LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: David Hildenbrand <email@example.com>
To: Michal Hocko <firstname.lastname@example.org>,
Anshuman Khandual <email@example.com>
Cc: Mark Rutland <firstname.lastname@example.org>,
email@example.com, firstname.lastname@example.org, email@example.com,
Subject: Re: [PATCH V3 2/4] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump
Date: Thu, 23 May 2019 10:40:05 +0200 [thread overview]
Message-ID: <firstname.lastname@example.org> (raw)
On 16.05.19 13:16, Michal Hocko wrote:
> On Thu 16-05-19 16:36:12, Anshuman Khandual wrote:
>> On 05/16/2019 03:53 PM, 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.
>>> 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.
>>>> 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()?
>> Still wondering how pfn_to_online_page() is applicable here. It validates
>> a given PFN and whether its online from sparse section mapping perspective
>> before giving it's struct page. IIUC it is used during a linear scanning
>> of a physical address range not for a page table walk. So how it can solve
>> the problem when a struct page which was used as an intermediate level page
>> table page gets released back to the buddy from another concurrent thread ?
> Well, my comment about pfn_to_online_page was more generic and it might
> not apply to this specific case. I meant to say that the code outside of
> the core MM shouldn't really care about the hotplug locking.
What am I missing, how is it guaranteed that a page doesn't get
offlined/removed without holding a lock here?
We would at least need some RCU mechnism or similar to sync against
pfn_to_online_page() assumes that somebody touches a page he doesn't
own. There has to be some way for core-mm to realize this and defer
David / dhildenb
next prev parent reply other threads:[~2019-05-23 8:40 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
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 [this message]
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).