linux-kernel - Re: [PATCH 1/1] mm/vmalloc: remove preempt

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Date:   Tue, 15 Oct 2019 11:54:47 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wagner <dwagner@...e.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Hillf Danton <hdanton@...a.com>,
        Matthew Wilcox <willy@...radead.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 1/1] mm/vmalloc: remove preempt_disable/enable when do
 preloading

> > > > > > : 	 * The preload is done in non-atomic context, thus it allows us
> > > > > > : 	 * to use more permissive allocation masks to be more stable under
> > > > > > : 	 * low memory condition and high memory pressure.
> > > > > > : 	 *
> > > > > > : 	 * Even if it fails we do not really care about that. Just proceed
> > > > > > : 	 * as it is. "overflow" path will refill the cache we allocate from.
> > > > > > : 	 */
> > > > > > : 	if (!this_cpu_read(ne_fit_preload_node)) {
> > > > > > 
> > > > > > Readability nit: local `pva' should be defined here, rather than having
> > > > > > function-wide scope.
> > > > > > 
> > > > > > : 		pva = kmem_cache_alloc_node(vmap_area_cachep, GFP_KERNEL, node);
> > > > > > 
> > > > > > Why doesn't this honour gfp_mask?  If it's not a bug, please add
> > > > > > comment explaining this.
> > > > > > 
> > > > But there is a comment, if understand you correctly:
> > > > 
> > > > <snip>
> > > > * Even if it fails we do not really care about that. Just proceed
> > > > * as it is. "overflow" path will refill the cache we allocate from.
> > > > <snip>
> > > 
> > > My point is that the alloc_vmap_area() caller passed us a gfp_t but
> > > this code ignores it, as does adjust_va_to_fit_type().  These *look*
> > > like potential bugs.  If not, they should be commented so they don't
> > > look like bugs any more ;)
> > > 
> > I got it, there was misunderstanding from my side :) I agree.
> > 
> > In the first case i should have used and respect the passed "gfp_mask",
> > like below:
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index f48cd0711478..880b6e8cdeae 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -1113,7 +1113,8 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
> >                  * Just proceed as it is. If needed "overflow" path
> >                  * will refill the cache we allocate from.
> >                  */
> > -               pva = kmem_cache_alloc_node(vmap_area_cachep, GFP_KERNEL, node);
> > +               pva = kmem_cache_alloc_node(vmap_area_cachep,
> > +                               gfp_mask & GFP_RECLAIM_MASK, node);
> >  
> >         spin_lock(&vmap_area_lock);
> > 
> > It should be sent as a separate patch, i think.
> 
> Yes. I do not think this would make any real difference because that
> battle is lost long ago. vmalloc is simply not gfp mask friendly. There
> are places like page table allocation which are hardcoded GFP_KERNEL so
> GFP_NOWAIT semantic is not going to work, really. The above makes sense
> from a pure aesthetic POV, though, I would say.
I agree. Then i will create a patch.

Thank you!

--
Vlad Rezki