From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: [patch 120/131] mm, mempolicy: fix up gup usage in lookup_node Date: Wed, 03 Jun 2020 16:03:25 -0700 Message-ID: <20200603230325.hku-0B8kS%akpm@linux-foundation.org> References: <20200603155549.e041363450869eaae4c7f05b@linux-foundation.org> Reply-To: linux-kernel@vger.kernel.org Return-path: Received: from mail.kernel.org ([198.145.29.99]:47790 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726013AbgFCXIg (ORCPT ); Wed, 3 Jun 2020 19:08:36 -0400 In-Reply-To: <20200603155549.e041363450869eaae4c7f05b@linux-foundation.org> Sender: mm-commits-owner@vger.kernel.org List-Id: mm-commits@vger.kernel.org To: akpm@linux-foundation.org, linux-mm@kvack.org, mhocko@suse.com, mm-commits@vger.kernel.org, peterx@redhat.com, torvalds@linux-foundation.org From: Michal Hocko Subject: mm, mempolicy: fix up gup usage in lookup_node ba841078cd05 ("mm/mempolicy: Allow lookup_node() to handle fatal signal") has added a special casing for 0 return value because that was a possible gup return value when interrupted by fatal signal. This has been fixed by ae46d2aa6a7f ("mm/gup: Let __get_user_pages_locked() return -EINTR for fatal signal") in the mean time so ba841078cd05 can be reverted. This patch however doesn't go all the way to revert it because the check for 0 is wrong and confusing here. Firstly it is inherently unsafe to access the page when get_user_pages_locked returns 0 (aka no page returned). Fortunatelly this will not happen because get_user_pages_locked will not return 0 when nr_pages > 0 unless FOLL_NOWAIT is specified which is not the case here. Document this potential error code in gup code while we are at it. Link: http://lkml.kernel.org/r/20200421071026.18394-1-mhocko@kernel.org Signed-off-by: Michal Hocko Cc: Peter Xu Signed-off-by: Andrew Morton --- mm/gup.c | 5 +++++ mm/mempolicy.c | 5 +---- 2 files changed, 6 insertions(+), 4 deletions(-) --- a/mm/gup.c~mm-mempolicy-fix-up-gup-usage-in-lookup_node +++ a/mm/gup.c @@ -989,6 +989,7 @@ static int check_vma_flags(struct vm_are * -- If nr_pages is >0, but no pages were pinned, returns -errno. * -- If nr_pages is >0, and some pages were pinned, returns the number of * pages pinned. Again, this may be less than nr_pages. + * -- 0 return value is possible when the fault would need to be retried. * * The caller is responsible for releasing returned @pages, via put_page(). * @@ -1265,6 +1266,10 @@ retry: } EXPORT_SYMBOL_GPL(fixup_user_fault); +/* + * Please note that this function, unlike __get_user_pages will not + * return 0 for nr_pages > 0 without FOLL_NOWAIT + */ static __always_inline long __get_user_pages_locked(struct task_struct *tsk, struct mm_struct *mm, unsigned long start, --- a/mm/mempolicy.c~mm-mempolicy-fix-up-gup-usage-in-lookup_node +++ a/mm/mempolicy.c @@ -927,10 +927,7 @@ static int lookup_node(struct mm_struct int locked = 1; err = get_user_pages_locked(addr & PAGE_MASK, 1, 0, &p, &locked); - if (err == 0) { - /* E.g. GUP interrupted by fatal signal */ - err = -EFAULT; - } else if (err > 0) { + if (err > 0) { err = page_to_nid(p); put_page(p); } _