From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>,
"Kirill A. Shutemov" <kirill@shutemov.name>,
Peter Zijlstra <peterz@infradead.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Johannes Weiner <hannes@cmpxchg.org>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
Mel Gorman <mgorman@techsingularity.net>,
Tony Luck <tony.luck@intel.com>, Vlastimil Babka <vbabka@suse.cz>,
Michal Hocko <mhocko@kernel.org>,
"hillf.zj" <hillf.zj@alibaba-inc.com>,
Hugh Dickins <hughd@google.com>, Oleg Nesterov <oleg@redhat.com>,
Rik van Riel <riel@redhat.com>,
Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
Vladimir Davydov <vdavydov.dev@gmail.com>,
Ingo Molnar <mingo@kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-mm <linux-mm@kvack.org>,
the arch/x86 maintainers <x86@kernel.org>
Subject: Re: [mm 4.15-rc8] Random oopses under memory pressure.
Date: Fri, 19 Jan 2018 22:12:43 +0000 [thread overview]
Message-ID: <20180119221243.GL13338@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CA+55aFwWCeFrhN+WJDD8u9nqBzmvknXk428Q0dVwwXAvwhg_-w@mail.gmail.com>
On Fri, Jan 19, 2018 at 10:42:18AM -0800, Linus Torvalds wrote:
> On Fri, Jan 19, 2018 at 4:55 AM, Matthew Wilcox <willy@infradead.org> wrote:
> >
> > So really we should be casting 'b' and 'a' to uintptr_t to be fully
> > compliant with the spec.
>
> That's an unnecessary technicality.
>
> Any compiler that doesn't get pointer inequality testing right is not
> worth even worrying about. We wouldn't want to use such a compiler,
> because it's intentionally generating garbage just to f*ck with us.
> Why would you go along with that?
>
> So the only real issue is that pointer subtraction case.
>
> I actually asked (long long ago) for an optinal compiler warning for
> "pointer subtraction with non-power-of-2 sizes". Not because of it
> being undefined, but simply because it's expensive. The
> divide->multiply thing doesn't always work, and a real divide is
> really quite expensive on many architectures.
>
> We *should* be careful about it. I guess sparse could be made to warn,
> but I'm afraid that we have so many of these things that a warning
> isn't reasonable.
You mean like -Wptr-subtraction-blows?
FWIW, allmodconfig on amd64 with C=2 CF=-Wptr-subtraction-blows is not too large
The tail (alphabetically sorted) is
lib/dynamic_debug.c:1013:9: warning: potentially expensive pointer subtraction
lib/extable.c:70:28: warning: potentially expensive pointer subtraction
mm/memory_hotplug.c:1530:13: warning: potentially expensive pointer subtraction
mm/memory_hotplug.c:734:13: warning: potentially expensive pointer subtraction
mm/memory_hotplug.c:831:41: warning: potentially expensive pointer subtraction
mm/page_owner.c:607:38: warning: potentially expensive pointer subtraction
mm/vmstat.c:1334:38: warning: potentially expensive pointer subtraction
net/core/net-sysfs.c:1040:19: warning: potentially expensive pointer subtraction
net/ipv4/ipmr.c:3026:32: warning: potentially expensive pointer subtraction
net/ipv6/ip6mr.c:458:32: warning: potentially expensive pointer subtraction
net/mac80211/tx.c:1307:41: warning: potentially expensive pointer subtraction
net/mac80211/tx.c:1351:44: warning: potentially expensive pointer subtraction
net/rds/ib_recv.c:345:49: warning: potentially expensive pointer subtraction
net/rds/ib_recv.c:861:38: warning: potentially expensive pointer subtraction
net/sched/cls_tcindex.c:622:43: warning: potentially expensive pointer subtraction
net/sched/sch_cbs.c:302:35: warning: potentially expensive pointer subtraction
net/sched/sch_hhf.c:367:23: warning: potentially expensive pointer subtraction
net/sched/sch_hhf.c:434:38: warning: potentially expensive pointer subtraction
net/sunrpc/svc_xprt.c:1377:43: warning: potentially expensive pointer subtraction
sound/pci/hda/hda_generic.c:301:20: warning: potentially expensive pointer subtraction
IOW it's not terribly noisy. Might be an interesting idea to teach sparse to
print the type in question... Aha - with
--- a/evaluate.c
+++ b/evaluate.c
@@ -848,7 +848,8 @@ static struct symbol *evaluate_ptr_sub(struct expression *expr)
if (value & (value-1)) {
if (Wptr_subtraction_blows)
- warning(expr->pos, "potentially expensive pointer subtraction");
+ warning(expr->pos, "[%s] potentially expensive pointer subtraction",
+ show_typename(lbase));
}
sub->op = '-';
we get things like
drivers/gpu/drm/i915/i915_gem_execbuffer.c:435:17: warning: [struct drm_i915_gem_exec_object2] potentially expensive pointer subtraction
OK, the top sources of that warning are:
91 struct cpufreq_frequency_table
36 struct Indirect (actually, that conflates ext2/ext4/minix/sysv)
21 struct ips_scb
18 struct runlist_element
13 struct zone
13 struct vring
11 struct usbhsh_device
10 struct xpc_partition
9 struct skge_element
9 struct lock_class
9 struct hstate
7 struct nvme_rdma_queue
7 struct iso_context
6 struct i915_power_well
6 struct hpet_dev
6 struct ext4_extent
6 struct esas2r_target
5 struct iio_chan_spec
5 struct hwspinlock
4 struct myri10ge_slice_state
4 struct ext4_extent_idx
everything beyond that is 3 instances or less...
