From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S968398AbdAFI1C (ORCPT <rfc822;w@1wt.eu>);
        Fri, 6 Jan 2017 03:27:02 -0500
Received: from mail-wm0-f67.google.com ([74.125.82.67]:36651 "EHLO
        mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S968250AbdAFI0w (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 6 Jan 2017 03:26:52 -0500
Date: Fri, 6 Jan 2017 11:18:44 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Keno Fischer <keno@juliacomputing.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        torvalds@linux-foundation.org, gthelen@google.com, npiggin@gmail.com,
        w@1wt.eu, oleg@redhat.com, keescook@chromium.org, luto@kernel.org,
        mhocko@suse.com, hughd@google.com
Subject: Re: [PATCH v2] mm: Respect FOLL_FORCE/FOLL_COW for thp
Message-ID: <20170106081844.GA4454@node.shutemov.name>
References: <20170106015025.GA38411@juliacomputing.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170106015025.GA38411@juliacomputing.com>
User-Agent: Mutt/1.5.23.1 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, Jan 05, 2017 at 08:50:25PM -0500, Keno Fischer wrote:
> In 19be0eaff ("mm: remove gup_flags FOLL_WRITE games from __get_user_pages()"),
> the mm code was changed from unsetting FOLL_WRITE after a COW was resolved to
> setting the (newly introduced) FOLL_COW instead. Simultaneously, the check in
> gup.c was updated to still allow writes with FOLL_FORCE set if FOLL_COW had
> also been set. However, a similar check in huge_memory.c was forgotten. As a
> result, remote memory writes to ro regions of memory backed by transparent huge
> pages cause an infinite loop in the kernel (handle_mm_fault sets FOLL_COW and
> returns 0 causing a retry, but follow_trans_huge_pmd bails out immidiately
> because `(flags & FOLL_WRITE) && !pmd_write(*pmd)` is true. While in this
> state the process is stil SIGKILLable, but little else works (e.g. no ptrace
> attach, no other signals). This is easily reproduced with the following
> code (assuming thp are set to always):
> 
>     #include <assert.h>
>     #include <fcntl.h>
>     #include <stdint.h>
>     #include <stdio.h>
>     #include <string.h>
>     #include <sys/mman.h>
>     #include <sys/stat.h>
>     #include <sys/types.h>
>     #include <sys/wait.h>
>     #include <unistd.h>
> 
>     #define TEST_SIZE 5 * 1024 * 1024
> 
>     int main(void) {
>       int status;
>       pid_t child;
>       int fd = open("/proc/self/mem", O_RDWR);
>       void *addr = mmap(NULL, TEST_SIZE, PROT_READ,
>                         MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
>       assert(addr != MAP_FAILED);
>       pid_t parent_pid = getpid();
>       if ((child = fork()) == 0) {
>         void *addr2 = mmap(NULL, TEST_SIZE, PROT_READ | PROT_WRITE,
>                            MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
>         assert(addr2 != MAP_FAILED);
>         memset(addr2, 'a', TEST_SIZE);
>         pwrite(fd, addr2, TEST_SIZE, (uintptr_t)addr);
>         return 0;
>       }
>       assert(child == waitpid(child, &status, 0));
>       assert(WIFEXITED(status) && WEXITSTATUS(status) == 0);
>       return 0;
>     }
> 
> Fix this by updating follow_trans_huge_pmd in huge_memory.c analogously to
> the update in gup.c in the original commit. The same pattern exists in
> follow_devmap_pmd. However, we should not be able to reach that check
> with FOLL_COW set, so add WARN_ONCE to make sure we notice if we ever
> do.
> 
> Signed-off-by: Keno Fischer <keno@juliacomputing.com>

Cc: stable@ ?

Acke-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
 Kirill A. Shutemov