Re: [PATCH RFC 0/2] mm/page_alloc: free_pcppages_bulk safeguard

[Chris Li <chrisl@kernel.org>]

Hi Mel,

Adding Alexei to the discussion.

On Mon, Aug 21, 2023 at 3:32 AM 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 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