From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f198.google.com (mail-qk0-f198.google.com [209.85.220.198]) by kanga.kvack.org (Postfix) with ESMTP id 62BDF6B0008 for ; Thu, 1 Mar 2018 09:40:02 -0500 (EST) Received: by mail-qk0-f198.google.com with SMTP id 26so4816176qkx.11 for ; Thu, 01 Mar 2018 06:40:02 -0800 (PST) Received: from mx0a-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com. [148.163.158.5]) by mx.google.com with ESMTPS id 21si1157732qtz.64.2018.03.01.06.40.01 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 01 Mar 2018 06:40:01 -0800 (PST) Received: from pps.filterd (m0098420.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w21Ecc29030277 for ; Thu, 1 Mar 2018 09:40:01 -0500 Received: from e06smtp15.uk.ibm.com (e06smtp15.uk.ibm.com [195.75.94.111]) by mx0b-001b2d01.pphosted.com with ESMTP id 2gek14sdvk-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Thu, 01 Mar 2018 09:40:00 -0500 Received: from localhost by e06smtp15.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 1 Mar 2018 14:39:58 -0000 Date: Thu, 1 Mar 2018 15:39:52 +0100 From: Martin Schwidefsky 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> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Message-Id: <20180301153952.668bfdc7@mschwideX1> Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Matthew Wilcox , linux-mm@kvack.org, Matthew Wilcox , 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. -- 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