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

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Message-ID: <20191030141259.GE31513@dhcp22.suse.cz>
Date:   Wed, 30 Oct 2019 15:12:59 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>
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

[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?

-- 
Michal Hocko
SUSE Labs