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

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Date:   Wed, 30 Oct 2019 16:31:50 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     Vincent Whitchurch <vincent.whitchurch@...s.com>,
        "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 30-10-19 11:20:44, Pavel Tatashin wrote:
> On Wed, Oct 30, 2019 at 10:13 AM Michal Hocko <mhocko@...nel.org> wrote:
> >
> > [Add Pavel - the email thread starts http://lkml.kernel.org/r/20191030131122.8256-1-vincent.whitchurch@axis.com
> >  but it used your old email address]
> >
> > On Wed 30-10-19 15:02:16, Vincent Whitchurch wrote:
> > > 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()?
> >
> > I am not 100% sure TBH. Aligning makes some sense when mapping the
> > memmaps to page tables but that would suggest that sparse_buffer_init
> > is using a wrong alignment then. It is quite wasteful to allocate
> > alarge misaligned block like that.
> >
> > Your patch still makes sense but this is something to look into.
> >
> > Pavel?
> 
> I remember thinking about this large alignment, as it looked out of
> place to me also.
> It was there to keep memmap in single chunks on larger x86 machines.
> Perhaps it can be revisited now.

Don't we need 2MB aligned memmaps for their PMD mappings?

> The patch that introduced this alignment:
> 9bdac914240759457175ac0d6529a37d2820bc4d
> 
> vmemmap_alloc_block_buf
> +       ptr = (void *)ALIGN((unsigned long)vmemmap_buf, size);
> 
> Pasha

-- 
Michal Hocko
SUSE Labs