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

From: Michal Hocko
Date: Tue Apr 24 2018 - 13:02:51 EST

On Tue 24-04-18 12:48:50, Chunyu Hu wrote:
> ----- Original Message -----
> > From: "Michal Hocko" <mhocko@xxxxxxxxxx>
> > To: "Chunyu Hu" <chuhu.ncepu@xxxxxxxxx>
> > Cc: "Dmitry Vyukov" <dvyukov@xxxxxxxxxx>, "Catalin Marinas" <catalin.marinas@xxxxxxx>, "Chunyu Hu"
> > <chuhu@xxxxxxxxxx>, "LKML" <linux-kernel@xxxxxxxxxxxxxxx>, "Linux-MM" <linux-mm@xxxxxxxxx>
> > Sent: Tuesday, April 24, 2018 9:20:57 PM
> > Subject: Re: [RFC] mm: kmemleak: replace __GFP_NOFAIL to GFP_NOWAIT in gfp_kmemleak_mask
> >
> > On Mon 23-04-18 12:17:32, Chunyu Hu wrote:
> > [...]
> > > So if there is a new flag, it would be the 25th bits.
> >
> > No new flags please. Can you simply store a simple bool into fail_page_alloc
> > and have save/restore api for that?
> Hi Michal,
> I still don't get your point. The original NOFAIL added in kmemleak was
> for skipping fault injection in page/slab allocation for kmemleak object,
> since kmemleak will disable itself until next reboot, whenever it hit an
> allocation failure, in that case, it will lose effect to check kmemleak
> in errer path rose by fault injection. But NOFAULT's effect is more than
> skipping fault injection, it's also for hard allocation. So a dedicated flag
> for skipping fault injection in specified slab/page allocation was mentioned.

I am not familiar with the kmemleak all that much, but fiddling with the
gfp_mask is a wrong way to achieve kmemleak specific action. I might be
easilly wrong but I do not see any code that would restore the original
gfp_mask down the kmem_cache_alloc path.

> d9570ee3bd1d ("kmemleak: allow to coexist with fault injection")
> Do you mean something like below, with the save/store api? But looks like
> to make it possible to skip a specified allocation, not global disabling,
> a bool is not enough, and a gfp_flag is also needed. Maybe I missed something?

Yes, this is essentially what I meant. It is still a global thing which
is not all that great and if it matters then you can make it per
task_struct. That really depends on the code flow here.

Michal Hocko