Re: [RFC PATCH v2 2/3] xpfo: Only put previous userspace pages into the hot cache

From	Juerg Haefliger <juerg.haefliger@hpe.com>
Newsgroups	linux.kernel
Subject	Re: [RFC PATCH v2 2/3] xpfo: Only put previous userspace pages into the hot cache
Date	2016-09-05 14:00 +0200
Message-ID	<se0Jb-3fd-1@gated-at.bofh.it> (permalink)
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On 09/02/2016 10:39 PM, Dave Hansen wrote:
> On 09/02/2016 04:39 AM, Juerg Haefliger wrote:
>> Allocating a page to userspace that was previously allocated to the
>> kernel requires an expensive TLB shootdown. To minimize this, we only
>> put non-kernel pages into the hot cache to favor their allocation.
> 
> But kernel allocations do allocate from these pools, right?

Yes.

> Does this
> just mean that kernel allocations usually have to pay the penalty to
> convert a page?

Only pages that are allocated for userspace (gfp & GFP_HIGHUSER == GFP_HIGHUSER) which were
previously allocated for the kernel (gfp & GFP_HIGHUSER != GFP_HIGHUSER) have to pay the penalty.

> So, what's the logic here?  You're assuming that order-0 kernel
> allocations are more rare than allocations for userspace?

The logic is to put reclaimed kernel pages into the cold cache to postpone their allocation as long
as possible to minimize (potential) TLB flushes.

...Juerg

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[RFC PATCH v2 0/3] Add support for eXclusive Page Frame Ownership (XPFO) Juerg Haefliger <juerg.haefliger@hpe.com> - 2016-09-02 13:40 +0200
  [RFC PATCH v2 1/3] Add support for eXclusive Page Frame Ownership (XPFO) Juerg Haefliger <juerg.haefliger@hpe.com> - 2016-09-02 13:40 +0200
  [RFC PATCH v2 3/3] block: Always use a bounce buffer when XPFO is enabled Juerg Haefliger <juerg.haefliger@hpe.com> - 2016-09-02 13:50 +0200
  [RFC PATCH v2 2/3] xpfo: Only put previous userspace pages into the hot cache Juerg Haefliger <juerg.haefliger@hpe.com> - 2016-09-02 13:50 +0200
    Re: [RFC PATCH v2 2/3] xpfo: Only put previous userspace pages into  the hot cache Dave Hansen <dave.hansen@intel.com> - 2016-09-02 22:40 +0200
      Re: [RFC PATCH v2 2/3] xpfo: Only put previous userspace pages into  the hot cache Juerg Haefliger <juerg.haefliger@hpe.com> - 2016-09-05 14:00 +0200

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