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

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Message-ID: <CAF8kJuOsWo5RfDcfnWZfnqYXjf6bkkxdXG1JCwjaEZ1nn29AaA@mail.gmail.com>
Date:   Tue, 22 Aug 2023 10:48:42 -0700
From:   Chris Li <chrisl@...nel.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Kemeng Shi <shikemeng@...weicloud.com>,
        baolin.wang@...ux.alibaba.com, Michal Hocko <mhocko@...e.com>,
        david@...hat.com, willy@...radead.org, linux-mm@...ck.org,
        Namhyung Kim <namhyung@...gle.com>,
        Greg Thelen <gthelen@...gle.com>, 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 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?"

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 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