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

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Message-Id: <20191115155535.2a9da68ad58bb787a0ac7833@linux-foundation.org>
Date:   Fri, 15 Nov 2019 15:55:35 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        "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 Tue, 5 Nov 2019 09:43:52 +0100 Michal Hocko <mhocko@...nel.org> wrote:

> On Mon 04-11-19 16:51:26, Vincent Whitchurch wrote:
> > On Thu, Oct 31, 2019 at 08:25:55AM +0100, Michal Hocko wrote:
> > > On Wed 30-10-19 18:31:23, Michal Hocko wrote:
> > > [...]
> > > > What about this? It still aligns to the size but that should be
> > > > correctly done to the section size level.
> > > > 
> > > > diff --git a/mm/sparse.c b/mm/sparse.c
> > > > index 72f010d9bff5..ab1e6175ac9a 100644
> > > > --- a/mm/sparse.c
> > > > +++ b/mm/sparse.c
> > > > @@ -456,8 +456,7 @@ struct page __init *__populate_section_memmap(unsigned long pfn,
> > > >  	if (map)
> > > >  		return map;
> > > >  
> > > > -	map = memblock_alloc_try_nid(size,
> > > > -					  PAGE_SIZE, addr,
> > > > +	map = memblock_alloc_try_nid(size, size, addr,
> > > >  					  MEMBLOCK_ALLOC_ACCESSIBLE, nid);
> > > >  	if (!map)
> > > >  		panic("%s: Failed to allocate %lu bytes align=0x%lx nid=%d from=%pa\n",
> > > > @@ -474,8 +473,13 @@ static void __init sparse_buffer_init(unsigned long size, int nid)
> > > >  {
> > > >  	phys_addr_t addr = __pa(MAX_DMA_ADDRESS);
> > > >  	WARN_ON(sparsemap_buf);	/* forgot to call sparse_buffer_fini()? */
> > > > +	/*
> > > > +	 * Pre-allocated buffer is mainly used by __populate_section_memmap
> > > > +	 * and we want it to be properly aligned to the section size - this is
> > > > +	 * especially the case for VMEMMAP which maps memmap to PMDs
> > > > +	 */
> > > >  	sparsemap_buf =
> > > > -		memblock_alloc_try_nid_raw(size, PAGE_SIZE,
> > > > +		memblock_alloc_try_nid_raw(size, section_map_size(),
> > > >  						addr,
> > > >  						MEMBLOCK_ALLOC_ACCESSIBLE, nid);
> > > >  	sparsemap_buf_end = sparsemap_buf + size;
> > >
> > > Vincent, could you give this a try please? It would be even better if
> > > you could add some debugging to measure the overhead. Let me know if you
> > > need any help with a debugging patch.
> > 
> > I've tested this patch and it works on my platform:  The allocations
> > from sparse_buffer_alloc() now succeed and the fallback path is not
> > taken.
> 
> Thanks a lot. I will try to prepare the full patch with a proper
> changelog sometimes this week.
> 

We're late in -rc7.  Should we run with Vincent's original for now?

And I'm wondering why this is -stable -material?  You said

: Anyway the patch is OK.  Even though this is not a bug strictly
: speaking it is certainly a suboptimal behavior because zeroying takes
: time so I would flag this for a stable tree 4.19+.  There is no clear
: Fixes tag to apply (35fd1eb1e8212 would get closest I guess).

I'm not seeing any description of any runtime effect of the bug at
present.  When would unzeroed sparsemem pageframes cause a problem? 
Could they be visible during deferred initialization or mem hotadd?