From 2d3a36a47964371101d9a71691c18d59ee611e87 Mon Sep 17 00:00:00 2001 From: Michal Hocko Date: Wed, 3 Jun 2020 16:03:25 -0700 Subject: [PATCH] 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. Signed-off-by: Michal Hocko Signed-off-by: Andrew Morton Cc: Peter Xu Link: http://lkml.kernel.org/r/20200421071026.18394-1-mhocko@kernel.org Signed-off-by: Linus Torvalds --- mm/gup.c | 5 +++++ mm/mempolicy.c | 5 +---- 2 files changed, 6 insertions(+), 4 deletions(-) diff --git a/mm/gup.c b/mm/gup.c index 61ee405ec558..e19ff770eb4c 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -989,6 +989,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags) * -- 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, diff --git a/mm/mempolicy.c b/mm/mempolicy.c index 48ba9729062e..1965e2681877 100644 --- a/mm/mempolicy.c +++ b/mm/mempolicy.c @@ -927,10 +927,7 @@ static int lookup_node(struct mm_struct *mm, unsigned long addr) 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); } -- 2.34.1