From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1031475AbeCAOkC (ORCPT ); Thu, 1 Mar 2018 09:40:02 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:57556 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1031055AbeCAOkB (ORCPT ); Thu, 1 Mar 2018 09:40:01 -0500 Date: Thu, 1 Mar 2018 15:39:52 +0100 From: Martin Schwidefsky To: "Kirill A. Shutemov" 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 In-Reply-To: <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> <20180301142855.emaa5x65oj2hkwsm@node.shutemov.name> X-Mailer: Claws Mail 3.13.2 (GTK+ 2.24.30; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 x-cbid: 18030114-0020-0000-0000-000003FD5F6C X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18030114-0021-0000-0000-000042918DE4 Message-Id: <20180301153952.668bfdc7@mschwideX1> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2018-03-01_07:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=2 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1803010185 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 1 Mar 2018 17:28:55 +0300 "Kirill A. Shutemov" wrote: > 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. Ah, ok. But this is not a problem as the page is freed after both bits for the two 2K pieces havbe been set to zero. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.