Re: [PATCH] Revert "perf machine: Fix paranoid check in machine__set_kernel_mmap()"
From: Namhyung Kim
Date: Fri Apr 13 2018 - 03:40:13 EST
On Thu, Apr 12, 2018 at 07:09:54PM -0500, Kim Phillips wrote:
> On Thu, 12 Apr 2018 10:57:56 +0900
> Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> > On Wed, Apr 11, 2018 at 07:29:40PM -0500, Kim Phillips wrote:
> > > On Thu, 12 Apr 2018 08:31:09 +0900
> > > Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> > > > overflow can create it.. could you please show me the mmap event?
> > > >
> > > > $ perf script --show-mmap-events
> > >
> > > Here you go:
> > >
> > > swapper 0 [000] 0.000000: PERF_RECORD_MMAP -1/0: [0xffff200008080000(0xdffff7f7ffff) @ 0xffff200008080000]: x [kernel.kallsyms]_text
> >
> > 0xffff200008080000
> > + 0xdffff7f7ffff
> > --------------------
> > 0xffffffffffffffff
> >
> > So it should have ~0ULL of the 'end' already. I don't know why it
> > caused the problem.. Was it from the `perf.good`?
>
> There's no difference between the output of perf.good and perf.bad (the
> difference between them is this reversion patch, .good being the one
> that includes it).
Strange.. I have no idea what's the problem if the end is already
~0ULL then.
>
> > > swapper 0 [000] 0.000000: PERF_RECORD_MMAP -1/0: [0xffff2000021e0000(0x18000) @ 0]: x /lib/modules/4.16.0+/kernel/drivers/bus/arm-ccn.ko
> >
> > Hmm.. also it seems arm64 loads modules under the kernel. Then the
> > fixup logic in machine__create_kernel_maps() won't work. Also as we
>
> you mean this:
>
> void __map_groups__fixup_end(struct map_groups *mg, enum map_type type)
> ...
> if (!curr->end)
> curr->end = next->start;
>
> ?
Right. For the fixup routine to work, a map should have 0 end.
> Module symbol resolution is working, however. Maybe because this
> 'paranoid' check was setting all the end addresses to ~0ULL?
Only if the end was 0. Modules should have non-zero end.
> Does the
> design assume what maps__first() returns always has the lowest address,
> and they're in sorted order? If so, what's the fix, to re-sort
> map_groups afterwards?
I think it's another problem but agree with you to fix it sorted.
>
> > don't use the map_groups__fixup_end() anymore
>
> ? I see map_groups__fixup_end() still being called.
Oops, sorry. I looked a wrong branch (which I removed it but never
sent to the LKML).
>
> > I think it's ok just checking the end address.
>
> Where? In machine__set_kernel_mmap(), like this reversion does?
Yes, the reason is all other maps (i.e. modules) are already have
non-zero end and we can fixup the end of kernel manually.
>
> The original commit says:
>
> The machine__set_kernel_mmap() is to setup addresses of the kernel map
> using external info. But it has a check when the address is given from
> an incorrect input which should have the start and end address of 0
> (i.e. machine__process_kernel_mmap_event).
>
> But we also use the end address of 0 for a valid input so change it to
> check both start and end addresses.
>
> yet the comment above the code says:
>
> * Be a bit paranoid here, some perf.data file came with
> * a zero sized synthesized MMAP event for the kernel.
>
> I tracked the first comment insertion down to this commit:
>
> commit 10fe12ef631a7e85022ed26304a37f033a6a95b8
> Author: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Date: Sat Feb 20 19:53:13 2010 -0200
>
> perf symbols: Fix up map end too on modular kernels with no modules installed
>
> In 2161db9 we stopped failing when not finding modules when
> asked too, but then the kernel maps (just one, for vmlinux)
> wasn't having its ->end field correctly set up, so symbols were
> not being found for the vmlinux map because its range was 0-0.
>
> which, when reading that last part, one would assume start == end == 0,
> therefore size also == 0, the comment only talks about a zero *sized*
> event...so did the original commit 1d12cec6ce99 "perf machine: Fix
> paranoid check in machine__set_kernel_mmap()" really fix that case?
> Because it checks start == 0, not necessarily the size...
As I said I thought it only cares the start == end == 0 case and now
the problem is fixed. So I added the start check for the fixup
routine to take care of the end address. Otherwise kernel might have
an overlap with some module(s).
>
> If not, can it be reverted until we figure out what's going on with
> arm64? It's causing a regression in the meantime...
I think the right solution would be
1. sort map groups
2. set kernel end to ~0ULL in machine__create_kernel_maps()
3. fix the kernel end to the start of next module (if any) manually
instead of calling map_groups__fixup_end()
Thanks,
Namhyung