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Date:   Fri, 7 Jul 2023 12:45:07 -0400
From:   "Liam R. Howlett" <Liam.Howlett@Oracle.com>
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        John Hsu <John.Hsu@mediatek.com>, stable@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH stable v6.1] mm/mmap: Fix extra maple tree write
Message-ID: <20230707164507.mfekysya2jyupghp@revolver>
Mail-Followup-To: "Liam R. Howlett" <Liam.Howlett@Oracle.com>,
        Greg KH <gregkh@linuxfoundation.org>, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        John Hsu <John.Hsu@mediatek.com>, stable@vger.kernel.org,
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References: <20230706185135.2235532-1-Liam.Howlett@oracle.com>
 <2023070748-confiding-abnormal-b7e3@gregkh>
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* Greg KH <gregkh@linuxfoundation.org> [230707 11:55]:
> On Thu, Jul 06, 2023 at 02:51:35PM -0400, Liam R. Howlett wrote:
> > commit 0503ea8f5ba73eb3ab13a81c1eefbaf51405385a upstream.
> > 
> > This was inadvertently fixed during the removal of __vma_adjust().
> > 
> > When __vma_adjust() is adjusting next with a negative value (pushing
> > vma->vm_end lower), there would be two writes to the maple tree.  The
> > first write is unnecessary and uses all allocated nodes in the maple
> > state.  The second write is necessary but will need to allocate nodes
> > since the first write has used the allocated nodes.  This may be a
> > problem as it may not be safe to allocate at this time, such as a low
> > memory situation.  Fix the issue by avoiding the first write and only
> > write the adjusted "next" VMA.
> 
> Are you sure this is the same git id?  The one you reference above is
> _VERY_ different from your 2 line change below.
> 
> And the changelog text is not the same.

Yes, but I am not sure I've indicated what happened correctly.

The bug exists in the older __vma_adjust() function, but I removed
__vma_adjust() and inadvertently fixed the bug.  So the bug doesn't
exist upstream *because* of that commit:

0503ea8f5ba7 ("mm/mmap: remove __vma_adjust()")

My comment after the commit id indicates what happened, but the
documentation wasn't clear to me on how to specify what happened.

Does this answer your question?

Thanks,
Liam