linux-kernel - Re: [PATCH RFC 0/2] mm/page_alloc: free_pcppages

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Message-ID: <CAF8kJuOFcVsAWt_sMKvSgw_vpikX0-+T4388pJn_F2-2sW_4Kg@mail.gmail.com>
Date:   Tue, 22 Aug 2023 14:34:40 -0700
From:   Chris Li <chrisl@...nel.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kemeng Shi <shikemeng@...weicloud.com>,
        baolin.wang@...ux.alibaba.com, Michal Hocko <mhocko@...e.com>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-mm <linux-mm@...ck.org>,
        Namhyung Kim <namhyung@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        John Sperbeck <jsperbeck@...gle.com>,
        Huang Ying <ying.huang@...el.com>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH RFC 0/2] mm/page_alloc: free_pcppages_bulk safeguard

Hi Alexei,

On Tue, Aug 22, 2023 at 11:57 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Tue, Aug 22, 2023 at 10:48 AM Chris Li <chrisl@...nel.org> wrote:
> >
> > Hi Mel,
> >
> > Adding Alexei to the discussion.
> >
> > On Mon, Aug 21, 2023 at 3:32 AM Mel Gorman <mgorman@...hsingularity.net> wrote:
> > >
> > > On Thu, Aug 17, 2023 at 11:05:22PM -0700, Chris Li wrote:
> > > > In this patch series I want to safeguard
> > > > the free_pcppage_bulk against change in the
> > > > pcp->count outside of this function. e.g.
> > > > by BPF program inject on the function tracepoint.
> > > >
> > > > I break up the patches into two seperate patches
> > > > for the safeguard and clean up.
> > > >
> > > > Hopefully that is easier to review.
> > > >
> > > > Signed-off-by: Chris Li <chrisl@...nel.org>
> > >
> > > This sounds like a maintenance nightmare if internal state can be arbitrary
> > > modified by a BPF program and still expected to work properly in all cases.
> > > Every review would have to take into account "what if a BPF script modifies
> > > state behind our back?"
>
> Where did this concern come from?
> Since when BPF can modify arbitrary state?
>
> But I wasn't cc-ed on the original patch, so not sure what it attempts to do.
> Maybe concern is valid.

Sorry I did not CC you on the original patch submission.  I should.

Here is the link for the 1/2 patch, which has the step to reproduce.

https://lore.kernel.org/linux-mm/20230817-free_pcppages_bulk-v1-1-c14574a9f80c@kernel.org/

It is using an older version of the BPF program. That spinlock
allocation was fixed
in  commit c66a36af7ba3a628.

Chris


>
> > Thanks for the feedback.
> >
> > I agree that it is hard to support if we allow BPF to change any internal
> > stage as a rule.  That is why it is a RFC. Would you consider it case
> > by case basis?
> > The kernel panic is bad, the first patch is actually very small. I can
> > also change it
> > to generate warnings if we detect the inconsistent state.
>
> panic and warns because of bpf prog?!


> bpf infra takes all the precaution to make sure that bpf progs
> can never cause such damage.
>
> >
> > How about the second (clean up) patch or Keming's clean up version? I can modify
> > it to take out the pcp->count if the verdict is just not supporting
> > BPF changing internal
> > state at all. I do wish to get rid of the pindex_min and pindex_max.
> >
> > Thanks
> >
> > Chris
>