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

[Alexei Starovoitov]

On Tue, Aug 22, 2023 at 2:34 PM Chris Li <chrisl@xxxxxxxxxx> wrote:
>
> Hi Alexei,
>
> On Tue, Aug 22, 2023 at 11:57 AM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > On Tue, Aug 22, 2023 at 10:48 AM Chris Li <chrisl@xxxxxxxxxx> wrote:
> > >
> > > Hi Mel,
> > >
> > > Adding Alexei to the discussion.
> > >
> > > On Mon, Aug 21, 2023 at 3:32 AM Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxxx/
>
> It is using an older version of the BPF program. That spinlock
> allocation was fixed
> in  commit c66a36af7ba3a628.

No. It was a temp workaround. It was fixed on bpf local storage side later.