Re: [RFC] mm: kmemleak: replace __GFP_NOFAIL to GFP_NOWAIT in gfp_kmemleak_mask

[Chunyu Hu <chuhu@redhat.com>]

----- Original Message -----
> From: "Catalin Marinas" <catalin.marinas@arm.com>
> To: "Chunyu Hu" <chuhu@redhat.com>
> Cc: "Michal Hocko" <mhocko@kernel.org>, "Chunyu Hu" <chuhu.ncepu@gmail.com>, "Dmitry Vyukov" <dvyukov@google.com>,
> "LKML" <linux-kernel@vger.kernel.org>, "Linux-MM" <linux-mm@kvack.org>
> Sent: Thursday, April 26, 2018 8:56:35 PM
> Subject: Re: [RFC] mm: kmemleak: replace __GFP_NOFAIL to GFP_NOWAIT in gfp_kmemleak_mask
> 
> On Thu, Apr 26, 2018 at 08:23:19AM -0400, Chunyu Hu wrote:
> > kmemleak is using kmem_cache to record every pointers returned from kernel
> > mem
> > allocation activities such as kmem_cache_alloc(). every time an object from
> > slab allocator is returned, a following new kmemleak object is allocated.
> > 
> > And when a slab object is freed, then the kmemleak object which contains
> > the ptr will also be freed.
> > 
> > and kmemleak scan thread will run in period to scan the kernel data, stack,
> > and per cpu areas to check that every pointers recorded by kmemleak has at
> > least
> > one reference in those areas beside the one recorded by kmemleak. If there
> > is no place in the memory acreas recording the ptr, then it's possible a
> > leak.
> > 
> > so once a kmemleak object allocation failed, it has to disable itself,
> > otherwise
> > it would lose track of some object pointers, and become less meaningful to
> > continue record and scan the kernel memory for the pointers. So disable
> > it forever. so this is why kmemleak can't tolerate a slab alloc fail (from
> > fault injection)
> > 
> > @Catalin,
> > 
> > Is this right? If something not so correct or precise, please correct me.
> 
> That's a good description, thanks.
> 
> > I'm thinking about, is it possible that make kmemleak don't disable itself
> > when fail_page_alloc is enabled?  I can't think clearly what would happen
> > if several memory allocation missed by kmelkeak trace, what's the bad
> > result?
> 
> Take for example a long linked list. If kmemleak doesn't track an object
> in such list (because the metadata allocation failed), such list_head is
> never scanned and the subsequent objects in the list (pointed at by
> 'next') will be reported as leaks. Kmemleak pretty much becomes unusable
> with a high number of false positives.

Thanks for the example, one object may contain many pointers, so loose one,
means many false reports. I'm clear now. 

> 
> --
> Catalin
> 

-- 
Regards,
Chunyu Hu