linux-kernel - Re: [PATCH v4 4/4] Use 2GB memory block size on large-memory x86-64 systems

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Date:	Fri, 21 Aug 2015 13:50:33 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	Daniel J Blueman <daniel@...ascale.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Steffen Persvold <sp@...ascale.com>
Subject: Re: [PATCH v4 4/4] Use 2GB memory block size on large-memory x86-64 systems

On Fri, Aug 21, 2015 at 1:27 PM, Luck, Tony <tony.luck@...el.com> wrote:
> On Fri, Aug 21, 2015 at 11:38:13AM -0700, Yinghai Lu wrote:
>> That commit could be reverted.
>> According to
>> https://lkml.org/lkml/2014/11/10/123
>
> Do we really need to force the MIN_MEMORY_BLOCK_SIZE on small
> systems?

That is introduced in commit 982792c7 ("x86, mm: probe memory block
size for generic x86 64bit
").
that patch is used to make boot faster why create less entries
in /sys/device/system/memory/.
On system with less 64G ram, that will not have too many entries
even with MIN_MEMORY_BLOCK_SIZE.

>
> What about this patch - which just uses max_pfn to choose
> the block size.
>
> It seems that many systems with large amounts of memory
> will have a nicely aligned max_pfn ... so they will get
> the 2GB block size.  If they don't have a well aligned
> max_pfn, then they need to use a smaller size to avoid
> the crash I saw.

Good to me.

> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 3fba623e3ba5..e14e90fd1cf8 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -1195,15 +1195,6 @@ static unsigned long probe_memory_block_size(void)
>         /* start from 2g */
>         unsigned long bz = 1UL<<31;
>
> -       if (totalram_pages >= (64ULL << (30 - PAGE_SHIFT))) {
> -               pr_info("Using 2GB memory block size for large-memory system\n");
> -               return 2UL * 1024 * 1024 * 1024;
> -       }
> -
> -       /* less than 64g installed */
> -       if ((max_pfn << PAGE_SHIFT) < (16UL << 32))
> -               return MIN_MEMORY_BLOCK_SIZE;
> -
>         /* get the tail size */
>         while (bz > MIN_MEMORY_BLOCK_SIZE) {
>                 if (!((max_pfn << PAGE_SHIFT) & (bz - 1)))
--
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