1105a2fc02
In our armv8a server(QDF2400), I noticed lots of WARN_ON caused by PAGE_SIZE unaligned for rmap_item->address under memory pressure tests(start 20 guests and run memhog in the host). WARNING: CPU: 4 PID: 4641 at virt/kvm/arm/mmu.c:1826 kvm_age_hva_handler+0xc0/0xc8 CPU: 4 PID: 4641 Comm: memhog Tainted: G W 4.17.0-rc3+ #8 Call trace: kvm_age_hva_handler+0xc0/0xc8 handle_hva_to_gpa+0xa8/0xe0 kvm_age_hva+0x4c/0xe8 kvm_mmu_notifier_clear_flush_young+0x54/0x98 __mmu_notifier_clear_flush_young+0x6c/0xa0 page_referenced_one+0x154/0x1d8 rmap_walk_ksm+0x12c/0x1d0 rmap_walk+0x94/0xa0 page_referenced+0x194/0x1b0 shrink_page_list+0x674/0xc28 shrink_inactive_list+0x26c/0x5b8 shrink_node_memcg+0x35c/0x620 shrink_node+0x100/0x430 do_try_to_free_pages+0xe0/0x3a8 try_to_free_pages+0xe4/0x230 __alloc_pages_nodemask+0x564/0xdc0 alloc_pages_vma+0x90/0x228 do_anonymous_page+0xc8/0x4d0 __handle_mm_fault+0x4a0/0x508 handle_mm_fault+0xf8/0x1b0 do_page_fault+0x218/0x4b8 do_translation_fault+0x90/0xa0 do_mem_abort+0x68/0xf0 el0_da+0x24/0x28 In rmap_walk_ksm, the rmap_item->address might still have the STABLE_FLAG, then the start and end in handle_hva_to_gpa might not be PAGE_SIZE aligned. Thus it will cause exceptions in handle_hva_to_gpa on arm64. This patch fixes it by ignoring (not removing) the low bits of address when doing rmap_walk_ksm. IMO, it should be backported to stable tree. the storm of WARN_ONs is very easy for me to reproduce. More than that, I watched a panic (not reproducible) as follows: page:ffff7fe003742d80 count:-4871 mapcount:-2126053375 mapping: (null) index:0x0 flags: 0x1fffc00000000000() raw: 1fffc00000000000 0000000000000000 0000000000000000 ffffecf981470000 raw: dead000000000100 dead000000000200 ffff8017c001c000 0000000000000000 page dumped because: nonzero _refcount CPU: 29 PID: 18323 Comm: qemu-kvm Tainted: G W 4.14.15-5.hxt.aarch64 #1 Hardware name: <snip for confidential issues> Call trace: dump_backtrace+0x0/0x22c show_stack+0x24/0x2c dump_stack+0x8c/0xb0 bad_page+0xf4/0x154 free_pages_check_bad+0x90/0x9c free_pcppages_bulk+0x464/0x518 free_hot_cold_page+0x22c/0x300 __put_page+0x54/0x60 unmap_stage2_range+0x170/0x2b4 kvm_unmap_hva_handler+0x30/0x40 handle_hva_to_gpa+0xb0/0xec kvm_unmap_hva_range+0x5c/0xd0 I even injected a fault on purpose in kvm_unmap_hva_range by seting size=size-0x200, the call trace is similar as above. So I thought the panic is similarly caused by the root cause of WARN_ON. Andrea said: : It looks a straightforward safe fix, on x86 hva_to_gfn_memslot would : zap those bits and hide the misalignment caused by the low metadata : bits being erroneously left set in the address, but the arm code : notices when that's the last page in the memslot and the hva_end is : getting aligned and the size is below one page. : : I think the problem triggers in the addr += PAGE_SIZE of : unmap_stage2_ptes that never matches end because end is aligned but : addr is not. : : } while (pte++, addr += PAGE_SIZE, addr != end); : : x86 again only works on hva_start/hva_end after converting it to : gfn_start/end and that being in pfn units the bits are zapped before : they risk to cause trouble. Jia He said: : I've tested by myself in arm64 server (QDF2400,46 cpus,96G mem) Without : this patch, the WARN_ON is very easy for reproducing. After this patch, I : have run the same benchmarch for a whole day without any WARN_ONs Link: http://lkml.kernel.org/r/1525403506-6750-1-git-send-email-hejianet@gmail.com Signed-off-by: Jia He <jia.he@hxt-semitech.com> Reviewed-by: Andrea Arcangeli <aarcange@redhat.com> Tested-by: Jia He <hejianet@gmail.com> Cc: Suzuki K Poulose <Suzuki.Poulose@arm.com> Cc: Minchan Kim <minchan@kernel.org> Cc: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com> Cc: Arvind Yadav <arvind.yadav.cs@gmail.com> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.