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Date:   Thu, 1 Mar 2018 15:04:20 +0100
From:   Martin Schwidefsky <schwidefsky@de.ibm.com>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Matthew Wilcox <willy@infradead.org>, linux-mm@kvack.org,
        Matthew Wilcox <mawilcox@microsoft.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 1/4] s390: Use _refcount for pgtables
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On Thu, 1 Mar 2018 15:53:10 +0300
"Kirill A. Shutemov" <kirill@shutemov.name> wrote:

> 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.
 
2^^24 put_page_testzero calls for page table pages? I don't think so.

-- 
blue skies,
   Martin.

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