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

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Date:   Tue, 22 Aug 2023 14:37:02 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
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 Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-mm <linux-mm@...ck.org>,
        Namhyung Kim <namhyung@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        LKML <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 2:34 PM Chris Li <chrisl@...nel.org> wrote:
>
> Hi Alexei,
>
> On Tue, Aug 22, 2023 at 11:57 AM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > On Tue, Aug 22, 2023 at 10:48 AM Chris Li <chrisl@...nel.org> 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?"
> >
> > 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@kernel.org/
>
> 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.