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Date:   Thu, 1 Mar 2018 15:53:10 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     linux-mm@kvack.org, Matthew Wilcox <mawilcox@microsoft.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 1/4] s390: Use _refcount for pgtables
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On Wed, Feb 28, 2018 at 02:31:54PM -0800, Matthew Wilcox wrote:
> From: Matthew Wilcox <mawilcox@microsoft.com>
> 
> s390 borrows the storage used for _mapcount in struct page in order to
> account whether the bottom or top half is being used for 2kB page
> tables.  I want to use that for something else, so use the top byte of
> _refcount instead of the bottom byte of _mapcount.  _refcount may
> temporarily be incremented by other CPUs that see a stale pointer to
> this page in the page cache, but each CPU can only increment it by one,
> and there are no systems with 2^24 CPUs today, so they will not change
> the upper byte of _refcount.  We do have to be a little careful not to
> lose any of their writes (as they will subsequently decrement the
> counter).

Hm. I'm more worried about false-negative put_page_testzero().
Are you sure it won't lead to leaks. I cannot say from the code changes.

And for page-table pages should have planty space in other fields.
IIRC page->mapping is unused there.

-- 
 Kirill A. Shutemov