From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wj0-f198.google.com (mail-wj0-f198.google.com [209.85.210.198]) by kanga.kvack.org (Postfix) with ESMTP id ADB316B0069 for ; Fri, 6 Jan 2017 03:18:49 -0500 (EST) Received: by mail-wj0-f198.google.com with SMTP id qs7so71047077wjc.4 for ; Fri, 06 Jan 2017 00:18:49 -0800 (PST) Received: from mail-wm0-x242.google.com (mail-wm0-x242.google.com. [2a00:1450:400c:c09::242]) by mx.google.com with ESMTPS id n9si88574026wjg.148.2017.01.06.00.18.46 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 06 Jan 2017 00:18:46 -0800 (PST) Received: by mail-wm0-x242.google.com with SMTP id c85so3140977wmi.1 for ; Fri, 06 Jan 2017 00:18:46 -0800 (PST) Date: Fri, 6 Jan 2017 11:18:44 +0300 From: "Kirill A. Shutemov" 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> Sender: owner-linux-mm@kvack.org List-ID: To: Keno Fischer 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 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 > #include > #include > #include > #include > #include > #include > #include > #include > #include > > #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 Cc: stable@ ? Acke-by: Kirill A. Shutemov -- Kirill A. Shutemov -- 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