linux-kernel - Re: [PATCH] mm/mmu_notifier: Fix mmget() assert in __mmu_interval_notifier

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Message-ID: <20200901002634.GD24045@ziepe.ca>
Date:   Mon, 31 Aug 2020 21:26:34 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Jann Horn <jannh@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH] mm/mmu_notifier: Fix mmget() assert in
 __mmu_interval_notifier_insert

On Tue, Sep 01, 2020 at 02:01:43AM +0200, Jann Horn wrote:
> The comment talks about having to hold mmget() (which means mm_users), but
> the actual check is on mm_count (which would be mmgrab()).
> 
> Given that MMU notifiers are torn down in
> mmput() -> __mmput() -> exit_mmap() -> mmu_notifier_release(),
> I believe that the comment is correct and the check should be on
> mm->mm_users. Fix it up accordingly.
> 
> Fixes: 99cb252f5e68 ("mm/mmu_notifier: add an interval tree notifier")
> Signed-off-by: Jann Horn <jannh@...gle.com>
> ---
> Can someone please double-check this? I'm like 90% sure that I fixed
> this the right way around, but it'd be good if someone more familiar
> with mmu notifiers could confirm.

Hmm.. I added the comment but copied the expression from older code.

The intent is certainly that the caller must have a mmget, this
function must not race with __mmu_notifier_release() and the mmget is
what prevents that.

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason