linux-kernel - Re: [PATCH 1/1] mm: lock VMAs skipped by a failed queue_pages

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Message-ID: <ZQlhaO1C5kf8zGjf@dhcp22.suse.cz>
Date:   Tue, 19 Sep 2023 10:52:56 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, willy@...radead.org, hughd@...gle.com,
        shy828301@...il.com, vbabka@...e.cz,
        syzkaller-bugs@...glegroups.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        syzbot+b591856e0f0139f83023@...kaller.appspotmail.com
Subject: Re: [PATCH 1/1] mm: lock VMAs skipped by a failed queue_pages_range()

On Mon 18-09-23 14:16:08, Suren Baghdasaryan wrote:
> When queue_pages_range() encounters an unmovable page, it terminates
> its page walk. This walk, among other things, locks the VMAs in the range.
> This termination might result in some VMAs being left unlock after
> queue_pages_range() completes. Since do_mbind() continues to operate on
> these VMAs despite the failure from queue_pages_range(), it will encounter
> an unlocked VMA.
> This mbind() behavior has been modified several times before and might
> need some changes to either finish the page walk even in the presence
> of unmovable pages or to error out immediately after the failure to
> queue_pages_range(). However that requires more discussions, so to
> fix the immediate issue, explicitly lock the VMAs in the range if
> queue_pages_range() failed. The added condition does not save much
> but is added for documentation purposes to understand when this extra
> locking is needed.

The semantic of the walk in this case is really clear as mud. I was
trying to reconstruct the whole picture and it really hurts... Then I
found http://lkml.kernel.org/r/CAHbLzkrmTaqBRmHVdE2kyW57Uoghqd_E+jAXC9cB5ofkhL-uvw@mail.gmail.com
and that helped a lot. Let's keep it a reference at least in the email
thread here for future.

> Fixes: 49b0638502da ("mm: enable page walking API to lock vmas during the walk")
> Reported-by: syzbot+b591856e0f0139f83023@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/all/000000000000f392a60604a65085@google.com/
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>

I cannot say I like the patch (it looks like a potential double locking
unless you realize this lock is special) but considering this might be just
temporal I do not mind. 

Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!

> ---
>  mm/mempolicy.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 42b5567e3773..cbc584e9b6ca 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1342,6 +1342,9 @@ static long do_mbind(unsigned long start, unsigned long len,
>  	vma_iter_init(&vmi, mm, start);
>  	prev = vma_prev(&vmi);
>  	for_each_vma_range(vmi, vma, end) {
> +		/* If queue_pages_range failed then not all VMAs might be locked */
> +		if (ret)
> +			vma_start_write(vma);
>  		err = mbind_range(&vmi, vma, &prev, start, end, new);
>  		if (err)
>  			break;
> -- 
> 2.42.0.459.ge4e396fd5e-goog

-- 
Michal Hocko
SUSE Labs