16:42 ` Mark Rutland
2019-05-24 6:06 ` Anshuman Khandual
2019-05-27 7:20 ` Michal Hocko
2019-05-28 14:09 ` Mark Rutland
2019-05-16 11:06 ` Anshuman Khandual
2019-05-16 11:16 ` Michal Hocko
2019-05-23 8:40 ` David Hildenbrand
2019-05-24 5:43 ` Anshuman Khandual
2019-05-14 9:00 ` [PATCH V3 3/4] arm64/mm: Inhibit huge-vmap with ptdump Anshuman Khandual
2019-05-14 9:20 ` David Hildenbrand
2019-05-16 8:38 ` Ard Biesheuvel
2019-05-14 9:00 ` [PATCH V3 4/4] arm64/mm: Enable memory hot remove Anshuman Khandual
2019-05-14 9:08 ` David Hildenbrand
2019-05-15 11:49 ` Mark Rutland
2019-05-16 5:34 ` Anshuman Khandual
2019-05-16 10:57 ` Mark Rutland
2019-05-17 3:15 ` Anshuman Khandual
2019-05-14 9:10 ` [PATCH V3 0/4] " David Hildenbrand
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--subject='Re: [PATCH V3 2/4] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump' \
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).