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=-9.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT autolearn=unavailable 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 5EFFFC4363A for ; Mon, 12 Oct 2020 13:42:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2A5FF22227 for ; Mon, 12 Oct 2020 13:42:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1602510149; bh=S+YFtUA5XDumNX/F9yPoBb0FU8aoU0vSWe/PnjTLB2Y=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=wF2GX6qvSGd6eq1FKm9R1QVv6qnkDGe1oi2MvCbC1+tdZxB9xOiSJAo6uEXqrxOFw BjDQxbyDFuvV5RfTnNdOIsvcD6S2RsycepsyI4k1a42rQET+KgZHzoxyC0rsrFefo8 LXSeSDcbsIPhODTYBJcTcHpiwCYNw6Aqvun6cfpo= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389263AbgJLNm2 (ORCPT ); Mon, 12 Oct 2020 09:42:28 -0400 Received: from mail.kernel.org ([198.145.29.99]:46214 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731077AbgJLNlh (ORCPT ); Mon, 12 Oct 2020 09:41:37 -0400 Received: from localhost (83-86-74-64.cable.dynamic.v4.ziggo.nl [83.86.74.64]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 81DDF221FC; Mon, 12 Oct 2020 13:41:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1602510087; bh=S+YFtUA5XDumNX/F9yPoBb0FU8aoU0vSWe/PnjTLB2Y=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=axl+PVe0dAuJa1xMc5/mUkzcK85QXTz09UbuxNzErx2nvnEu9YYTFWKtrdxoOYdPy swzrDb9T0palA014EZG5MAsWdBu9QjNXPV0jo/qAFtE6czONY6VhQb4MSTMhxGJJ/E trsXULNK5UzaLHsw7V+kVHGumqjDoKm2xIXLEyTM= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Denis Lisov , Qian Cai , Hugh Dickins , "Matthew Wilcox (Oracle)" , Linus Torvalds , Suren Baghdasaryan Subject: [PATCH 5.4 34/85] mm/khugepaged: fix filemap page_to_pgoff(page) != offset Date: Mon, 12 Oct 2020 15:26:57 +0200 Message-Id: <20201012132634.499402561@linuxfoundation.org> X-Mailer: git-send-email 2.28.0 In-Reply-To: <20201012132632.846779148@linuxfoundation.org> References: <20201012132632.846779148@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Hugh Dickins commit 033b5d77551167f8c24ca862ce83d3e0745f9245 upstream. There have been elusive reports of filemap_fault() hitting its VM_BUG_ON_PAGE(page_to_pgoff(page) != offset, page) on kernels built with CONFIG_READ_ONLY_THP_FOR_FS=y. Suren has hit it on a kernel with CONFIG_READ_ONLY_THP_FOR_FS=y and CONFIG_NUMA is not set: and he has analyzed it down to how khugepaged without NUMA reuses the same huge page after collapse_file() failed (whereas NUMA targets its allocation to the respective node each time). And most of us were usually testing with CONFIG_NUMA=y kernels. collapse_file(old start) new_page = khugepaged_alloc_page(hpage) __SetPageLocked(new_page) new_page->index = start // hpage->index=old offset new_page->mapping = mapping xas_store(&xas, new_page) filemap_fault page = find_get_page(mapping, offset) // if offset falls inside hpage then // compound_head(page) == hpage lock_page_maybe_drop_mmap() __lock_page(page) // collapse fails xas_store(&xas, old page) new_page->mapping = NULL unlock_page(new_page) collapse_file(new start) new_page = khugepaged_alloc_page(hpage) __SetPageLocked(new_page) new_page->index = start // hpage->index=new offset new_page->mapping = mapping // mapping becomes valid again // since compound_head(page) == hpage // page_to_pgoff(page) got changed VM_BUG_ON_PAGE(page_to_pgoff(page) != offset) An initial patch replaced __SetPageLocked() by lock_page(), which did fix the race which Suren illustrates above. But testing showed that it's not good enough: if the racing task's __lock_page() gets delayed long after its find_get_page(), then it may follow collapse_file(new start)'s successful final unlock_page(), and crash on the same VM_BUG_ON_PAGE. It could be fixed by relaxing filemap_fault()'s VM_BUG_ON_PAGE to a check and retry (as is done for mapping), with similar relaxations in find_lock_entry() and pagecache_get_page(): but it's not obvious what else might get caught out; and khugepaged non-NUMA appears to be unique in exposing a page to page cache, then revoking, without going through a full cycle of freeing before reuse. Instead, non-NUMA khugepaged_prealloc_page() release the old page if anyone else has a reference to it (1% of cases when I tested). Although never reported on huge tmpfs, I believe its find_lock_entry() has been at similar risk; but huge tmpfs does not rely on khugepaged for its normal working nearly so much as READ_ONLY_THP_FOR_FS does. Reported-by: Denis Lisov Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=206569 Link: https://lore.kernel.org/linux-mm/?q=20200219144635.3b7417145de19b65f258c943%40linux-foundation.org Reported-by: Qian Cai Link: https://lore.kernel.org/linux-xfs/?q=20200616013309.GB815%40lca.pw Reported-and-analyzed-by: Suren Baghdasaryan Fixes: 87c460a0bded ("mm/khugepaged: collapse_shmem() without freezing new_page") Signed-off-by: Hugh Dickins Cc: stable@vger.kernel.org # v4.9+ Reviewed-by: Matthew Wilcox (Oracle) Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- mm/khugepaged.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -832,6 +832,18 @@ static struct page *khugepaged_alloc_hug static bool khugepaged_prealloc_page(struct page **hpage, bool *wait) { + /* + * If the hpage allocated earlier was briefly exposed in page cache + * before collapse_file() failed, it is possible that racing lookups + * have not yet completed, and would then be unpleasantly surprised by + * finding the hpage reused for the same mapping at a different offset. + * Just release the previous allocation if there is any danger of that. + */ + if (*hpage && page_count(*hpage) > 1) { + put_page(*hpage); + *hpage = NULL; + } + if (!*hpage) *hpage = khugepaged_alloc_hugepage(wait);