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Date:   Fri, 15 Feb 2019 19:53:12 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND PATCH net] mm: page_alloc: fix ref bias in
 page_frag_alloc() for 1-byte allocs

On Fri, Feb 15, 2019 at 7:35 PM Alexander Duyck
<alexander.duyck@...il.com> wrote:
> On Fri, Feb 15, 2019 at 6:10 AM Jann Horn <jannh@...gle.com> wrote:
> > On Thu, Feb 14, 2019 at 11:21 PM David Miller <davem@...emloft.net> wrote:
> > > From: Jann Horn <jannh@...gle.com>
> > > Date: Thu, 14 Feb 2019 22:26:22 +0100
> > >
> > > > On Thu, Feb 14, 2019 at 6:13 PM David Miller <davem@...emloft.net> wrote:
> > > >> From: Jann Horn <jannh@...gle.com>
> > > >> 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 <jannh@...gle.com>
> > > >>
> > > >> 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?
> > >
> > > Yes, please send a follow-up.  Sorry about that.
> >
> > @Alexander Do you want to do that? It was your idea and I don't think
> > I can reasonably judge the usefulness of the change.
>
> I'll take care of it. I'm kind of annoyed that you resubmitted this to
> netdev before anyone had a chance to even provide review comments
> though.

Ah, I wasn't aware that there's a convention against resubmitting a
patch immediately if the first recipient list pointed to the wrong
maintainer. The only recipient I removed for the resend was akpm, so I
assumed that anyone wanting to comment on the patch could just as well
do that on the resent patch. Also, akpm had already put it in his
tree, and I wasn't sure whether that meant that I had to send it to
davem quickly enough to make it land in davem's tree before akpm sends
his next pull request to Linus...

So next time this happens, should I add a note below the resend
pointing out that the resend is of a recent patch and it hasn't had
time to be reviewed yet, or something like that?

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