From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.6 required=3.0 tests=DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,T_DKIM_INVALID autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0FEA2C5CFC1 for ; Tue, 19 Jun 2018 09:03:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AFC862075E for ; Tue, 19 Jun 2018 09:03:05 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="dqXVPibe" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org AFC862075E Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S937499AbeFSJDD (ORCPT ); Tue, 19 Jun 2018 05:03:03 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:57990 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756178AbeFSJDB (ORCPT ); Tue, 19 Jun 2018 05:03:01 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=J0rYBzZdgJuklfSmS0e1qKxSub3Cx5Lqcz2HUrE9S7g=; b=dqXVPibe5aTXn+LKzTfTylAla tCDh74WvAh7TCroDSjM+RJwQpXPTDv/1qZjidbVJobx6JOsVgflG+xJ+o+UpyoYgraNaaCndc3PDX yimhR9SzLsUZ5IPW/32gZAww27FXATtW5ObkL3LSwbWYEm4a4/cZL+9anZ8MWX5UkAiMVXmE6OHY8 UbU6Jf2TlEQqFA1e+gOBdrfx4sDv0N4fVXXObj6lqZkrIrwx32tCmCrNbO7jPJduiTm7ba/TyMumU fQ+lEGUId1bm3PtMPj9A82/Esn9BUN3FgnM54KsF5cbCuw/VizdC3ypxzdw/XvOZ6iOqHoMhf5Ogf CJd17sa2w==; Received: from willy by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1fVCXP-0008U6-Sw; Tue, 19 Jun 2018 09:02:55 +0000 Date: Tue, 19 Jun 2018 02:02:55 -0700 From: Matthew Wilcox To: Jan Kara Cc: John Hubbard , Dan Williams , Christoph Hellwig , Jason Gunthorpe , John Hubbard , Michal Hocko , Christopher Lameter , Linux MM , LKML , linux-rdma Subject: Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user_pages*() Message-ID: <20180619090255.GA25522@bombadil.infradead.org> References: <20180617200432.krw36wrcwidb25cj@ziepe.ca> <311eba48-60f1-b6cc-d001-5cc3ed4d76a9@nvidia.com> <20180618081258.GB16991@lst.de> <3898ef6b-2fa0-e852-a9ac-d904b47320d5@nvidia.com> <0e6053b3-b78c-c8be-4fab-e8555810c732@nvidia.com> <20180619082949.wzoe42wpxsahuitu@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180619082949.wzoe42wpxsahuitu@quack2.suse.cz> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 19, 2018 at 10:29:49AM +0200, Jan Kara wrote: > And for record, the problem with page cache pages is not only that > try_to_unmap() may unmap them. It is also that page_mkclean() can > write-protect them. And once PTEs are write-protected filesystems may end > up doing bad things if DMA then modifies the page contents (DIF/DIX > failures, data corruption, oopses). As such I don't think that solutions > based on page reference count have a big chance of dealing with the > problem. > > And your page flag approach would also need to take page_mkclean() into > account. And there the issue is that until the flag is cleared (i.e., we > are sure there are no writers using references from GUP) you cannot > writeback the page safely which does not work well with your idea of > clearing the flag only once the page is evicted from page cache (hint, page > cache page cannot get evicted until it is written back). > > So as sad as it is, I don't see an easy solution here. Pages which are "got" don't need to be on the LRU list. They'll be marked dirty when they're put, so we can use page->lru for fun things like a "got" refcount. If we use bit 1 of page->lru for PageGot, we've got 30/62 bits in the first word and a full 64 bits in the second word.