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

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Message-ID: <ZOT+RYe22TkkqCgP@casper.infradead.org>
Date:   Tue, 22 Aug 2023 19:28:21 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Chris Li <chrisl@...nel.org>
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@...hat.com, 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

On Tue, Aug 22, 2023 at 10:48:42AM -0700, Chris Li 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?"
> 
> 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.

We wouldn't allow C code that hooks spinlocks (eg lockdep) to allocate
memory.  I don't understand why BPF code should allocate memory either.