From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030594AbeCAMxZ (ORCPT ); Thu, 1 Mar 2018 07:53:25 -0500 Received: from mail-wm0-f53.google.com ([74.125.82.53]:35392 "EHLO mail-wm0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030504AbeCAMxX (ORCPT ); Thu, 1 Mar 2018 07:53:23 -0500 X-Google-Smtp-Source: AG47ELsEdfIUk9H8b3k6g/hlE0bUoM4NxVe6PHODat4pA6hYSl2eBK2Q/S9IV+yqCWiD4iZRM0AJgA== Date: Thu, 1 Mar 2018 15:53:10 +0300 From: "Kirill A. Shutemov" To: Matthew Wilcox Cc: linux-mm@kvack.org, Matthew Wilcox , Martin Schwidefsky , linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 1/4] s390: Use _refcount for pgtables Message-ID: <20180301125310.jx6c5dypk5axrmum@node.shutemov.name> References: <20180228223157.9281-1-willy@infradead.org> <20180228223157.9281-2-willy@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180228223157.9281-2-willy@infradead.org> User-Agent: NeoMutt/20180223 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 28, 2018 at 02:31:54PM -0800, Matthew Wilcox wrote: > From: Matthew Wilcox > > 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