From: Peter Xu <peterx@redhat.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: linux-kernel@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
syzbot+693dc11fcb53120b5559@syzkaller.appspotmail.com
Subject: Re: [PATCH 1/2] mm/mempolicy: Allow lookup_node() to handle fatal signal
Date: Thu, 9 Apr 2020 08:52:58 -0400 [thread overview]
Message-ID: <20200409125258.GA362416@xz-x1> (raw)
In-Reply-To: <20200409070253.GB18386@dhcp22.suse.cz>
On Thu, Apr 09, 2020 at 09:02:53AM +0200, Michal Hocko wrote:
> This patch has been merged and it is actually wrong after ae46d2aa6a7f
> has been merged. We can either revert or I suggest just handling >0,
> like the patch below:
>
> From 03fbe30ec61e65b0927d0d41bccc7dff5f7eafd8 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Thu, 9 Apr 2020 08:26:57 +0200
> 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 0 return
> value is impossible. We always get an error or 1 for a single page
> request.
>
> Fixes: ba841078cd05 ("mm/mempolicy: Allow lookup_node() to handle fatal signal")
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
> mm/mempolicy.c | 5 +----
> 1 file changed, 1 insertion(+), 4 deletions(-)
>
> 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);
> }
Hi, Michal,
I'm totally not against this, but note that get_user_pages_locked()
could still return zero. Although I'm not 100% sure now on whether
npages==0 will be the only case, it won't hurt to keep this ret==0
check until we consolidate the whole gup code to never return zero.
Assuming there's another case (even possible for a future gup bug)
that could return a zero, your patch will let err be anything (which
you didn't initialize err with your patch), then the function will
return a random value. So even if you really want this change, I
would suggest you initialize err to some error code.
I just don't see much gain we get from removing that check.
Thanks,
--
Peter Xu
next prev parent reply other threads:[~2020-04-09 12:53 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-08 1:40 [PATCH 0/2] mm: Two small fixes for recent syzbot reports Peter Xu
2020-04-08 1:40 ` [PATCH 1/2] mm/mempolicy: Allow lookup_node() to handle fatal signal Peter Xu
2020-04-08 10:21 ` Michal Hocko
2020-04-08 14:20 ` Peter Xu
2020-04-08 14:30 ` Michal Hocko
2020-04-08 15:24 ` Peter Xu
2020-04-08 15:26 ` Michal Hocko
2020-04-09 7:02 ` Michal Hocko
2020-04-09 12:52 ` Peter Xu [this message]
2020-04-09 13:00 ` Peter Xu
2020-04-09 13:53 ` Michal Hocko
2020-04-09 16:42 ` Linus Torvalds
2020-04-14 11:04 ` Michal Hocko
2020-04-14 13:49 ` Peter Xu
2020-04-14 14:18 ` Michal Hocko
2020-04-20 12:47 ` Michal Hocko
2020-04-20 17:31 ` Linus Torvalds
2020-04-21 7:09 ` Michal Hocko
2020-04-08 1:40 ` [PATCH 2/2] mm/gup: Mark lock taken only after a successful retake Peter Xu
2020-04-09 0:47 ` [PATCH 0/2] mm: Two small fixes for recent syzbot reports Andrew Morton
2020-04-09 11:49 ` Matthew Wilcox
2020-04-09 13:00 ` Dmitry Vyukov
2020-04-09 18:16 ` Andrew Morton
2020-04-09 18:53 ` Linus Torvalds
2020-04-09 19:12 ` Andrew Morton
2020-04-09 19:46 ` Linus Torvalds
2020-04-09 19:56 ` Matthew Wilcox
2020-04-09 19:58 ` Linus Torvalds
2020-04-09 20:27 ` Eric Biggers
2020-04-09 20:34 ` Linus Torvalds
2020-04-09 23:34 ` Stephen Rothwell
2020-04-10 1:11 ` Theodore Y. Ts'o
2020-04-09 12:55 ` Dmitry Vyukov
2020-04-09 16:32 ` Linus Torvalds
2020-04-09 16:58 ` Qian Cai
2020-04-09 17:05 ` Linus Torvalds
2020-04-09 17:58 ` Qian Cai
2020-04-09 18:06 ` Linus Torvalds
2020-04-09 21:14 ` Qian Cai
2020-04-10 13:12 ` Tetsuo Handa
2020-04-10 14:26 ` Qian Cai
2020-04-10 17:26 ` Andrew Morton
2020-04-10 19:46 ` Qian Cai
2020-04-09 23:29 ` Stephen Rothwell
2020-04-13 22:06 ` Qian Cai
2020-04-13 23:05 ` Jens Axboe
2020-04-14 11:12 ` Dmitry Vyukov
2020-04-14 11:59 ` Qian Cai
2020-04-14 12:05 ` Dmitry Vyukov
2020-04-14 19:28 ` Dan Rue
2020-04-15 11:09 ` Dmitry Vyukov
2020-04-15 16:23 ` Dan Rue
2020-04-16 0:34 ` Stephen Rothwell
2020-05-11 15:29 ` Dmitry Vyukov
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