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From: Matthew Wilcox <willy@infradead.org>
To: Michal Hocko <mhocko@kernel.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
Date: Thu, 22 Feb 2018 04:22:54 -0800	[thread overview]
Message-ID: <20180222122254.GA22703@bombadil.infradead.org> (raw)
In-Reply-To: <20180222065943.GA30681@dhcp22.suse.cz>

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!

> 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.

  reply	other threads:[~2018-02-22 12:23 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-01-23 10:55 [PATCH 1/4] vmalloc: add vm_flags argument to internal __vmalloc_node() Konstantin Khlebnikov
2018-01-23 10:55 ` [PATCH 2/4] vmalloc: add __vmalloc_area() Konstantin Khlebnikov
2018-01-23 10:55 ` [PATCH 3/4] kernel/fork: switch vmapped stack callation to __vmalloc_area() Konstantin Khlebnikov
2018-01-23 13:57   ` Konstantin Khlebnikov
2018-02-21  0:16     ` Andrew Morton
2018-02-21  7:23       ` Konstantin Khlebnikov
2018-02-21 16:35         ` Andy Lutomirski
2018-01-23 10:55 ` [PATCH 4/4] kernel/fork: add option to use virtually mapped stacks as fallback Konstantin Khlebnikov
2018-02-21 15:42   ` Use higher-order pages in vmalloc Matthew Wilcox
2018-02-21 16:11     ` Andy Lutomirski
2018-02-21 16:50       ` Matthew Wilcox
2018-02-21 16:16     ` Dave Hansen
2018-02-21 17:01       ` Matthew Wilcox
2018-02-22  6:59         ` Michal Hocko
2018-02-22 12:22           ` Matthew Wilcox [this message]
2018-02-22 13:36             ` Michal Hocko
2018-02-22 19:01               ` Andy Lutomirski
2018-02-22 19:19                 ` Dave Hansen
2018-02-22 19:27                   ` Andy Lutomirski
2018-02-22 19:36                     ` Dave Hansen
2018-02-23 12:13                 ` Michal Hocko
2018-03-01 18:16                   ` Eric Dumazet
2018-02-21 12:24 ` [PATCH 1/4] vmalloc: add vm_flags argument to internal __vmalloc_node() Matthew Wilcox
2018-02-21 12:39   ` Andrey Ryabinin

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