linux-kernel - Re: linux-next: manual merge of the mm tree with Linus' tree

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Message-ID: <20230728102022.7593856b@canb.auug.org.au>
Date:   Fri, 28 Jul 2023 10:21:00 +1000
From:   Stephen Rothwell <sfr@...b.auug.org.au>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: linux-next: manual merge of the mm tree with Linus' tree

Hi Matthew,

On Fri, 28 Jul 2023 00:49:50 +0100 Matthew Wilcox <willy@...radead.org> wrote:
>
> On Fri, Jul 28, 2023 at 09:18:49AM +1000, Stephen Rothwell wrote:
> > diff --cc mm/memory.c
> > index ca632b58f792,271982fab2b8..000000000000
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@@ -5392,32 -5597,18 +5597,21 @@@ retry
> >   	if (!vma)
> >   		goto inval;
> >   
> > - 	/* Only anonymous and tcp vmas are supported for now */
> > - 	if (!vma_is_anonymous(vma) && !vma_is_tcp(vma))
> >  -	/* find_mergeable_anon_vma uses adjacent vmas which are not locked */
> >  -	if (vma_is_anonymous(vma) && !vma->anon_vma)
> > --		goto inval;
> > --
> >   	if (!vma_start_read(vma))
> >   		goto inval;
> >   
> >  +	/*
> >  +	 * find_mergeable_anon_vma uses adjacent vmas which are not locked.
> >  +	 * This check must happen after vma_start_read(); otherwise, a
> >  +	 * concurrent mremap() with MREMAP_DONTUNMAP could dissociate the VMA
> >  +	 * from its anon_vma.
> >  +	 */
> > - 	if (unlikely(!vma->anon_vma && !vma_is_tcp(vma)))
> > - 		goto inval_end_read;
> > - 
> > - 	/*
> > - 	 * Due to the possibility of userfault handler dropping mmap_lock, avoid
> > - 	 * it for now and fall back to page fault handling under mmap_lock.
> > - 	 */
> > - 	if (userfaultfd_armed(vma))
> > ++	if (unlikely(vma_is_anonymous(vma) && !vma_is_tcp(vma)))
> >  +		goto inval_end_read;
> >  +  
> 
> No, this isn't right.  It should be:
> 
> 	if (unlikely(vma_is_anonymous(vma) && !vma->anon_vma))
> 		goto inval_end_read;

Yeah, see my second attempt.

> I'm not sure about the userfaultfd_armed() clause.  My patch wasn't
> intended to affect that.

That was removed by commit

  69f6bbd1317f ("mm: handle userfaults under VMA lock")

in the mm branch.

-- 
Cheers,
Stephen Rothwell

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