linux-kernel - Re: [PATCH] mm/percpu.c: use smarter memory allocation for struct pcpu_alloc

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Date:   Tue, 3 Oct 2017 18:29:49 -0400 (EDT)
From:   Nicolas Pitre <nicolas.pitre@...aro.org>
To:     Tejun Heo <tj@...nel.org>
cc:     Christoph Lameter <cl@...ux.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/percpu.c: use smarter memory allocation for struct
 pcpu_alloc_info

On Tue, 3 Oct 2017, Tejun Heo wrote:

> On Tue, Oct 03, 2017 at 04:57:44PM -0400, Nicolas Pitre wrote:
> > This can be much smaller than a page on very small memory systems. 
> > Always rounding up the size to a page is wasteful in that case, and 
> > required alignment is smaller than the memblock default. Let's round 
> > things up to a page size only when the actual size is >= page size, and 
> > then it makes sense to page-align for a nicer allocation pattern.
> 
> Isn't that a temporary area which gets freed later during boot?

Hmmm...

It may get freed through 3 different paths where 2 of them are error 
paths. What looks like a non-error path is in pcpu_embed_first_chunk() 
called from setup_per_cpu_areas(). But there are two versions of 
setup_per_cpu_areas(): one for SMP and one for !SMP. And the !SMP case 
never calls pcpu_free_alloc_info() currently.

I'm not sure i understand that code fully, but maybe the following patch 
could be a better fit:

----- >8
Subject: [PATCH] percpu: don't forget to free the temporary struct pcpu_alloc_info

Unlike the SMP case, the !SMP case does not free the memory for struct 
pcpu_alloc_info allocated in setup_per_cpu_areas(). And to give it a 
chance of being reused by the page allocator later, align it to a page 
boundary just like its size.

Signed-off-by: Nicolas Pitre <nico@...aro.org>

diff --git a/mm/percpu.c b/mm/percpu.c
index 434844415d..caab63375b 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1416,7 +1416,7 @@ struct pcpu_alloc_info * __init pcpu_alloc_alloc_info(int nr_groups,
 			  __alignof__(ai->groups[0].cpu_map[0]));
 	ai_size = base_size + nr_units * sizeof(ai->groups[0].cpu_map[0]);
 
-	ptr = memblock_virt_alloc_nopanic(PFN_ALIGN(ai_size), 0);
+	ptr = memblock_virt_alloc_nopanic(PFN_ALIGN(ai_size), PAGE_SIZE);
 	if (!ptr)
 		return NULL;
 	ai = ptr;
@@ -2295,6 +2295,7 @@ void __init setup_per_cpu_areas(void)
 
 	if (pcpu_setup_first_chunk(ai, fc) < 0)
 		panic("Failed to initialize percpu areas.");
+	pcpu_free_alloc_info(ai);
 }
 
 #endif	/* CONFIG_SMP */