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
Date: Thu, 1 Mar 2018 15:53:10 +0300 [thread overview]
Message-ID: <20180301125310.jx6c5dypk5axrmum@node.shutemov.name> (raw)
In-Reply-To: <20180228223157.9281-2-willy@infradead.org>
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
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next prev parent reply other threads:[~2018-03-01 12:53 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-28 22:31 [PATCH v3 0/4] Split page_type out from mapcount Matthew Wilcox
2018-02-28 22:31 ` [PATCH v3 1/4] s390: Use _refcount for pgtables Matthew Wilcox
2018-03-01 12:53 ` Kirill A. Shutemov [this message]
2018-03-01 14:04 ` Martin Schwidefsky
2018-03-01 14:28 ` Kirill A. Shutemov
2018-03-01 14:39 ` Martin Schwidefsky
2018-02-28 22:31 ` [PATCH v3 2/4] mm: Split page_type out from _map_count Matthew Wilcox
2018-02-28 22:31 ` [PATCH v3 3/4] mm: Mark pages allocated through vmalloc Matthew Wilcox
2018-02-28 22:31 ` [PATCH v3 4/4] mm: Mark pages in use for page tables Matthew Wilcox
2018-02-28 23:22 ` [PATCH v3 0/4] Split page_type out from mapcount Randy Dunlap
2018-03-01 1:31 ` Matthew Wilcox
2018-03-01 7:17 ` Martin Schwidefsky
2018-03-01 8:00 ` Martin Schwidefsky
2018-03-01 12:44 ` Kirill A. Shutemov
2018-03-01 14:47 ` Martin Schwidefsky
2018-03-01 14:50 ` Matthew Wilcox
2018-03-01 15:02 ` Martin Schwidefsky
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