Re: [PATCH] mm/percpu.c: use smarter memory allocation for struct pcpu_alloc_info

From: Nicolas Pitre
Date: Tue Oct 03 2017 - 18:29:58 EST

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?


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@xxxxxxxxxx>

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,
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 */