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Date: Mon, 4 Jul 2016 14:29:55 +0900
From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] mm/page_owner: track page free call chain
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On Mon, Jul 04, 2016 at 02:07:30PM +0900, Sergey Senozhatsky wrote:
> Hello,
> 
> On (07/04/16 13:57), Joonsoo Kim wrote:
> > On Sun, Jul 03, 2016 at 01:16:56AM +0900, Sergey Senozhatsky wrote:
> > > Introduce PAGE_OWNER_TRACK_FREE config option to extend page owner with
> > > free_pages() tracking functionality. This adds to the dump_page_owner()
> > > output an additional backtrace, that tells us what path has freed the
> > > page.
> > 
> > Hmm... Do you have other ideas to use this feature? Following example is
> > just to detect use-after-free and we have other good tools for it
> > (KASAN or DEBUG_PAGEALLOC) so I'm not sure whether it's useful or not.
> 
> there is no kasan for ARM32, for example (apart from the fact that
> it's really hard to use kasan sometimes due to its cpu cycles and
> memory requirements).

Hmm... for debugging purpose, KASAN provides many more things so IMO it's
better to implement/support KASAN in ARM32 rather than expand
PAGE_OWNER for free.

> 
> educate me, will DEBUG_PAGEALLOC tell us what path has triggered the
> extra put_page()? hm... does ARM32 provide ARCH_SUPPORTS_DEBUG_PAGEALLOC?

Hmm... Now, I notice that PAGE_OWNER_TRACK_FREE will detect
double-free rather than use-after-free. DEBUG_PAGEALLOC doesn't catch
double-free but it can be implemented easily. In this case, we can
show call path for second free.

AFAIK, ARM32 doesn't support ARCH_SUPPORTS_DEBUG_PAGEALLOC.

Thanks.