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mm, mempolicy: fix up gup usage in lookup_node

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ba841078cd ("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
ae46d2aa6a ("mm/gup: Let __get_user_pages_locked() return -EINTR for
fatal signal") in the mean time so ba841078cd 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 <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Xu <peterx@redhat.com>
Link: http://lkml.kernel.org/r/20200421071026.18394-1-mhocko@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Michal Hocko 2020-06-03 16:03:25 -07:00 committed by Linus Torvalds
parent 5b94ce2fca
commit 2d3a36a479
2 changed files with 6 additions and 4 deletions
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5
mm/gup.c
View File

@ -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,

5
mm/mempolicy.c
View File

@ -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);
}