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

[Michal Hocko <mhocko@kernel.org>]

On Fri 20-04-18 18:50:24, Catalin Marinas wrote:
> On Sat, Apr 21, 2018 at 12:58:33AM +0800, Chunyu Hu wrote:
> > __GFP_NORETRY and  __GFP_NOFAIL are combined in gfp_kmemleak_mask now.
> > But it's a wrong combination. As __GFP_NOFAIL is blockable, but
> > __GFP_NORETY is not blockable, make it self-contradiction.
> > 
> > __GFP_NOFAIL means 'The VM implementation _must_ retry infinitely'. But
> > it's not the real intention, as kmemleak allow alloc failure happen in
> > memory pressure, in that case kmemleak just disables itself.
> 
> Good point. The __GFP_NOFAIL flag was added by commit d9570ee3bd1d
> ("kmemleak: allow to coexist with fault injection") to keep kmemleak
> usable under fault injection.
> 
> > commit 9a67f6488eca ("mm: consolidate GFP_NOFAIL checks in the allocator
> > slowpath") documented that what user wants here should use GFP_NOWAIT, and
> > the WARN in __alloc_pages_slowpath caught this weird usage.
> > 
> >  <snip>
> >  WARNING: CPU: 3 PID: 64 at mm/page_alloc.c:4261 __alloc_pages_slowpath+0x1cc3/0x2780
> [...]
> > Replace the __GFP_NOFAIL with GFP_NOWAIT in gfp_kmemleak_mask, __GFP_NORETRY
> > and GFP_NOWAIT are in the gfp_kmemleak_mask. So kmemleak object allocaion
> > is no blockable and no reclaim, making kmemleak less disruptive to user
> > processes in pressure.
> 
> It doesn't solve the fault injection problem for kmemleak (unless we
> change __should_failslab() somehow, not sure yet). An option would be to
> replace __GFP_NORETRY with __GFP_NOFAIL in kmemleak when fault injection
> is enabled.

Cannot we simply have a disable_fault_injection knob around the
allocation rather than playing this dirty tricks with gfp flags which do
not make any sense?

> BTW, does the combination of NOWAIT and NORETRY make kmemleak
> allocations more likely to fail?

NOWAIT + NORETRY simply doesn't make much sesne. It is equivalent to
NOWAIT.

-- 
Michal Hocko
SUSE Labs