From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f72.google.com (mail-wm0-f72.google.com [74.125.82.72]) by kanga.kvack.org (Postfix) with ESMTP id D8AB46B000C for ; Thu, 1 Mar 2018 07:53:23 -0500 (EST) Received: by mail-wm0-f72.google.com with SMTP id e74so3445295wmg.0 for ; Thu, 01 Mar 2018 04:53:23 -0800 (PST) Received: from mail-sor-f41.google.com (mail-sor-f41.google.com. [209.85.220.41]) by mx.google.com with SMTPS id k23sor2509102edb.7.2018.03.01.04.53.22 for (Google Transport Security); Thu, 01 Mar 2018 04:53:22 -0800 (PST) Date: Thu, 1 Mar 2018 15:53:10 +0300 From: "Kirill A. Shutemov" 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> Sender: owner-linux-mm@kvack.org List-ID: To: Matthew Wilcox Cc: linux-mm@kvack.org, Matthew Wilcox , Martin Schwidefsky , 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org