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Date:   Thu, 14 Feb 2019 22:26:22 +0100
From:   Jann Horn <>
To:     David Miller <>,
        Alexander Duyck <>
Cc:     Network Development <>,
        kernel list <>
Subject: Re: [RESEND PATCH net] mm: page_alloc: fix ref bias in
 page_frag_alloc() for 1-byte allocs

On Thu, Feb 14, 2019 at 6:13 PM David Miller <> wrote:
> From: Jann Horn <>
> Date: Wed, 13 Feb 2019 22:45:59 +0100
> > The basic idea behind ->pagecnt_bias is: If we pre-allocate the maximum
> > number of references that we might need to create in the fastpath later,
> > the bump-allocation fastpath only has to modify the non-atomic bias value
> > that tracks the number of extra references we hold instead of the atomic
> > refcount. The maximum number of allocations we can serve (under the
> > assumption that no allocation is made with size 0) is nc->size, so that's
> > the bias used.
> >
> > However, even when all memory in the allocation has been given away, a
> > reference to the page is still held; and in the `offset < 0` slowpath, the
> > page may be reused if everyone else has dropped their references.
> > This means that the necessary number of references is actually
> > `nc->size+1`.
> >
> > Luckily, from a quick grep, it looks like the only path that can call
> > page_frag_alloc(fragsz=1) is TAP with the IFF_NAPI_FRAGS flag, which
> > requires CAP_NET_ADMIN in the init namespace and is only intended to be
> > used for kernel testing and fuzzing.
> >
> > To test for this issue, put a `WARN_ON(page_ref_count(page) == 0)` in the
> > `offset < 0` path, below the virt_to_page() call, and then repeatedly call
> > writev() on a TAP device with IFF_TAP|IFF_NO_PI|IFF_NAPI_FRAGS|IFF_NAPI,
> > with a vector consisting of 15 elements containing 1 byte each.
> >
> > Signed-off-by: Jann Horn <>
> Applied and queued up for -stable.

I had sent a v2 at Alexander Duyck's request an hour before you
applied the patch (with a minor difference that, in Alexander's
opinion, might be slightly more efficient). I guess the net tree
doesn't work like the mm tree, where patches can get removed and
replaced with newer versions? So if Alexander wants that change
(s/size/PAGE_FRAG_CACHE_MAX_SIZE/ in the refcount), someone has to
send that as a separate patch?

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