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_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 19A61C433E1 for ; Mon, 18 May 2020 15:26:05 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id D28B220758 for ; Mon, 18 May 2020 15:26:04 +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="JMphm1HF" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D28B220758 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 5E050900003; Mon, 18 May 2020 11:26:04 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 591F9900002; Mon, 18 May 2020 11:26:04 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 4A78A900003; Mon, 18 May 2020 11:26:04 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0089.hostedemail.com [216.40.44.89]) by kanga.kvack.org (Postfix) with ESMTP id 324E0900002 for ; Mon, 18 May 2020 11:26:04 -0400 (EDT) Received: from smtpin02.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id E9B2540C0 for ; Mon, 18 May 2020 15:26:03 +0000 (UTC) X-FDA: 76830215406.02.story53_5204b8a59f62b X-HE-Tag: story53_5204b8a59f62b X-Filterd-Recvd-Size: 3798 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) by imf17.hostedemail.com (Postfix) with ESMTP for ; Mon, 18 May 2020 15:26:03 +0000 (UTC) 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; bh=5JgLj2+DCv1+0yfBDC+57Xxa+VbNaCmUQuN65CNtdGs=; b=JMphm1HFkx34iCMwxbaxGU6ONt PKTFL0wcixP1KXZMFUWMjiN36Q75Ohiwq17R3+IYkAm8KUBIBCmcXb3rDtnDRouUUdrkEDA7iVrHw 4hQdNZL1lDz0qIyW7+DYDdG8dRBwDMbH+qcwGvqAZaz9R/WkLA2kBzcLh4UBGP3T6nrcl6PK9ACP2 Xys01wrdcxVHilBEJKsWr2Dcg0mkuYgCQF2R4hWA2DR1yg6HbDay4/6doT1cFsIgYQ4PoMeG4HBA5 3nng6OOy9XbauYvK/BWvzNaMJtS7IYLcio2u7QNPX9g4izcv4qmOtttmSJgVoaCLKSsn+0ACH+VXF 4Uglmmhg==; Received: from willy by bombadil.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1jaheT-0005ZQ-KN; Mon, 18 May 2020 15:26:01 +0000 Date: Mon, 18 May 2020 08:26:01 -0700 From: Matthew Wilcox To: Miklos Szeredi Cc: linux-fsdevel@vger.kernel.org, fuse-devel , linux-mm , miklos , Gabriel Krisman Bertazi , =?iso-8859-1?Q?Andr=E9?= Almeida Subject: Re: [fuse-devel] fuse: trying to steal weird page Message-ID: <20200518152601.GU16070@bombadil.infradead.org> References: <87a72qtaqk.fsf@vostro.rath.org> <877dxut8q7.fsf@vostro.rath.org> <20200503032613.GE29705@bombadil.infradead.org> <87368hz9vm.fsf@vostro.rath.org> <20200503102742.GF29705@bombadil.infradead.org> <20200518144853.GT16070@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, May 18, 2020 at 04:58:05PM +0200, Miklos Szeredi wrote: > On Mon, May 18, 2020 at 4:48 PM Matthew Wilcox wrote: > > > > On Mon, May 18, 2020 at 02:45:02PM +0200, Miklos Szeredi wrote: > > > > page_cache_pipe_buf_steal() calls remove_mapping() which calls > > > page_ref_unfreeze(page, 1). That sets the refcount to 1, right? > > > > > > What am I missing? > > > > find_get_entry() calling page_cache_get_speculative(). > > > > In a previous allocation, this page belonged to the page cache. Then it > > was freed, but another thread is in the middle of a page cache lookup and > > has already loaded the pointer. It is now delayed by a few clock ticks. > > > > Now the page is allocated to FUSE, which calls page_ref_unfreeze(). > > And then the refcount gets bumped to 2 by page_cache_get_speculative(). > > find_get_entry() calls xas_reload() and discovers this page is no longer > > at that index, so it calls put_page(), but in that narrow window, FUSE > > checks the refcount and finds it's not 1. > > What if that page_cache_get_speculative() happens just before > page_ref_unfreeze()? The speculative reference would be lost and on > put_page() the page would be freed, even though fuse is still holding > the pointer. static inline void page_ref_unfreeze(struct page *page, int count) { VM_BUG_ON_PAGE(page_count(page) != 0, page); VM_BUG_ON(count == 0); atomic_set_release(&page->_refcount, count); static inline int __page_cache_add_speculative(struct page *page, int count) { if (unlikely(!page_ref_add_unless(page, count, 0))) { return 0;