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next prev parent reply other threads:[~2018-01-19 22:13 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-05 14:45 [x86? mm? fs? 4.15-rc6] Random oopses by simple write under memory pressure Tetsuo Handa
2018-01-09 10:39 ` [mm? 4.15-rc7] " Tetsuo Handa
2018-01-10 11:49 ` [mm? 4.15-rc7] Random oopses " Tetsuo Handa
2018-01-10 12:45 ` Michal Hocko
2018-01-10 13:37 ` Tetsuo Handa
2018-01-11 13:57 ` Michal Hocko
2018-01-11 14:11 ` Tetsuo Handa
2018-01-11 14:21 ` Michal Hocko
2018-01-11 14:37 ` Tetsuo Handa
2018-01-12 1:31 ` [mm " Tetsuo Handa
2018-01-12 1:42 ` Linus Torvalds
2018-01-12 11:22 ` Tetsuo Handa
2018-01-14 11:54 ` Tetsuo Handa
2018-01-15 23:05 ` Linus Torvalds
2018-01-16 1:15 ` [mm 4.15-rc8] " Tetsuo Handa
2018-01-16 2:14 ` Linus Torvalds
2018-01-16 8:06 ` Dave Hansen
2018-01-16 8:37 ` Ingo Molnar
2018-01-16 19:30 ` Linus Torvalds
2018-01-16 17:33 ` Tetsuo Handa
2018-01-16 19:34 ` Linus Torvalds
2018-01-17 11:08 ` Tetsuo Handa
2018-01-17 21:39 ` Linus Torvalds
2018-01-17 21:51 ` Linus Torvalds
2018-01-17 22:04 ` Dave Hansen
2018-01-17 22:00 ` Dave Hansen
2018-01-17 22:15 ` Linus Torvalds
2018-01-18 8:12 ` Tetsuo Handa
2018-01-18 12:25 ` Kirill A. Shutemov
2018-01-18 13:12 ` Kirill A. Shutemov
2018-01-18 14:34 ` Kirill A. Shutemov
2018-01-18 14:38 ` Dave Hansen
2018-01-18 14:45 ` Kirill A. Shutemov
2018-01-18 14:51 ` Dave Hansen
2018-01-18 16:58 ` Linus Torvalds
2018-01-18 14:45 ` Dave Hansen
2018-01-18 14:58 ` Andrea Arcangeli
2018-01-18 16:56 ` Kirill A. Shutemov
2018-01-18 17:26 ` Luck, Tony
2018-01-18 17:28 ` Linus Torvalds
2018-01-18 17:26 ` Linus Torvalds
2018-01-18 23:49 ` Kirill A. Shutemov
2018-01-19 12:55 ` Matthew Wilcox
2018-01-19 18:42 ` Linus Torvalds
2018-01-19 22:12 ` Al Viro [this message]
2018-01-19 22:53 ` Linus Torvalds
2018-01-20 2:02 ` Al Viro
2018-01-20 5:24 ` Al Viro
2018-01-20 9:38 ` Luc Van Oostenryck
2018-01-18 15:40 ` Kirill A. Shutemov
2018-01-18 17:22 ` Michal Hocko
2018-01-19 10:02 ` Kirill A. Shutemov
2018-01-19 10:33 ` Michal Hocko
2018-01-19 11:49 ` Kirill A. Shutemov
2018-01-19 12:07 ` Michal Hocko
2018-01-19 12:30 ` Kirill A. Shutemov
2018-01-19 2:01 ` Tetsuo Handa
2018-01-11 18:11 ` [mm? 4.15-rc7] " Linus Torvalds
2018-01-11 20:59 ` Tetsuo Handa
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