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Date:   Thu, 22 Feb 2018 14:36:43 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        linux-kernel@vger.kernel.org,
        Christoph Hellwig <hch@infradead.org>, linux-mm@kvack.org,
        Andy Lutomirski <luto@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>
Subject: Re: Use higher-order pages in vmalloc
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On Thu 22-02-18 04:22:54, Matthew Wilcox wrote:
> On Thu, Feb 22, 2018 at 07:59:43AM +0100, Michal Hocko wrote:
> > On Wed 21-02-18 09:01:29, Matthew Wilcox wrote:
> > > Right.  It helps with fragmentation if we can keep higher-order
> > > allocations together.
> > 
> > Hmm, wouldn't it help if we made vmalloc pages migrateable instead? That
> > would help the compaction and get us to a lower fragmentation longterm
> > without playing tricks in the allocation path.
> 
> I was wondering about that possibility.  If we want to migrate a page
> then we have to shoot down the PTE across all CPUs, copy the data to the
> new page, and insert the new PTE.  Copying 4kB doesn't take long; if you
> have 12GB/s (current example on Wikipedia: dual-channel memory and one
> DDR2-800 module per channel gives a theoretical bandwidth of 12.8GB/s)
> then we should be able to copy a page in 666ns).  So there's no problem
> holding a spinlock for it.
> 
> But we can't handle a fault in vmalloc space today.  It's handled in
> arch-specific code, see vmalloc_fault() in arch/x86/mm/fault.c
> If we're going to do this, it'll have to be something arches opt into
> because I'm not taking on the job of fixing every architecture!

yes.

> > Maybe we should consider kvmalloc for the kernel stack?
> 
> We'd lose the guard page, so it'd have to be something we let the
> sysadmin decide to do.

ohh, right, I forgot about the guard page.
-- 
Michal Hocko
SUSE Labs