From: Heiko Carstens <heiko.carstens@de.ibm.com>
To: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: linux-kernel@vger.kernel.org, peterz@infradead.org,
akpm@linux-foundation.org, linux-mm@kvack.org,
Joonsoo Kim <iamjoonsoo.kim@lge.com>
Subject: Re: [PATCH] mm/debug_pagealloc: Ask users for default setting of debug_pagealloc
Date: Mon, 25 Jan 2016 11:02:48 +0100 [thread overview]
Message-ID: <20160125100248.GB4298@osiris> (raw)
In-Reply-To: <56A5EECE.90607@de.ibm.com>
On Mon, Jan 25, 2016 at 10:45:50AM +0100, Christian Borntraeger wrote:
> >> + By default this option will be almost for free and can be activated
> >> + in distribution kernels. The overhead and the debugging can be enabled
> >> + by DEBUG_PAGEALLOC_ENABLE_DEFAULT or the debug_pagealloc command line
> >> + parameter.
> >
> > Sorry, but it's not almost for free and should not be used by distribution
> > kernels. If we have DEBUG_PAGEALLOC enabled, at least on s390 we will not
> > make use of 2GB and 1MB pagetable entries for the identy mapping anymore.
> > Instead we will only use 4K mappings.
>
> Hmmm, can we change these code areas to use debug_pagealloc_enabled? I guess
> this evaluated too late?
Yes, that should be possible. "debug_pagealloc" is an early_param, which
will be evaluated before we call paging_init() (both in
arch/s390/kernel/setup.c).
So it looks like this can be trivially changed. (replace the ifdefs in
arch/s390/mm/vmem.c with debug_pagealloc_enabled()).
> > I assume this is true for all architectures since freeing pages can happen
> > in any context and therefore we can't allocate memory in order to split
> > page tables.
> >
> > So enabling this will cost memory and put more pressure on the TLB.
>
> So I will change the description and drop the "if unsure" statement.
Well, given that we can change it like above... I don't care anymore ;)
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next prev parent reply other threads:[~2016-01-25 10:02 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-25 9:19 [PATCH] mm/debug_pagealloc: Ask users for default setting of debug_pagealloc Christian Borntraeger
2016-01-25 9:41 ` Heiko Carstens
2016-01-25 9:45 ` Christian Borntraeger
2016-01-25 10:02 ` Heiko Carstens [this message]
2016-01-25 10:07 ` Christian Borntraeger
2016-01-25 11:10 ` Christian Borntraeger
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