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

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Date:   Wed, 30 Oct 2019 18:31:23 +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 12:53:41, Pavel Tatashin wrote:
> On Wed, Oct 30, 2019 at 11:31 AM Michal Hocko <mhocko@...nel.org> wrote:
> >
> > 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?
> 
> Yes, PMD_SIZE should be the alignment here. It just does not make
> sense to align to size.

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;

-- 
Michal Hocko
SUSE Labs