Messages in this thread | | | Date | Thu, 16 May 2019 13:16:07 +0200 | From | Michal Hocko <> | Subject | Re: [PATCH V3 2/4] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump |
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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. -- Michal Hocko SUSE Labs
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