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Date: Wed, 5 May 2021 15:09:12 +0200
From: Peter Zijlstra <>
To: Mike Rapoport <>
Cc: Rick Edgecombe <>,,,,,,,,,,
Subject: Re: [PATCH RFC 3/9] x86/mm/cpa: Add grouped page allocations

On Wed, May 05, 2021 at 03:08:27PM +0300, Mike Rapoport wrote:
> On Tue, May 04, 2021 at 05:30:26PM -0700, Rick Edgecombe wrote:
> > For x86, setting memory permissions on the direct map results in fracturing
> > large pages. Direct map fracturing can be reduced by locating pages that
> > will have their permissions set close together.
> > 
> > Create a simple page cache that allocates pages from huge page size
> > blocks. Don't guarantee that a page will come from a huge page grouping,
> > instead fallback to non-grouped pages to fulfill the allocation if
> > needed. Also, register a shrinker such that the system can ask for the
> > pages back if needed. Since this is only needed when there is a direct
> > map, compile it out on highmem systems.
> I only had time to skim through the patches, I like the idea of having a
> simple cache that allocates larger pages with a fallback to basic page
> size.
> I just think it should be more generic and closer to the page allocator.
> I was thinking about adding a GFP flag that will tell that the allocated
> pages should be removed from the direct map. Then alloc_pages() could use
> such cache whenever this GFP flag is specified with a fallback for lower
> order allocations.

That doesn't provide enough information I think. Removing from direct
map isn't the only consideration, you also want to group them by the
target protection bits such that we don't get to use 4k pages quite so

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