From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1031577AbeCAO3K (ORCPT ); Thu, 1 Mar 2018 09:29:10 -0500 Received: from mail-wm0-f68.google.com ([74.125.82.68]:51807 "EHLO mail-wm0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1031397AbeCAO3H (ORCPT ); Thu, 1 Mar 2018 09:29:07 -0500 X-Google-Smtp-Source: AG47ELsOpyfsGDACTa9v6/LvQpMfTavxLx+f1NaVL/y7SJe2Swslv/yPzY3IWNCyLuPPRbdedNSCpQ== Date: Thu, 1 Mar 2018 17:28:55 +0300 From: "Kirill A. Shutemov" To: Martin Schwidefsky Cc: Matthew Wilcox , linux-mm@kvack.org, Matthew Wilcox , linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 1/4] s390: Use _refcount for pgtables Message-ID: <20180301142855.emaa5x65oj2hkwsm@node.shutemov.name> References: <20180228223157.9281-1-willy@infradead.org> <20180228223157.9281-2-willy@infradead.org> <20180301125310.jx6c5dypk5axrmum@node.shutemov.name> <20180301150420.19a14fd3@mschwideX1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180301150420.19a14fd3@mschwideX1> User-Agent: NeoMutt/20180223 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 01, 2018 at 03:04:20PM +0100, Martin Schwidefsky wrote: > On Thu, 1 Mar 2018 15:53:10 +0300 > "Kirill A. Shutemov" wrote: > > > 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. > > 2^^24 put_page_testzero calls for page table pages? I don't think so. No, I mean oposite: we don't free the page when we should. 2^24 is not zero and page won't be freed if the acctual refcount (without the flag in upper bits) drops to zero. -- Kirill A. Shutemov