linux-kernel - Re: [PATCH] mm/sparse: Consistently do not zero memmap

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Date:   Wed, 30 Oct 2019 15:02:16 +0100
From:   Vincent Whitchurch <vincent.whitchurch@...s.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "osalvador@...e.de" <osalvador@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/sparse: Consistently do not zero memmap

On Wed, Oct 30, 2019 at 02:29:58PM +0100, Michal Hocko wrote:
> On Wed 30-10-19 14:11:22, Vincent Whitchurch wrote:
> > (I noticed this because on my ARM64 platform, with 1 GiB of memory the
> >  first [and only] section is allocated from the zeroing path while with
> >  2 GiB of memory the first 1 GiB section is allocated from the
> >  non-zeroing path.)
> 
> Do I get it right that sparse_buffer_init couldn't allocate memmap for
> the full node for some reason and so sparse_init_nid would have to
> allocate one for each memory section?

Not quite.  The sparsemap_buf is successfully allocated with the correct
size in sparse_buffer_init(), but sparse_buffer_alloc() fails to
allocate the same size from it.

The reason it fails is that sparse_buffer_alloc() for some reason wants
to return a pointer which is aligned to the allocation size.  But the
sparsemap_buf was only allocated with PAGE_SIZE alignment so there's not
enough space to align it.

I don't understand the reason for this alignment requirement since the
fallback path also allocates with PAGE_SIZE alignment.  I'm guessing the
alignment is for the VMEMAP code which also uses sparse_buffer_alloc()?