linux-kernel - Re: [RESEND PATCH net] mm: page_alloc: fix ref bias in page_frag

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Date:   Fri, 15 Feb 2019 15:10:18 +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 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.