Re: [tip:x86/mm] x86/mm/gup: Switch GUP to the generic get_user_page_fast() implementation

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Apr 20, 2017 at 02:46:51PM -0700, Dan Williams wrote:
> On Sat, Mar 18, 2017 at 2:52 AM, tip-bot for Kirill A. Shutemov
> <tipbot@zytor.com> wrote:
> > Commit-ID:  2947ba054a4dabbd82848728d765346886050029
> > Gitweb:     http://git.kernel.org/tip/2947ba054a4dabbd82848728d765346886050029
> > Author:     Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > AuthorDate: Fri, 17 Mar 2017 00:39:06 +0300
> > Committer:  Ingo Molnar <mingo@kernel.org>
> > CommitDate: Sat, 18 Mar 2017 09:48:03 +0100
> >
> > x86/mm/gup: Switch GUP to the generic get_user_page_fast() implementation
> >
> > This patch provides all required callbacks required by the generic
> > get_user_pages_fast() code and switches x86 over - and removes
> > the platform specific implementation.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Aneesh Kumar K . V <aneesh.kumar@linux.vnet.ibm.com>
> > Cc: Borislav Petkov <bp@alien8.de>
> > Cc: Catalin Marinas <catalin.marinas@arm.com>
> > Cc: Dann Frazier <dann.frazier@canonical.com>
> > Cc: Dave Hansen <dave.hansen@intel.com>
> > Cc: H. Peter Anvin <hpa@zytor.com>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Rik van Riel <riel@redhat.com>
> > Cc: Steve Capper <steve.capper@linaro.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: linux-arch@vger.kernel.org
> > Cc: linux-mm@kvack.org
> > Link: http://lkml.kernel.org/r/20170316213906.89528-1-kirill.shutemov@linux.intel.com
> > [ Minor readability edits. ]
> > Signed-off-by: Ingo Molnar <mingo@kernel.org>
> 
> I'm still trying to spot the bug, but bisect points to this patch as
> the point at which my unit tests start failing with the following
> signature:

I can't find the issue either.

Is it something reproducible without hardware? In KVM?

If yes, could you share the test-case?

> [   35.423841] WARNING: CPU: 8 PID: 245 at lib/percpu-refcount.c:155
> percpu_ref_switch_to_atomic_rcu+0x1f5/0x200
> [   35.425328] percpu ref (dax_pmem_percpu_release [dax_pmem]) <= 0
> (0) after switching to atomic
> [   35.425329] Modules linked in: ip6t_rpfilter ip6t_REJECT
> nf_reject_ipv6 xt_conntrack ebtable_nat ebtable_broute bridge stp llc
> ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip
> 6table_mangle ip6table_raw ip6table_security iptable_nat
> nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack
> iptable_mangle iptable_raw iptable_security ebtable_filter ebtables
> ip6table_filter ip6_tables crct10dif_pclmul crc32_pclmul crc32c_intel
> ghash_clmulni_intel nd_pmem(O) dax_pmem(O) nd_btt(O) dax(O) serio_raw
> nfit(O) nd_e820(O) libnvdimm(O) tpm_tis tpm_tis_co
> re tpm nfit_test_iomap(O) nfsd nfs_acl
> [   35.433683] CPU: 8 PID: 245 Comm: rcuos/29 Tainted: G           O
>  4.11.0-rc2+ #55
> [   35.435538] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
> BIOS 1.9.3-1.fc25 04/01/2014
> [   35.437500] Call Trace:
> [   35.438270]  dump_stack+0x86/0xc3
> [   35.439156]  __warn+0xcb/0xf0
> [   35.439995]  warn_slowpath_fmt+0x5f/0x80
> [   35.440962]  ? rcu_nocb_kthread+0x27a/0x500
> [   35.441957]  ? dax_pmem_percpu_exit+0x50/0x50 [dax_pmem]
> [   35.443107]  percpu_ref_switch_to_atomic_rcu+0x1f5/0x200
> [   35.444251]  ? percpu_ref_exit+0x60/0x60
> [   35.445206]  rcu_nocb_kthread+0x327/0x500
> [   35.446186]  ? rcu_nocb_kthread+0x27a/0x500
> [   35.447188]  kthread+0x10c/0x140
> [   35.448058]  ? rcu_eqs_enter+0x50/0x50
> [   35.448990]  ? kthread_create_on_node+0x60/0x60
> [   35.450038]  ret_from_fork+0x31/0x40
> [   35.450976] ---[ end trace eaa40898a09519b5 ]---
> 
> This is similar to the backtrace when we were not properly handling
> pud faults and was fixed with this commit: 220ced1676c4 "mm: fix
> get_user_pages() vs device-dax pud mappings"
> 
> I've found some missing _devmap checks in the generic
> get_user_pages_fast() path, but this does not fix the regression:

I don't see these in x86 GUP. Was the bug there too?

-- 
 Kirill A. Shutemov