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Date:	Mon, 16 Jun 2014 12:40:06 +1000
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Denis Kirjanov <kda@...ux-powerpc.org>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Paul Mackerras <paulus@...ba.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: kmemleak: Unable to handle kernel paging request

On Fri, 2014-06-13 at 14:26 +0400, Denis Kirjanov wrote:
> On 6/13/14, Catalin Marinas <catalin.marinas@....com> wrote:
> > On Fri, Jun 13, 2014 at 08:12:08AM +0100, Denis Kirjanov wrote:
> >> On 6/12/14, Catalin Marinas <catalin.marinas@....com> wrote:
> >> > On Thu, Jun 12, 2014 at 01:00:57PM +0100, Denis Kirjanov wrote:
> >> >> On 6/12/14, Denis Kirjanov <kda@...ux-powerpc.org> wrote:
> >> >> > On 6/12/14, Catalin Marinas <catalin.marinas@....com> wrote:
> >> >> >> On 11 Jun 2014, at 21:04, Denis Kirjanov <kda@...ux-powerpc.org>
> >> >> >> wrote:
> >> >> >>> On 6/11/14, Catalin Marinas <catalin.marinas@....com> wrote:
> >> >> >>>> On Wed, Jun 11, 2014 at 04:13:07PM +0400, Denis Kirjanov wrote:
> >> >> >>>>> I got a trace while running 3.15.0-08556-gdfb9454:
> >> >> >>>>>
> >> >> >>>>> [  104.534026] Unable to handle kernel paging request for data
> >> >> >>>>> at
> >> >> >>>>> address 0xc00000007f000000
> >> >> >>>>
> >> >> >>>> Were there any kmemleak messages prior to this, like "kmemleak
> >> >> >>>> disabled"? There could be a race when kmemleak is disabled
> >> >> >>>> because
> >> >> >>>> of
> >> >> >>>> some fatal (for kmemleak) error while the scanning is taking
> >> >> >>>> place
> >> >> >>>> (which needs some more thinking to fix properly).
> >> >> >>>
> >> >> >>> No. I checked for the similar problem and didn't find anything
> >> >> >>> relevant.
> >> >> >>> I'll try to bisect it.
> >> >> >>
> >> >> >> Does this happen soon after boot? I guess it’s the first scan
> >> >> >> (scheduled at around 1min after boot). Something seems to be
> >> >> >> telling
> >> >> >> kmemleak that there is a valid memory block at 0xc00000007f000000.
> >> >> >
> >> >> > Yeah, it happens after a while with a booted system so that's the
> >> >> > first kmemleak scan.
> >> >>
> >> >> I've bisected to this commit: d4c54919ed86302094c0ca7d48a8cbd4ee753e92
> >> >> "mm: add !pte_present() check on existing hugetlb_entry callbacks".
> >> >> Reverting the commit fixes the issue
> >> >
> >> > I can't figure how this causes the problem but I have more questions.
> >> > Is
> >> > 0xc00000007f000000 address always the same in all crashes? If yes, you
> >> > could comment out start_scan_thread() in kmemleak_late_init() to avoid
> >> > the scanning thread starting. Once booted, you can run:
> >> >
> >> >   echo dump=0xc00000007f000000 > /sys/kernel/debug/kmemleak
> >> >
> >> > and check the dmesg for what kmemleak knows about that address, when it
> >> > was allocated and whether it should be mapped or not.
> >>
> >> The address is always the same.
> >>
> >> [  179.466239] kmemleak: Object 0xc00000007f000000 (size 16777216):
> >> [  179.466503] kmemleak:   comm "swapper/0", pid 0, jiffies 4294892300
> >> [  179.466508] kmemleak:   min_count = 0
> >> [  179.466512] kmemleak:   count = 0
> >> [  179.466517] kmemleak:   flags = 0x1
> >> [  179.466522] kmemleak:   checksum = 0
> >> [  179.466526] kmemleak:   backtrace:
> >> [  179.466531]      [<c000000000afc3dc>]
> >> .memblock_alloc_range_nid+0x68/0x88
> >> [  179.466544]      [<c000000000afc444>] .memblock_alloc_base+0x20/0x58
> >> [  179.466553]      [<c000000000ae96cc>] .alloc_dart_table+0x5c/0xb0
> >> [  179.466561]      [<c000000000aea300>] .pmac_probe+0x38/0xa0
> >> [  179.466569]      [<000000000002166c>] 0x2166c
> >> [  179.466579]      [<0000000000ae0e68>] 0xae0e68
> >> [  179.466587]      [<0000000000009bc4>] 0x9bc4
> >
> > OK, so that's the DART table allocated via alloc_dart_table(). Is
> > dart_tablebase removed from the kernel linear mapping after allocation?
> > If that's the case, we need to tell kmemleak to ignore this block (see
> > patch below, untested). But I still can't explain how commit
> > d4c54919ed863020 causes this issue.
> >
> > (also cc'ing the powerpc list and maintainers)
> 
> Ok, your path fixes the oops.
> 
> Ben, can you shed some light on this issue?

(I'm not Ben)

Yes, the memory for dart_tablebase is removed from the linear mapping. In fact
it's never mapped, see htab_initialize().

I don't easily see how commit d4c54919ed8, could have exposed this, but I don't
know enough of the kmemleak internals to say for sure.

cheers


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