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From:   Chris Li <chrisl@kernel.org>
Date:   Tue, 22 Aug 2023 10:48:42 -0700
Message-ID: <CAF8kJuOsWo5RfDcfnWZfnqYXjf6bkkxdXG1JCwjaEZ1nn29AaA@mail.gmail.com>
Subject: Re: [PATCH RFC 0/2] mm/page_alloc: free_pcppages_bulk safeguard
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Kemeng Shi <shikemeng@huaweicloud.com>,
        baolin.wang@linux.alibaba.com, Michal Hocko <mhocko@suse.com>,
        david@redhat.com, willy@infradead.org, linux-mm@kvack.org,
        Namhyung Kim <namhyung@google.com>,
        Greg Thelen <gthelen@google.com>, linux-kernel@vger.kernel.org,
        John Sperbeck <jsperbeck@google.com>,
        Huang Ying <ying.huang@intel.com>,
        Alexei Starovoitov <ast@kernel.org>
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Hi Mel,

Adding Alexei to the discussion.

On Mon, Aug 21, 2023 at 3:32=E2=80=AFAM Mel Gorman <mgorman@techsingularity=
.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@kernel.org>
>
> This sounds like a maintenance nightmare if internal state can be arbitra=
ry
> modified by a BPF program and still expected to work properly in all case=
s.
> Every review would have to take into account "what if a BPF script modifi=
es
> state behind our back?"

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.

How about the second (clean up) patch or Keming's clean up version? I can m=
odify
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