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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190516111607.GR16651@dhcp22.suse.cz> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.43]); Thu, 23 May 2019 08:40:10 +0000 (UTC) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190523_014012_008464_BFC1F266 X-CRM114-Status: GOOD ( 25.56 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , ira.weiny@intel.com, catalin.marinas@arm.com, cai@lca.pw, will.deacon@arm.com, linux-kernel@vger.kernel.org, logang@deltatee.com, james.morse@arm.com, cpandya@codeaurora.org, arunks@codeaurora.org, akpm@linux-foundation.org, osalvador@suse.de, mgorman@techsingularity.net, dan.j.williams@intel.com, linux-arm-kernel@lists.infradead.org, robin.murphy@arm.com Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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 pages vanishing. 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 offlining/removinf. -- Thanks, David / dhildenb _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel