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Date:   Thu, 1 Mar 2018 16:02:58 +0100
From:   Martin Schwidefsky <schwidefsky@de.ibm.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     "Kirill A. Shutemov" <kirill@shutemov.name>, linux-mm@kvack.org,
        Matthew Wilcox <mawilcox@microsoft.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 0/4] Split page_type out from mapcount
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On Thu, 1 Mar 2018 06:50:58 -0800
Matthew Wilcox <willy@infradead.org> wrote:

> On Thu, Mar 01, 2018 at 03:44:12PM +0300, Kirill A. Shutemov wrote:
> > On Thu, Mar 01, 2018 at 08:17:50AM +0100, Martin Schwidefsky wrote:  
> > > Yeah, that is a nasty bit of code. On s390 we have 2K page tables (pte)
> > > but 4K pages. If we use full pages for the pte tables we waste 2K of
> > > memory for each of the tables. So we allocate 4K and split it into two
> > > 2K pieces. Now we have to keep track of the pieces to be able to free
> > > them again.  
> > 
> > Have you considered to use slab for page table allocation instead?
> > IIRC some architectures practice this already.  
> 
> You're not allowed to do that any more.  Look at pgtable_page_ctor(),
> or rather ptlock_init().

Oh yes, I forgot about the ptl. This takes up some fields in struct page
which the slab/slub cache want to use as well.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.