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

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Date:   Mon, 14 Oct 2019 16:27:19 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        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>,
        Michal Hocko <mhocko@...e.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

On Fri, Oct 11, 2019 at 04:55:15PM -0700, Andrew Morton wrote:
> On Thu, 10 Oct 2019 17:17:49 +0200 Uladzislau Rezki <urezki@...il.com> wrote:
> 
> > > > : 	 * 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.

As for adjust_va_to_fit_type(), i can add a comment, since we can not
sleep there and the case is one per 1000000 or even lower with your proposal.

Does it sound good?

Thank you!

--
Vlad Rezki