Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7BE45C433FE for ; Wed, 24 Nov 2021 20:37:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237483AbhKXUk0 (ORCPT ); Wed, 24 Nov 2021 15:40:26 -0500 Received: from mail.kernel.org ([198.145.29.99]:50408 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231846AbhKXUkY (ORCPT ); Wed, 24 Nov 2021 15:40:24 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id AD72460E05; Wed, 24 Nov 2021 20:37:12 +0000 (UTC) Date: Wed, 24 Nov 2021 20:37:09 +0000 From: Catalin Marinas To: Matthew Wilcox Cc: Linus Torvalds , Josef Bacik , David Sterba , Andreas Gruenbacher , Al Viro , Andrew Morton , Will Deacon , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-btrfs@vger.kernel.org Subject: Re: [PATCH 3/3] btrfs: Avoid live-lock in search_ioctl() on hardware with sub-page faults Message-ID: References: <20211124192024.2408218-1-catalin.marinas@arm.com> <20211124192024.2408218-4-catalin.marinas@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 24, 2021 at 08:03:58PM +0000, Matthew Wilcox wrote: > On Wed, Nov 24, 2021 at 07:20:24PM +0000, Catalin Marinas wrote: > > +++ b/fs/btrfs/ioctl.c > > @@ -2223,7 +2223,8 @@ static noinline int search_ioctl(struct inode *inode, > > > > while (1) { > > ret = -EFAULT; > > - if (fault_in_writeable(ubuf + sk_offset, *buf_size - sk_offset)) > > + if (fault_in_exact_writeable(ubuf + sk_offset, > > + *buf_size - sk_offset)) > > break; > > > > ret = btrfs_search_forward(root, &key, path, sk->min_transid); > > Couldn't we avoid all of this nastiness by doing ... I had a similar attempt initially but I concluded that it doesn't work: https://lore.kernel.org/r/YS40qqmXL7CMFLGq@arm.com > @@ -2121,10 +2121,9 @@ static noinline int copy_to_sk(struct btrfs_path *path, > * problem. Otherwise we'll fault and then copy the buffer in > * properly this next time through > */ > - if (copy_to_user_nofault(ubuf + *sk_offset, &sh, sizeof(sh))) { > - ret = 0; > + ret = __copy_to_user_nofault(ubuf + *sk_offset, &sh, sizeof(sh)); > + if (ret) There is no requirement for the arch implementation to be exact and copy the maximum number of bytes possible. It can fail early while there are still some bytes left that would not fault. The only requirement is that if it is restarted from where it faulted, it makes some progress (on arm64 there is one extra byte). > goto out; > - } > > *sk_offset += sizeof(sh); > @@ -2196,6 +2195,7 @@ static noinline int search_ioctl(struct inode *inode, > int ret; > int num_found = 0; > unsigned long sk_offset = 0; > + unsigned long next_offset = 0; > > if (*buf_size < sizeof(struct btrfs_ioctl_search_header)) { > *buf_size = sizeof(struct btrfs_ioctl_search_header); > @@ -2223,7 +2223,8 @@ static noinline int search_ioctl(struct inode *inode, > > while (1) { > ret = -EFAULT; > - if (fault_in_writeable(ubuf + sk_offset, *buf_size - sk_offset)) > + if (fault_in_writeable(ubuf + sk_offset + next_offset, > + *buf_size - sk_offset - next_offset)) > break; > > ret = btrfs_search_forward(root, &key, path, sk->min_transid); > @@ -2235,11 +2236,12 @@ static noinline int search_ioctl(struct inode *inode, > ret = copy_to_sk(path, &key, sk, buf_size, ubuf, > &sk_offset, &num_found); > btrfs_release_path(path); > - if (ret) > + if (ret > 0) > + next_offset = ret; So after this point, ubuf+sk_offset+next_offset is writeable by fault_in_writable(). If copy_to_user() was attempted on ubuf+sk_offset+next_offset, all would be fine, but copy_to_sk() restarts the copy from ubuf+sk_offset, so it returns exacting the same ret as in the previous iteration. -- Catalin