Re: [PATCH 15/28] mm, page_alloc: Move might_sleep_if check to the allocator slowpath

[Vlastimil Babka]

On 04/26/2016 04:50 PM, Mel Gorman wrote:
> On Tue, Apr 26, 2016 at 03:41:22PM +0200, Vlastimil Babka wrote:
> > On 04/15/2016 11:07 AM, Mel Gorman wrote:
> > >There is a debugging check for callers that specify __GFP_DIRECT_RECLAIM
> > >from a context that cannot sleep. Triggering this is almost certainly
> > >a bug but it's also overhead in the fast path.
> >
> > For CONFIG_DEBUG_ATOMIC_SLEEP, enabling is asking for the overhead. But for
> > CONFIG_PREEMPT_VOLUNTARY which turns it into _cond_resched(), I guess it's
> > not.
>
> Either way, it struck me as odd. It does depend on the config and it's
> marginal so if there is a problem then I can drop it.

What I tried to say is that it makes sense, but it's perhaps non-obvious :)

> > >Move the check to the slow
> > >path. It'll be harder to trigger as it'll only be checked when watermarks
> > >are depleted but it'll also only be checked in a path that can sleep.
> >
> > Hmm what about zone_reclaim_mode=1, should the check be also duplicated to
> > that part of get_page_from_freelist()?
>
> zone_reclaim has a !gfpflags_allow_blocking() check, does not call
> cond_resched() before that check so it does not fall into an accidental
> sleep path. I'm not seeing why the check is necessary there.

Hmm I thought the primary purpose of this might_sleep_if() is to catch those 
(via the DEBUG_ATOMIC_SLEEP) that do pass __GFP_DIRECT_RECLAIM (which means 
gfpflags_allow_blocking() will be true and zone_reclaim will proceed), but do so 
from the wrong context. Am I getting that wrong?