linux-kernel - Re: [PATCH 1/2] mm/mempolicy: Allow lookup

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Date:   Thu, 9 Apr 2020 08:52:58 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        syzbot+693dc11fcb53120b5559@...kaller.appspotmail.com
Subject: Re: [PATCH 1/2] mm/mempolicy: Allow lookup_node() to handle fatal
 signal

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@...e.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@...e.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